Acids are sour in taste.

Source: Chapter 2, Introduction

The examiner expects a single, direct word/phrase — "sour." Quoting from the textbook ("sour and bitter tastes of food are due to acids and bases, respectively") confirms this. Never write that tasting acids is recommended as a testing method; the textbook warns against tasting chemicals directly.

Bases are bitter in taste.

Source: Acids, Bases and Salts (Chapter 2), Introduction

This is a direct recall question. The textbook explicitly states "bases are bitter." Examiners expect the single keyword bitter. Do not confuse with acids, which are sour. One word is sufficient, but writing a complete sentence is better exam practice.

Blue litmus paper turns red when dipped in an acid.

Source: Chapter 2, Section 2.1 (Introduction)

This is a direct fact-based question. The key point examiners look for is the word "red". Remember the simple rule: acids turn blue litmus red; bases turn red litmus blue.

When red litmus paper is dipped in a base, it turns blue.

Source: Chapter 2, Section 2.1.1

This is a standard 1-mark factual question. Examiners expect just the colour change: red → blue. Remember the simple rule: bases turn red litmus blue; acids turn blue litmus red. No extra explanation is needed for 1 mark.

Litmus solution is extracted from lichens (a plant-like organism belonging to the Lichenes group).

Examiners expect the single word "lichens." Litmus is a natural dye obtained from lichens. Note that lichens are symbiotic organisms (fungi + algae), but for CBSE Class 10, the expected answer is simply "lichens."

When litmus solution is neither acidic nor basic (i.e., neutral), its colour is purple.

Source: Introduction, Chapter 2

The textbook explicitly states: "Litmus solution is a purple dye... When the litmus solution is neither acidic nor basic, its colour is purple." Students often only remember that acids turn litmus red and bases turn it blue, forgetting the neutral (original) purple colour. This is a direct one-line fact question — just state the colour clearly.

Two natural acid-base indicators are litmus (extracted from lichen) and turmeric.

The introduction of Chapter 2 explicitly mentions litmus and turmeric as natural indicators. Litmus is extracted from lichen (a plant of division Thallophyta). Coloured petals of flowers like Hydrangea, Petunia, and Geranium, as well as red cabbage leaves, are also acceptable answers — any two from this list will earn the mark.

Two synthetic indicators mentioned in the chapter are phenolphthalein and methyl orange.

The chapter lists these two synthetic indicators in Table 2.1 (Activity 2.1), where they are used alongside litmus to test acids and bases. Examiners expect both names stated clearly. Litmus is also a synthetic indicator but is already implied by red/blue litmus; phenolphthalein and methyl orange are the two specifically highlighted synthetic ones.

Olfactory indicators are substances whose smell (odour) changes in acidic or basic conditions. They are used to detect the presence of acids or bases.

Example: Onion and clove oil are olfactory indicators. The smell of onion disappears in a basic solution, while it remains in an acidic solution.

Source: Chapter 2, Acids, Bases and Salts

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• The term "olfactory" relates to smell (olfactory receptors detect smell, as mentioned in Chapter 6).
• CBSE expects students to define the term clearly and give at least one valid example. Onion and clove oil are the standard textbook examples for olfactory indicators.
• Do not confuse with litmus or turmeric (those are colour-change indicators, not smell-based).
• For 2 marks: 1 mark for correct definition + 1 mark for correct example.

(c) Turmeric

Turmeric is a natural acid-base indicator. Methyl orange and phenolphthalein are synthetic indicators, while universal indicator is a mixture of several synthetic indicators.

Source: Chapter 2, Introduction

The textbook explicitly states: "Litmus is a natural indicator, turmeric is another such indicator." Methyl orange and phenolphthalein are mentioned as synthetic indicators. Universal indicator is also synthetic (a mixture of dyes). Examiners expect students to know the distinction between natural and synthetic indicators.

Acid + Metal → Salt + Hydrogen gas

Source: Acids, Bases and Salts, Chapter 2

Examiners expect the exact word-equation format. The two key products are a salt (named after the acid and metal used) and hydrogen gas. Do not write a specific example — the question asks for the general equation. One line is sufficient for 1 mark.

When zinc granules react with dilute sulphuric acid, hydrogen gas (H₂) is evolved.

$$\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$$

Test: A burning candle or matchstick is brought near the mouth of the test tube. Hydrogen gas burns with a pop sound, confirming its presence.

Source: Chapter 1, Section 1.1.2; Chapter 2

• The first mark is for correctly naming hydrogen gas (and ideally writing the equation).
• The second mark is for the test — examiners specifically want: burning splint/candle + pop sound. Both details are needed for full credit.
• Do not confuse the test for H₂ (pop sound) with CO₂ (turns lime water milky) — a common mistake.

When zinc reacts with sodium hydroxide solution, it produces sodium zincate and hydrogen gas. The balanced chemical equation is:

$$\text{Zn}(s) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{ZnO}_2(aq) + \text{H}_2(g)$$

Zinc displaces hydrogen from sodium hydroxide, forming sodium zincate (Na₂ZnO₂) and hydrogen gas. This shows zinc is amphoteric — it reacts with both acids and bases.

• The examiner expects the correctly balanced equation (1 mark) and state symbols or identification of products (1 mark).
• Key point: Zinc is amphoteric; it reacts with NaOH (a base) just as it reacts with dilute acids, releasing H₂ gas both times.
• Products to remember: sodium zincate (Na₂ZnO₂) and hydrogen gas (H₂) — students often forget the zincate formula.
• Coefficients: 2 NaOH is needed to balance Na and H on both sides.

The product formed when zinc reacts with sodium hydroxide is sodium zincate (Na₂ZnO₂).

Zinc is an amphoteric metal, so it reacts with both acids and bases. With NaOH, it forms sodium zincate and hydrogen gas: Zn + 2NaOH → Na₂ZnO₂ + H₂↑. Examiners expect the name sodium zincate; mentioning hydrogen gas as the other product can earn full credit in some mark schemes.

Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

This is a direct recall from Chapter 2 (Acids, Bases and Salts). Examiners expect the exact word equation with all three products listed. Do not omit CO₂ or water — both are essential for full marks.

When carbon dioxide gas is passed through lime water, it turns milky due to the formation of white precipitate of calcium carbonate (CaCO₃).

$$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$

Source: Chapter 1, Section 1.2.1 Combination Reaction

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• The key observation (milky appearance) and the reason (CaCO₃ precipitate) must both be stated for full marks.
• Write the balanced chemical equation with correct state symbols — examiners specifically check for (s), (aq), (g), (l).
• This reaction is also the standard test for CO₂ gas — worth mentioning if asked in a broader question.

When excess CO₂ is passed through milky lime water, the white precipitate (calcium carbonate) dissolves and the solution turns clear/colourless. This happens because excess CO₂ reacts with CaCO₃ and water to form calcium bicarbonate, which is soluble in water:

$$\text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \rightarrow \text{Ca(HCO}_3)_2(aq)$$

Source: Chapter 1, Section 1.2.1 (Combination Reaction)

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• The examiner expects two things: the observation (turns clear) and the reason (CaCO₃ converts to soluble Ca(HCO₃)₂).
• Writing the balanced equation earns a mark and shows understanding.
• This is a classic two-step lime water reaction frequently asked in CBSE exams — milky due to CaCO₃ formation, then clear due to Ca(HCO₃)₂ formation on excess CO₂.

The three naturally occurring forms of calcium carbonate mentioned in the chapter are marble, the thin layer formed on whitewashed walls, and calcium carbonate deposited in nature.

(As per the chapter, marble and the calcium carbonate layer on whitewashed walls share the formula CaCO₃. The chapter specifically names marble as a naturally occurring form.)

Correction: The chapter mentions only marble explicitly as a naturally occurring form with formula CaCO₃. The passage does not list three naturally occurring forms.

> Based strictly on the source passage: The chapter mentions marble (CaCO₃) as a naturally occurring form of calcium carbonate.

Source: Chapter 1, Section 1.2.1

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The source passage only explicitly names marble as a naturally occurring form of CaCO₃. The question asks for three, but the provided passages do not support three distinct naturally occurring forms. If this question appears in your textbook exercise, it likely refers to marble, limestone, and chalk — all CaCO₃ — which are standard NCERT knowledge, but these are not all named in the given passage. In the exam, if the passage is the only source, write what the passage supports. If it is a general knowledge question from the chapter, the expected answer is: marble, limestone, and chalk.

A neutralisation reaction is the reaction between an acid and a base to form salt and water.

General equation:

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

Example:
$$\text{NaOH}(aq) + \text{HCl}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$$

Source: Acids, Bases and Salts, Chapter 2

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• 1 mark for the correct definition (acid + base → salt + water).
• 1 mark for the general/symbolic equation (or a correct example equation).
• Examiners look for the key words: acid, base, salt, and water. Missing any one of these loses the mark.
• A balanced example equation (like NaOH + HCl) can substitute for or support the general equation — include both to be safe.

$$\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$$

Source: Chapter 2, Section 2.1.4

This is a neutralisation reaction — acid + base → salt + water. Examiners expect the balanced equation with state symbols. NaCl (common salt) and water are the products. One mark is awarded for the correct balanced equation; state symbols add precision but the equation itself is the key requirement.

Phenolphthalein turns pink (magenta) in a basic solution.

Source: Acids, Bases and Salts (Chapter 2), Section 2.1.4

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Activity 2.6 in the textbook shows that when phenolphthalein is added to NaOH (a base), the solution turns pink. When acid is added and the base is neutralised, the pink colour disappears. Examiners accept either "pink" or "magenta" — both are correct. Do not write "red"; the exact word from the textbook activity is pink.

Metal oxide + Acid → Salt + Water

(e.g., CuO + H₂SO₄ → CuSO₄ + H₂O)

Source: Acids, Bases and Salts, Chapter 2

The examiner expects the general word equation and ideally one example with a chemical equation. "Metal oxide + Acid → Salt + Water" is the core answer and must be written exactly in this form. The example strengthens the answer but the general equation alone is sufficient for 1 mark.

Metallic oxides are called basic oxides because they react with water to form bases (alkalies) and react with acids to form salt and water.

For example:
$$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \text{ (a base)}$$
$$\text{CaO} + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O}$$

Thus, metallic oxides show basic nature, hence they are called basic oxides.

Source: Chapter 3, Section 3.1 (Activity 3.8)

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Examiners expect you to state the reason clearly — metallic oxides are basic because they dissolve in water to give bases and/or neutralise acids. Supporting with at least one chemical equation earns the second mark. Mentioning the exception (amphoteric oxides like Al₂O₃ and ZnO) is a bonus but not required for 2 marks.

Non-metallic oxides are considered acidic in nature because when they dissolve in water, they produce H⁺(aq) ions in solution. For example, sulphur dioxide dissolves in water to form sulphurous acid (H₂SO₃), which releases H⁺ ions. Since acidic nature of a substance is due to the formation of H⁺(aq) ions in solution, non-metallic oxides are acidic.

Source: Chapter 3, What you have learnt; Chapter 2, Section 2.2

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• The key link examiners expect: non-metallic oxides + water → acid → releases H⁺(aq) ions.
• The textbook states non-metals form oxides that are acidic or neutral, and acidic nature is due to H⁺(aq) ion formation in solution.
• Give one example (SO₂ → H₂SO₃, or CO₂ → H₂CO₃) to support the answer — this often fetches the second mark.
• Do not confuse with metals, which form basic oxides (except amphoteric ones like Al₂O₃, ZnO).

The solution turns blue-green in colour, due to the formation of copper(II) chloride (CuCl₂).

Source: Chapter 2, Section 2.1.5 – Reaction of Metallic Oxides with Acids

The textbook explicitly states in Activity 2.7 that "the colour of the solution becomes blue-green" when copper oxide dissolves in dilute HCl, forming copper(II) chloride. Examiners expect the exact colour "blue-green" and may also award marks for naming the product formed.

Glucose solution does not conduct electricity, which means it does not produce ions in solution. This tells us that glucose does not dissociate into H⁺ ions in water. Therefore, although glucose contains hydrogen, it is not an acid — the presence of hydrogen alone does not make a substance acidic.

Source: Chapter 2, Section 2.2 (Activity 2.8)

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The examiner wants two clear points: (1) glucose solution does not conduct electricity → no ions are produced, and (2) therefore glucose is not an acid despite containing hydrogen. This directly addresses the concept that all acids produce H⁺(aq) ions, but glucose does not. Avoid writing a long explanation of how HCl conducts — focus on what the glucose result tells us. These two points together earn both marks.

H⁺(aq) ions (hydrogen ions or hydronium ions, H₃O⁺) are responsible for the acidic properties of an acid in aqueous solution.

Source: Acids, Bases and Salts, Section 2.2

The examiner expects the specific ion name — H⁺(aq) or hydronium ion (H₃O⁺). Either form is acceptable, but mentioning both shows complete understanding. Avoid writing just "hydrogen" without the ionic notation, as that will likely lose the mark.

A hydronium ion (H₃O⁺) is formed when a hydrogen ion (H⁺) released by an acid combines with a water molecule. Since H⁺ ions cannot exist freely in solution, they attach to water to form hydronium ions.

Equation for its formation:

$$\text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+$$

Source: Acids, Bases and Salts, Chapter 2

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• Examiners expect the definition (what it is) and the equation — both are needed for full 2 marks.
• Key point: H⁺ ions do not exist independently in aqueous solution; they always associate with water to form H₃O⁺. Mentioning this shows conceptual clarity.
• The term "hydronium ion" and its formula H₃O⁺ must be written correctly.
• The equation must be balanced with correct formulae.

When a base dissolves in water, it produces hydroxide ions, OH⁻(aq), in solution. The formation of OH⁻(aq) ions is responsible for the basic nature of a substance.

The examiner expects you to name OH⁻ (hydroxide) ions specifically. The source passage directly states: "Formation of OH⁻(aq) ions in solution is responsible for the basic nature of a substance." Writing the ion with its aqueous notation [OH⁻(aq)] shows precision and earns full marks.

Source: Chapter 2, What you have learnt (summary points)

An alkali is a base that dissolves in water and produces OH⁻ (hydroxide) ions in solution. For example, NaOH and KOH are alkalis.

Source: Chapter 2, Section 2.2

The key distinction examiners look for is: all alkalis are bases, but not all bases are alkalis — only water-soluble bases are alkalis. Mentioning OH⁻ ion production is essential for full marks.

This experiment proves that water is essential for acids to show acidic behaviour. Dry HCl gas does not ionise in the absence of water, so it cannot produce H⁺ ions and does not turn dry litmus red. When water is present (wet litmus), HCl ionises to produce H⁺ (hydronium) ions, turning it red:

$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$

Thus, H⁺ ions are produced by HCl only in the presence of water.

Source: Chapter 2, Section 2.2.1

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• Examiners expect two key points: (1) dry HCl has no H⁺ ions (no water to ionise it), and (2) water enables ionisation/dissociation to form H₃O⁺.
• Writing the equation earns a mark and shows understanding of the chemistry.
• The core conclusion — "acids show acidic character only in aqueous solution" — must be stated explicitly.

Mixing concentrated acid with water is a highly exothermic process. If water is added to a concentrated acid, the large amount of heat generated can cause the mixture to splash out violently, causing burns. The glass container may also break due to excessive local heating. Therefore, acid must always be added slowly to water with constant stirring, so heat is dissipated safely by the large volume of water.

Source: Chapter 2, Section 2.2.1

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• The key reason is exothermic reaction — examiners expect this term.
• Mention both consequences: splashing/burns AND possible breakage of glass container (each is worth a point in a 2-mark answer).
• The phrase "acid to water, never water to acid" is a standard lab safety rule — state the correct procedure too.
• Do not go into pH or ionisation; that is off-topic for this question.

Dilution is the process of mixing an acid or base with water, which results in a decrease in the concentration of $\text{H}_3\text{O}^+$ or $\text{OH}^-$ ions per unit volume.

Source: Chapter 2, Section 2.2.1

The textbook's exact definition is key here: dilution = adding water → decrease in ion concentration per unit volume. Examiners expect both components — the action (mixing with water) and the effect (decrease in $\text{H}_3\text{O}^+$/$\text{OH}^-$ concentration). Don't confuse dilution with neutralisation.

The 'p' in pH stands for 'potenz', a German word meaning power.

Source: Chapter 2, Section 2.3 — How Strong Are Acid or Base Solutions?

The examiner expects both pieces of information: the word 'potenz' and its language (German) and meaning (power). Missing either part may cost the mark. Keep it to one line.

The pH scale generally ranges from 0 to 14, where 0 represents a very acidic solution and 14 represents a very alkaline solution. A neutral solution has a pH of 7.

Source: Chapter 2, Section 2.3

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The key facts to state are the range (0–14), what the extremes mean, and the neutral value (7). Examiners expect all three points in a 1-mark answer — even one crisp sentence covering all three earns full marks. Do not write more than one or two lines.

The pH of a neutral solution is 7.

Source: Chapter 2, Section 2.3

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This is a direct one-line fact. The examiner expects the exact value "7." You may optionally add that values below 7 are acidic and above 7 are basic, but for 1 mark, just stating pH = 7 is sufficient. Do not write more than one line.

(c) Strongly acidic

A solution with pH = 3 is strongly acidic, as values much less than 7 on the pH scale represent high H⁺ ion concentration, indicating a strongly acidic solution.

The pH scale runs from 0 (most acidic) to 14 (most basic), with 7 being neutral. pH 3 is well below 7, indicating a strongly acidic solution — not weakly acidic (which would be closer to 7, e.g., pH 5–6). Remember: lower the pH, higher the H⁺ concentration and stronger the acid.

A universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions in a solution.

It is used to measure the pH of a solution (on a scale of 0–14), which indicates how acidic or basic a solution is. It is commonly used in the form of pH paper impregnated with the universal indicator.

Source: Chapter 2, Section 2.3

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• The question is worth 2 marks: one mark for defining universal indicator, one mark for its use.
• Key phrase: "mixture of several indicators" — examiners expect this exact description.
• Mention pH scale (0–14) and that it measures hydrogen ion concentration; this shows understanding of why it is used, not just what it measures.
• Avoid writing about pH importance in everyday life — that goes beyond what the question asks and wastes time.

The human body works within the pH range of 7.0 to 7.8.

Source: Chapter 2, Section 2.3.1 (Importance of pH in Everyday Life)

The examiner expects the exact numerical range (7.0 to 7.8) as stated in the textbook. Simply writing "neutral" or "slightly basic" without the numbers will not earn full marks. Since this is 1 mark, one line with the correct range is sufficient.

Acid rain is the rain water which has a pH less than 5.6. When oxides of nitrogen and sulphur (released by burning of fossil fuels) dissolve in rain water, they form nitric acid and sulphuric acid, making the rain acidic. Rain water with pH less than 5.6 is called acid rain.

Source: Chapter 2, Section 2.3.1

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• The definition must include two parts: what causes it (acidic gases dissolving in rain) and the pH value (< 5.6) — each part carries 1 mark.
• Examiners specifically look for the pH threshold of 5.6; writing "less than 7" will not earn the mark.
• Mentioning SO₂/NO₂ from fossil fuels adds accuracy but the pH value is the critical fact for the second mark.

Acid rain lowers the pH of river water, making it too acidic for aquatic organisms to survive, thus harming or killing aquatic life.

The source passages mention that living beings carry out metabolic activities within an optimal pH range (Chapter 2) and that human activities impact the environment (Chapter 13). Acid rain disrupts the pH of water bodies, directly threatening aquatic organisms. For a 1-mark answer, state the cause (lowered pH) and its effect (harm to aquatic life) concisely in one line.

Tooth decay starts when the pH of the mouth falls below 5.5. At this pH, the tooth enamel (calcium hydroxyapatite) gets corroded by acids produced by bacteria.

The key fact to state is the specific pH value: lower than 5.5. Examiners expect the exact threshold. Mentioning that enamel gets corroded and that bacteria produce the acid adds completeness but the pH value is the core 1-mark answer.

Tooth enamel is made of a hard, mineralised substance. Bacteria acting on sugars produce acids that demineralise (soften) the enamel, leading to dental caries (tooth decay). Enamel is important because it forms the protective outer covering of teeth; once it is softened or destroyed, microorganisms can invade deeper layers, causing infection.

Source: Chapter 5, Section 5.2.4 — "More to Know: Dental Caries"

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• The passage mentions enamel in the context of dental caries — it is demineralised by bacterial acids. The textbook does not explicitly state the chemical composition of enamel beyond calling it a hard mineralised layer, so do not over-specify.
• Examiners award 1 mark for stating what enamel is/does (hard protective covering, mineralised) and 1 mark for its importance (protects teeth; acid softens/demineralises it leading to decay and infection).
• Keep your answer focused on the passage; do not bring in outside chemistry (e.g., hydroxyapatite) unless taught in class.

Tooth decay is caused by bacteria acting on sugars to produce acids. These acids demineralise the tooth enamel. Bacteria and food particles form dental plaque, which covers teeth so saliva cannot neutralise the acid. Toothpaste helps by removing the plaque through brushing, preventing acid formation and thus protecting the enamel from decay.

Source: Chapter 5, Section 5.2.4 — "More to Know! Dental caries"

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• The key points examiners expect: (1) bacteria produce acid from sugars, (2) plaque blocks saliva from neutralising acid, (3) brushing with toothpaste removes plaque before acid is produced.
• Toothpaste itself isn't explicitly mentioned in the passage beyond brushing, but the question implies brushing with toothpaste = plaque removal — mention that connection.
• Do not write about digestion or other topics; keep the answer focused on dental caries only.

A bee sting injects formic acid (methanoic acid) into the skin, causing pain and irritation.

Remedy: A mild base such as baking soda (sodium hydrogencarbonate) solution should be applied to the affected area. It neutralises the acid, providing relief.

Source: Acids, Bases and Salts, Chapter 2

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• The examiner expects you to name the specific acid (formic/methanoic acid) for 1 mark and state the correct remedy with a brief reason (neutralisation) for the second mark.
• Simply saying "apply a base" without naming it may cost you half a mark — always name baking soda.
• The key concept being tested is neutralisation: acid + base → salt + water. Mentioning this word strengthens your answer.

The stinging hair of nettle leaves injects methanoic acid (formic acid) into the skin, causing a burning sensation.

This is a standard 1-mark factual question from Chapter 2. Examiners accept either name: methanoic acid (IUPAC) or formic acid. Note: this fact is from the NCERT textbook context for Chapter 2, even though it does not appear explicitly in the provided passages — it is a well-known "Do You Know?" fact from that chapter. Write just the one line; no extra detail needed.

Vinegar contains ethanoic acid (acetic acid), which is a carboxylic acid. It is a weak acid.

Source: Properties of Ethanoic Acid, Chapter 4

The examiner expects two pieces of information: the specific name (ethanoic/acetic acid) and its type (carboxylic acid). Mentioning it is a weak acid is a bonus but not strictly required for 1 mark. Avoid writing more than one line.

Tartaric acid is found in tamarind.

This is a straightforward factual question. The expected answer is simply the name of the acid — tartaric acid. Note: the source passages do not explicitly mention tamarind, but this is standard CBSE Class 10 science knowledge (Chapter 2). Do not confuse it with acetic acid (vinegar) or citric acid (lemon/citrus fruits).

Citric acid is present in lemon and orange.

This is a direct recall fact. The examiner expects the specific name "citric acid." No further detail is needed for 1 mark.

The acid produced in the human stomach that helps in digestion is hydrochloric acid (HCl). It creates an acidic medium that facilitates the action of the enzyme pepsin.

Source: Chapter 5, Section 5.2.4 – Nutrition in Human Beings

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The textbook explicitly states that gastric glands in the stomach wall release hydrochloric acid (HCl). Examiners expect the full name and the chemical formula for full marks. Mentioning its role (activating pepsin) strengthens the answer but is not strictly required for 1 mark. Avoid writing "stomach acid" alone — always name it as HCl.

Antacids are medicines used to neutralise excess hydrochloric acid produced in the stomach, thereby relieving indigestion and acidity. They are mild bases that react with the excess acid to form salt and water.

One mild base used as an antacid: Milk of Magnesia (Magnesium hydroxide, Mg(OH)₂).

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• The question has two parts: definition (1 mark) and example (1 mark). Keep it concise.
• Key points examiners look for: (1) antacids neutralise excess stomach acid, (2) they are mild/weak bases, (3) a correct named example. Mg(OH)₂ is the standard NCERT example; baking soda (NaHCO₃) is also acceptable.
• Do not write a long paragraph — two sentences plus the example is sufficient for 2 marks.

(c) Exactly 7

Salts of a strong acid and a strong base are neutral, with pH value of exactly 7.

Source: Acids, Bases and Salts, Section 2.4.2

The textbook directly states: "Salts of a strong acid and a strong base are neutral with pH value of 7." Example: NaCl (from HCl + NaOH). Remember — acidic salts (strong acid + weak base) have pH < 7, and basic salts (strong base + weak acid) have pH > 7.

The pH of a salt formed from a strong acid and a weak base is less than 7 (acidic in nature).

Source: Acids, Bases and Salts, Section 2.4.2

The textbook explicitly states: "salts of a strong acid and weak base are acidic with pH value less than 7." Example: NH₄Cl (from HCl + NH₄OH). Examiners expect the pH value (less than 7) and the word "acidic" — both are needed for full credit.

Rock salt is the naturally occurring solid form of sodium chloride (NaCl). It appears as large crystals, often brown due to impurities.

Formation of beds: Beds of rock salt were formed when seas of bygone ages dried up, leaving behind solid deposits of salt. These beds are mined like coal.

Source: Chemicals from Common Salt, Chapter 2

The question has two parts — definition and formation — each worth 1 mark. The key phrases examiners look for are: "solid salt / sodium chloride," "seas of bygone ages dried up," and "mined like coal." Keep the answer concise and use the exact textbook language for full marks.

The chlor-alkali process is the electrolysis of brine (common salt solution, NaCl). When electricity is passed through brine, it decomposes to give chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide (NaOH) solution is formed near the cathode.

$$\text{NaCl(aq)} \xrightarrow{\text{electrolysis}} \text{Na}^+\text{(aq)} + \text{Cl}^-\text{(aq)} \rightarrow \text{NaOH} + \text{Cl}_2 + \text{H}_2$$

All three products are useful industrially — chlorine for bleaching and disinfecting, hydrogen as fuel, and NaOH in paper and textile industries.

Source: Chapter 2 (Acids, Bases and Salts), Uses of salts section

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• The question is from Class 10 Chapter 2 (Acids, Bases and Salts), specifically the section on uses of sodium chloride (common salt) and its electrolysis.
• Examiners expect: the name brine, the term electrolysis, all three products (Cl₂, H₂, NaOH), and at least one use of each product.
• The source passages provided do not directly cover this topic, but the answer is grounded in the prescribed NCERT textbook content for Chapter 2.
• Do not confuse with the chlorination (substitution) reaction of hydrocarbons — that is a different chapter entirely.

The chlor-alkali process involves passing electricity through an aqueous solution of sodium chloride (brine). The balanced chemical equation is:

$$2\text{NaCl}(aq) + 2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{NaOH}(aq) + \text{Cl}_2(g) + \text{H}_2(g)$$

The products formed are sodium hydroxide (NaOH), chlorine gas (at anode), and hydrogen gas (at cathode).

Source: Chapter 2, Acids, Bases and Salts

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• Examiners expect the correct balanced equation with state symbols and the word "electricity" above the arrow — these earn the marks.
• The three products (NaOH, Cl₂, H₂) must all be mentioned; missing any one costs a mark.
• The name "chlor-alkali" comes from the products: chlor = chlorine, alkali = NaOH. Mentioning this context can help in descriptive parts but is not required here.

In the chlor-alkali process, electrolysis of brine (aqueous NaCl solution) produces:

• Anode (+): Chlorine gas (Cl₂)
• Cathode (−): Hydrogen gas (H₂)

(Sodium hydroxide solution is also formed near the cathode.)

Source: Acids, Bases and Salts, Chapter 2

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The examiner expects you to correctly identify which gas at which electrode — this is where marks are distributed (1 mark each). Remember the mnemonic: Anode → Chlorine (AC), Cathode → Hydrogen. NaOH is a bonus point sometimes asked separately. The source passages don't detail this reaction, but it is a standard NCERT Chapter 2 (Acids, Bases and Salts) topic on uses of sodium chloride — ensure you recall it from the textbook directly.

Bleaching powder is produced by passing chlorine gas over dry slaked lime [Ca(OH)₂].

$$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$

Slaked lime reacts with chlorine to form calcium oxychloride (CaOCl₂), known as bleaching powder, and water.

Source: Acids, Bases and Salts, Chapter 2

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• The examiner expects two things: the process (chlorine passed over dry slaked lime) and the balanced chemical equation — one mark each.
• Write the full names of reactants and products alongside the formula.
• "Dry" slaked lime is important — mention it to show accuracy.
• The chemical name of bleaching powder (calcium oxychloride) is a bonus detail that strengthens the answer.

The chemical formula of bleaching powder is CaOCl₂ (Calcium oxychloride / Calcium hypochlorite and chloride).

Bleaching powder is Ca(OCl)Cl, commonly written as CaOCl₂. This is a standard factual recall question from Chapter 2 (Acids, Bases and Salts). Note that the source passages provided do not directly mention bleaching powder, but this formula is part of the prescribed CBSE Class 10 Science syllabus (NCERT Chapter 2). Examiners expect the correct formula; writing the name alone will not fetch the mark.

Two uses of bleaching powder are:

1. It is used for disinfecting drinking water to make it free from germs.
2. It is used as an oxidising agent in chemical industries and for bleaching cotton and linen in the textile industry.

Bleaching powder (Ca(OCl)Cl) is a key topic from the chapter on Acids, Bases and Salts. Examiners expect any two distinct, correct uses. Common acceptable uses include: disinfecting water, bleaching cotton/linen/wood pulp, as an oxidising agent, and for making wool unshrinkable. Write two clearly separate points for full 2-mark credit. Note: the source passages provided do not cover bleaching powder directly — this answer is based on the standard NCERT Class 10 Science Chapter 2 content on bleaching powder.

The chemical name of baking soda is Sodium hydrogen carbonate (also called sodium bicarbonate). Its chemical formula is NaHCO₃.

Examiners expect both the chemical name and formula for full marks. "Sodium bicarbonate" is also accepted, but "sodium hydrogen carbonate" is the preferred IUPAC/NCERT name. Writing only the name or only the formula would likely lose half the credit.

Baking soda is sodium hydrogencarbonate (NaHCO₃). It is prepared from sodium chloride (common salt) by the following reaction:

$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$

The reactants are sodium chloride, water, carbon dioxide, and ammonia. The products formed are ammonium chloride and sodium hydrogencarbonate (baking soda).

Source: Chapter 2, Section 2.4.3 – Chemicals from Common Salt

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• Equation is essential for both marks — writing it correctly (balanced, with correct formulae) fetches full credit.
• Name the product: Examiners expect you to identify NaHCO₃ as sodium hydrogencarbonate/baking soda.
• Do not confuse this with the equation for washing soda or the chlor-alkali process — both involve NaCl but give different products.
• Writing the names of reactants/products below the equation (as done in the textbook) is good practice and shows understanding.

When baking soda (sodium hydrogen carbonate) is heated, it decomposes to form sodium carbonate, carbon dioxide, and water. This is a thermal decomposition reaction.

$$2\text{NaHCO}_3(s) \xrightarrow{\Delta} \text{Na}_2\text{CO}_3(s) + \text{CO}_2(g) + \text{H}_2\text{O}(g)$$

Source: Acids, Bases and Salts, Chapter 2

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• 1 mark for stating the products (sodium carbonate, CO₂, water) / identifying it as decomposition.
• 1 mark for the correctly balanced chemical equation with state symbols.
• Always include the heat symbol (Δ) above the arrow to show thermal decomposition.
• State symbols — (s), (g) — are expected and may cost a mark if missing.

Baking powder is a mixture of baking soda (sodium hydrogen carbonate, NaHCO₃) and a mild edible acid such as tartaric acid.

How it makes bread/cake soft and spongy:
When baking powder is heated during baking, the acid reacts with sodium hydrogen carbonate to release carbon dioxide (CO₂) gas:

$$\text{NaHCO}_3 + \text{acid} \xrightarrow{\Delta} \text{CO}_2 + \text{H}_2\text{O} + \text{salt}$$

The CO₂ gas produced gets trapped in the dough, forming bubbles. This causes the dough to rise and become soft and spongy. Baking powder is preferred over baking soda alone because no bitter taste is left — the acid neutralises the sodium carbonate formed.

Source: Chapter 2, Acids, Bases and Salts

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• Key points examiners look for: Definition of baking powder (mixture, name both components), the chemical reaction producing CO₂, and how CO₂ makes it spongy.
• The equation/reaction is important for full marks — don't skip it.
• Mentioning why baking powder is used instead of baking soda alone (no bitter taste) is a bonus point that can secure full marks.
• Note: The source passages provided did not directly cover this topic, but this is standard NCERT Chapter 2 content examiners expect.

Two uses of baking soda (NaHCO₃):

1. Antacid: It is used as a remedy for acidity (indigestion) because it neutralises excess hydrochloric acid present in the stomach.
2. Fire extinguisher: It is used in soda-acid fire extinguishers, where it reacts with acid to produce CO₂, which extinguishes the fire.

Source: Acids, Bases and Salts, Chapter 2

The question is directly from Exercise 15 of Chapter 2. Examiners expect two distinct, clearly stated uses — one mark each. Mentioning baking powder (used in cooking) is also acceptable as a third use, but the two above are most textbook-standard. Always name the use and briefly state the reason/reaction for full marks.

Washing soda is Sodium carbonate decahydrate, with the chemical formula Na₂CO₃·10H₂O.

The examiner expects both the chemical name and formula in one line. "Decahydrate" is important as it indicates the 10 water molecules of crystallisation. Omitting it may cost marks.

Baking soda (sodium hydrogencarbonate, NaHCO₃) is heated to obtain washing soda (sodium carbonate, Na₂CO₃). The reaction is:

$$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$

Na₂CO₃ is then recrystallised to get washing soda, Na₂CO₃·10H₂O.

Source: Acids, Bases and Salts, Chapter 2

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• Examiners expect the chemical name and formula of both baking soda and washing soda.
• The balanced chemical equation with heat symbol (Δ) is essential for full marks.
• Mentioning recrystallisation (addition of water of crystallisation) completes the answer and shows awareness that washing soda is the decahydrate.
• Do not confuse baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O) — both names and formulas must be correct.

Two uses of washing soda (Na₂CO₃·10H₂O):

1. It is used in the manufacture of glass, soap, and paper industries.
2. It is used as a cleaning agent for domestic purposes (removing grease and stains) and for softening hard water.

The source passages provided do not directly mention washing soda's uses, but this is standard CBSE Chapter 2 (Acids, Bases and Salts) content. Examiners expect any two distinct uses — industrial (glass/soap/paper manufacture) and domestic (cleaning agent/water softener) are the most commonly accepted. Write each use as a separate point for clear mark allocation.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, e.g., CuSO₄·5H₂O has 5 water molecules.

The definition is directly from the "What you have learnt" summary of Chapter 2. Examiners expect the exact phrase "fixed number of water molecules" and "one formula unit of a salt". Adding an example (like blue vitriol) strengthens a 1-mark answer. Do not write more than one line.

The chemical formula of hydrated copper sulphate is CuSO₄.5H₂O; five water molecules are present in one formula unit.

Source: Chapter 2, Section 2.4.4

The examiner expects both pieces of information: the correct formula CuSO₄.5H₂O and the number 5. Writing only the formula without stating "five water molecules" (or vice versa) will likely cost you the mark, as the question explicitly asks for both. Remember: the ".5H₂O" part represents the water of crystallisation — the fixed number of water molecules in one formula unit of the salt.

When copper sulphate crystals are heated strongly, two changes are observed:

1. Change in colour: The blue colour of copper sulphate crystals changes to white, as the crystals lose water of crystallisation (CuSO₄·5H₂O → CuSO₄ + 5H₂O).

1. Change in state: The hydrated crystals (solid with water) become anhydrous copper sulphate, which is a white powder (solid).

This is a thermal decomposition reaction.

Source: Chapter 1, Chemical Reactions and Equations

The question tests knowledge of the effect of heating a hydrated salt. Key points examiners look for:

• Correct colour change: blue → white (not grey or brown — those relate to other substances).
• Reason: loss of water of crystallisation.
• Mentioning it is a decomposition reaction adds value.
• Note: The source passages describe heating of ferrous sulphate in detail; copper sulphate's dehydration on heating is a standard NCERT-linked fact from the same chapter's context. Do not confuse with copper powder heating (which gives black CuO — a different reaction).

Gypsum is a soft mineral (hydrated calcium sulphate) used in the production of Plaster of Paris. Its chemical formula is CaSO₄·2H₂O.

This question comes from the chapter on acids, bases and salts (Chapter 2), though it is not covered in the source passages provided. Gypsum is the standard one-line answer expected: name + formula. Remember that Plaster of Paris (CaSO₄·½H₂O) is made by heating gypsum — examiners often test the distinction between the two formulas.

Plaster of Paris (POP) is a white powder with the chemical formula $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ (calcium sulphate hemihydrate). It is used in making casts for fractured bones, toys, and decorative materials.

Preparation from Gypsum:
Gypsum is $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ (calcium sulphate dihydrate). When gypsum is heated at 373 K (100°C), it loses three-fourths of its water of crystallisation to form Plaster of Paris:

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O}$$

On mixing with water, POP sets into a hard solid (gypsum) again.

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• 1 mark for definition/formula of POP.
• 1 mark for the correct condition (heating at 373 K) and starting material (gypsum with formula).
• 1 mark for the balanced chemical equation.
• Examiners expect the equation, both formulas, and the temperature 373 K — these are the most common marks lost. The reverse reaction (setting of POP) is a bonus line but not mandatory.

Plaster of Paris (calcium sulphate hemihydrate) reacts with water to form gypsum (calcium sulphate dihydrate). The chemical equation is:

$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}(s) + \frac{3}{2}\text{H}_2\text{O}(l) \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}(s)$$

$$\text{(Plaster of Paris)} \hspace{3cm} \text{(Gypsum)}$$

This reaction is exothermic and the mixture sets into a hard solid mass.

Source: Chapter 2, Acids, Bases and Salts

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• The examiner expects the correct chemical formula of Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$) and gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) — these formulas fetch the marks.
• Mention that the product is gypsum and that the mixture sets hard — this shows understanding and is often expected for full marks.
• This question is directly linked to Exercise Q13 of Chapter 2: "Plaster of Paris should be stored in a moisture-proof container" — the same reaction explains why (it absorbs moisture and sets hard, becoming useless).

Uses of Plaster of Paris:

1. It is used by doctors for setting fractured bones in the correct position (making plaster casts/splints).
2. It is used in making toys, decorative items, and for making smooth surfaces in construction (wall plaster/ceilings).

(Source: Chapter 2 – Acids, Bases and Salts)

The question is from Chapter 2 (Acids, Bases and Salts), specifically the section on Plaster of Paris (CaSO₄·½H₂O). The two most commonly expected uses in CBSE board exams are:

• Medical use (setting fractured/broken bones) — always mention this first.
• Construction/decorative use (making toys, statues, smooth wall surfaces).

Note: The source passages provided do not cover this topic, but these are the standard CBSE textbook answers. Write both points clearly for full 2 marks.

Plaster of Paris (CaSO₄·½H₂O) should be stored in a moisture-proof container because it readily reacts with water (moisture) present in air and gets converted back into gypsum (CaSO₄·2H₂O). This makes it set and harden prematurely, making it unfit for further use.

Source: Chapter 2 – Acids, Bases and Salts (Plaster of Paris section)

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• The examiner expects you to state what happens when POP absorbs moisture (it converts to gypsum/sets hard) and why that is a problem (becomes useless).
• Two clear points = 2 marks: (1) reacts with moisture → forms gypsum, (2) hardens prematurely / cannot be used.
• Note: The source passages provided do not cover this topic — this answer is drawn from the standard NCERT Class 10 Science Chapter 2 content on Plaster of Paris, which is the correct textbook reference for this question.

(c) Na₂CO₃·10H₂O

Washing soda is sodium carbonate decahydrate with the chemical formula Na₂CO₃·10H₂O.

Source: Chapter 2, Section 2.4.3

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The key distinction here is between Na₂CO₃ (anhydrous sodium carbonate) and Na₂CO₃·10H₂O (washing soda, which contains 10 water molecules of crystallisation). NaHCO₃ is baking soda — a common distractor. Examiners specifically test whether students know the water of crystallisation (·10H₂O) is part of the formula for washing soda.

(c) Antacid

Antacids are used to treat indigestion caused by excess acid in the stomach, as they neutralise the excess acid.

Source: Exercises, Chapter 2, Q.4

Antacids are mild bases (e.g., milk of magnesia, baking soda) that neutralise excess stomach acid via a neutralisation reaction. Antibiotics fight bacteria, analgesics relieve pain, and antiseptics prevent infection — none of these treat acidity.

(d) 10

A solution that turns red litmus blue is basic/alkaline. Basic solutions have pH greater than 7. Among the given options, only 10 is greater than 7, so the pH is most likely 10.

Source: Chapter 2, Section 2.3 (pH Scale)

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The key rule: pH > 7 → basic (turns red litmus blue); pH < 7 → acidic; pH = 7 → neutral. Options (a), (b), and (c) are all below 7 (acidic), so only (d) 10 is correct. Examiners award the mark for the correct option with a one-line justification.

The gastric glands in the stomach wall secrete mucus along with hydrochloric acid and pepsin. This mucus forms a protective lining on the inner wall of the stomach, shielding it from the corrosive action of the acid under normal conditions. Thus, the stomach does not get damaged by its own acid.

Source: Chapter 5, Section 5.2.4 – Nutrition in Human Beings

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• Examiners expect two key points: (1) mucus is secreted by gastric glands, and (2) mucus protects the inner lining from acid.
• The word "under normal conditions" is important — it also hints at why excess acid causes acidity/ulcers, which shows understanding.
• Avoid writing vague phrases like "the stomach is strong"; be specific about mucus as the protective agent.

Curd and sour substances contain acids. When stored in brass or copper vessels, these acids react with the metals (copper/zinc in brass) to form harmful salts (metal salts), which can be toxic if consumed. This contaminates the food and may cause food poisoning.

Source: Chapter 3, Metals and Non-metals

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• The key idea is acid + metal reaction: acidic foods react with copper/zinc to form poisonous compounds.
• Examiners expect: (1) statement that curd/sour substances are acidic, (2) they react with copper/brass, (3) forming harmful/toxic products.
• This question links Chapter 1 (acids react with metals) with Chapter 3 (properties of metals). Keep it concise — two clear points earn full 2 marks.

Electric current flows through a solution only when it contains free ions (charged particles) that can carry charge.

Distilled water is pure water with no dissolved salts or minerals. It does not contain any ions, so no current can flow through it — it does not conduct electricity.

Rainwater, however, dissolves gases from the atmosphere, especially carbon dioxide (CO₂), forming carbonic acid (H₂CO₃). This dissociates to produce ions (H⁺ and HCO₃⁻). It also contains dissolved salts and other impurities. These ions act as charge carriers, allowing electricity to be conducted.

Thus, the presence of ions in rainwater makes it a conductor, while the absence of ions in distilled water makes it a non-conductor.

Source: Chapter 2, Section 2.2; Chapter 11, Section 11.1

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• The key concept is ions as charge carriers — examiners expect you to explicitly state that distilled water lacks ions while rainwater has them due to dissolved CO₂/salts.
• Mention CO₂ dissolving to form carbonic acid (H₂CO₃) → dissociates into ions; this shows understanding beyond just "impurities."
• Do not write about covalent bonds of water extensively — that is off-topic here.
• For 3 marks, one line each for: (1) principle, (2) distilled water explanation, (3) rainwater explanation — is the ideal structure.

HCl produces more H⁺ ions than CH₃COOH of the same concentration.

HCl is a strong acid — it ionises completely in water:
$$\text{HCl} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$$

CH₃COOH is a weak acid — it ionises only partially in water, producing fewer H⁺ ions:
$$\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}$$

This tells us that acid strength depends on the degree of ionisation — acids that produce more H⁺ ions in solution are strong acids, and acids that produce fewer H⁺ ions are weak acids. This difference can be detected using a universal indicator, which shows a lower pH for HCl than for acetic acid of the same concentration.

Source: Chapter 2, Section 2.3; Chapter 4, Section 4.4.2

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• The key concept is degree of ionisation: HCl ionises completely (strong acid); CH₃COOH ionises partially (weak acid).
• Write the ionisation equations — examiners award marks for these.
• The reversible arrow (⇌) for acetic acid is important; it signals partial ionisation.
• Mention that a universal indicator/pH scale can distinguish them experimentally — this links theory to Activity 4.7 in the textbook.
• Do not confuse concentration with strength; both acids here have the same concentration but different strengths.

The pH of milk will decrease (become less than 6) as it turns into curd.

Reason: During the conversion of milk to curd, bacteria present in milk produce lactic acid (as seen in Table 2.3, sour milk/curd contains lactic acid). As lactic acid is formed, the concentration of H⁺ ions in the solution increases. Since higher H⁺ ion concentration corresponds to a lower pH value, the pH of the milk decreases, making curd more acidic than fresh milk.

Source: Chapter 2, Section 2.3 (Table 2.3); Chapter 2, Exercise Q.11

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• The key facts examiners expect: (1) pH decreases, (2) name the acid formed — lactic acid, (3) link acid formation → more H⁺ ions → lower pH. All three points together cover the 3 marks.
• Table 2.3 in Chapter 2 explicitly lists "Sour milk (Curd) → Lactic acid" — cite or reference this.
• Do not just say "it becomes acidic"; you must explain why using H⁺ ion concentration and the pH scale concept.

(a) Fresh milk has a pH of 6 (slightly acidic). The milkman adds baking soda (NaHCO₃) to shift the pH to slightly alkaline. This prevents the milk from turning sour quickly, as bacteria that cause souring (by producing lactic acid) are inhibited in an alkaline medium, thus increasing the shelf life of milk.

(b) Curd formation requires the action of lactic acid bacteria, which lower the pH of milk by producing lactic acid. Since the milk is now alkaline due to added baking soda, the bacteria need more time to first neutralise the base and then make the milk acidic enough to set into curd. Hence, this milk takes a longer time to set.

Source: Acids, Bases and Salts (Chapter 2), Exercise Q.12

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• (a) Key idea: alkaline pH inhibits the bacteria that cause souring, extending freshness. Examiners want: reason for shifting pH (prevent souring / spoilage).
• (b) Key idea: the lactic acid bacteria must first neutralise the added base before the pH can drop enough for curd to set — that extra step takes more time.
• Do not confuse the two parts: (a) is about why he makes it alkaline; (b) is about the consequence for curd formation.
• This is directly from Exercise Q.12 of Chapter 2; knowing Q.11 (pH drops as milk turns to curd) helps link both parts.

Fizzing will occur more vigorously in the test tube containing HCl (hydrochloric acid).

Reaction:
$$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\uparrow$$
$$\text{Mg} + 2\text{CH}_3\text{COOH} \rightarrow \text{(CH}_3\text{COO)}_2\text{Mg} + \text{H}_2\uparrow$$

Reason: HCl is a strong acid and completely dissociates in water, producing a higher concentration of H⁺ ions. CH₃COOH is a weak acid and partially dissociates, producing fewer H⁺ ions. Since the rate of reaction with magnesium depends on the concentration of H⁺ ions, more hydrogen gas is evolved per unit time with HCl, causing more vigorous fizzing.

Source: Chapter 2 (Acids, Bases and Salts), Exercise Q.10

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• Key concept: strong acid vs. weak acid — same molar concentration but different degree of ionisation.
• HCl → fully ionised; CH₃COOH → partially ionised. More H⁺ = faster reaction = more vigorous fizzing.
• Examiners expect you to name the type of acid (strong/weak), link it to H⁺ ion concentration, and connect that to the rate of reaction. Writing the balanced equations earns extra marks and shows conceptual clarity.

A farmer would treat his fields with quicklime (CaO) or slaked lime [Ca(OH)₂] when the soil is too acidic (i.e., has a low pH).

Reason: CaO and Ca(OH)₂ are basic in nature. When added to acidic soil, they neutralise the excess acid present in it:

$$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$$

Ca(OH)₂ then neutralises soil acids, raising the pH to a level suitable for crop growth. This process is called treatment of soil with bases to correct soil acidity.

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• The key concept tested is neutralisation — a base neutralises an acid.
• Examiner expects: (1) identification of soil condition (acidic/low pH), (2) reason that CaO/Ca(OH)₂ is basic and neutralises the acid in soil.
• Mentioning the reaction of CaO with water to form Ca(OH)₂ earns an extra point.
• Do not confuse with alkaline soil — lime is added only to acidic soil.

Yes, basic solutions do contain H⁺(aq) ions. However, in a basic solution, the concentration of OH⁻(aq) ions is greater than the concentration of H⁺(aq) ions. Since the basic nature of a solution depends on the relative excess of OH⁻ ions, the solution remains basic despite the presence of some H⁺ ions.

Source: Acids, Bases and Salts, Section 2.3

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• Examiners expect two clear points: (1) Yes, H⁺ ions are present, and (2) OH⁻ concentration exceeds H⁺ concentration, so the solution is still basic.
• Avoid simply saying "more OH⁻ ions are present" without linking it to why the solution is basic — the relative excess of OH⁻ is the key.
• Do not confuse "presence" with "dominance" — that distinction is the whole answer.

(a) Solution A (pH = 6) has more H⁺ ion concentration, because higher the H⁺ ion concentration, lower is the pH value.

(b) Solution A (pH = 6) is acidic (pH < 7), and Solution B (pH = 8) is basic (pH > 7).

Source: Chapter 2, Section 2.3 — How Strong Are Acid or Base Solutions?

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• The key rule from the textbook: Higher H⁺ concentration → lower pH. So between pH 6 and pH 8, pH 6 has more H⁺ ions.
• pH < 7 = acidic; pH = 7 = neutral; pH > 7 = basic/alkaline. Examiners expect you to state both classifications clearly.
• Always mention the pH values alongside your answer to justify your choice — this earns full marks.

The metal compound A is calcium carbonate (CaCO₃). It reacts with dilute HCl to produce CO₂ gas (which extinguishes a burning candle) and calcium chloride.

Word equation:
Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide

Balanced chemical equation:

$$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$$

This is a double displacement (and decomposition) reaction. CO₂ is heavier than air and does not support combustion, hence it extinguishes the candle.

Source: Chemical Reactions and Equations, Chapter 1

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• Identify compound A first: The clues are — reacts with HCl, gives a gas that extinguishes a candle (CO₂), and forms CaCl₂ → compound A must be CaCO₃.
• Balancing: Note the coefficient 2 before HCl — examiners specifically check this.
• 1 mark for identifying A, 1 mark for the correct balanced equation, 1 mark for stating the gas is CO₂ and why it extinguishes the candle.
• Do not write H₂CO₃ as a product; it immediately decomposes to H₂O and CO₂.

Activity 2.8 — Electrical Conductivity Test

Materials required: Solutions of glucose, alcohol, dilute HCl, dilute H₂SO₄; two nails fixed on a cork, a 100 mL beaker, a 6V battery, a bulb, and connecting wires.

Procedure:

1. Fix two nails on a cork and place it in a beaker. Connect the nails to a 6V battery through a bulb and switch.
2. Pour dilute HCl into the beaker and switch on the current. Observe whether the bulb glows.
3. Repeat with dilute H₂SO₄.
4. Repeat separately with glucose solution and alcohol solution.

Observation:

• The bulb glows with dilute HCl and H₂SO₄.
• The bulb does not glow with glucose and alcohol solutions.

Conclusion:
Acids (HCl, H₂SO₄) produce H⁺(aq) ions in solution, which carry electric current. Glucose and alcohol do not ionise in water — they produce no H⁺ ions — so they do not conduct electricity and are not acids, even though they contain hydrogen.

Source: Chapter 2, Activity 2.8, Section 2.2

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• Why the bulb test works: Current flows only when ions are present. Acids ionise → H⁺ + anion → bulb glows. Glucose/alcohol stay as molecules → no ions → bulb stays off.
• Key conclusion examiners want: Simply containing hydrogen is not enough to be an acid; the compound must produce H⁺(aq) ions in solution.
• Marks breakdown (5 marks): ~1 mark for materials/setup, ~1 mark for procedure steps, ~1 mark for observation (both cases), ~2 marks for conclusion with ionic explanation. Always state the inference explicitly.

Activity 2.9 (from NCERT textbook):

Materials required: Solid NaCl, concentrated H₂SO₄, delivery tube, dry and wet blue litmus paper.

Procedure:

1. Take about 1 g of solid NaCl in a clean, dry test tube.
2. Add some concentrated sulphuric acid to it. HCl gas is evolved.
3. Pass the gas first through dry blue litmus paper, then through wet blue litmus paper.

Observation:

• Dry blue litmus paper shows no change in colour.
• Wet blue litmus paper turns red.

Conclusion:

• Dry HCl gas does not ionise, so it shows no acidic character.
• When HCl dissolves in water (wet litmus), it ionises to produce H⁺ ions:

$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$

This shows that H⁺ ions are produced only in the presence of water. H⁺ ions cannot exist alone; they combine with water to form hydronium ions (H₃O⁺), which are responsible for the acidic nature.

Source: Chapter 2, Section 2.2.1, Activity 2.9

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• Examiners expect you to name the activity, list steps clearly, state both observations (dry vs. wet litmus), and write the ionisation equation.
• The key conclusion — "H⁺ ions are produced only in the presence of water" — must be explicitly stated; this is where marks are awarded.
• Writing H₃O⁺ (hydronium ion) instead of just H⁺ shows understanding and fetches full credit.
• Do not forget: dry HCl = no ionisation = no acidity; aqueous HCl = ionisation = acidic.

Chlor-Alkali Process:

When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes. This process is called the chlor-alkali process — chlor for chlorine and alkali for sodium hydroxide.

$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$

Products and their uses:

| Product | Where formed | One Use |
|---|---|---|
| Chlorine gas (Cl₂) | At anode | Used for manufacturing bleaching powder; also to disinfect drinking water |
| Hydrogen gas (H₂) | At cathode | Used as a fuel; also in the manufacture of margarine |
| Sodium hydroxide (NaOH) | Near cathode (in solution) | Used in making soap and detergents |

All three products are commercially very important and are obtained from common salt (NaCl).

Source: Chapter 2, Section 2.4.3

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• Examiners expect the balanced equation — don't skip it; it carries marks.
• Name all three products and give a use for each — the question specifically says "all products … one use of each," so missing even one product loses marks.
• Mentioning anode/cathode for Cl₂ and H₂ respectively shows understanding and is expected for full marks.
• The term "brine" for the NaCl solution should be used — it is the textbook term.

Common salt (NaCl) is the raw material for several important chemicals:

(i) Sodium Hydroxide:
Electrolysis of brine (aqueous NaCl) — the chlor-alkali process — gives NaOH, Cl₂, and H₂:
$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$

(ii) Bleaching Powder:
Cl₂ obtained above reacts with dry slaked lime:
$$2\text{Ca(OH)}_2 + 2\text{Cl}_2 \rightarrow \text{Ca(ClO)}_2 + \text{CaCl}_2 + 2\text{H}_2\text{O}$$

(iii) Baking Soda (NaHCO₃):
NaCl reacts with water, CO₂, and NH₃:
$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$

(iv) Washing Soda (Na₂CO₃·10H₂O):
Baking soda is heated to give Na₂CO₃, which on recrystallisation gives washing soda:
$$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$
$$\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}$$

Source: Chapter 2, Section 2.4.3

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• The question is essentially asking you to trace a chain: NaCl → NaOH/Cl₂ → Bleaching powder; NaCl → NaHCO₃ → Na₂CO₃ → Washing soda.
• Examiners expect all four reactions written and balanced — that's where most marks lie.
• Name the process (chlor-alkali) for NaOH — it's a frequently asked term.
• For washing soda, show two steps: heating baking soda first, then recrystallisation. Don't skip either.
• Keep prose minimal; let the equations do the work.

Importance of pH in Everyday Life

1. pH and our digestive system:
Our stomach produces hydrochloric acid (HCl) which helps in digestion. During indigestion, excess acid is produced causing pain. Antacids like Milk of Magnesia (magnesium hydroxide), which are mild bases, neutralise the excess acid and provide relief.

2. pH and tooth decay:
Bacteria in the mouth break down sugar to produce acids. When pH of the mouth falls below 5.5, the tooth enamel (calcium hydroxyapatite) gets corroded, causing tooth decay. Toothpastes are basic in nature and help neutralise this acid, preventing decay.

3. pH and acid rain/aquatic life:
Normal rain water has pH of about 5.6. When dissolved pollutants lower the pH below 5.6, it is called acid rain. When this flows into rivers, the pH of river water decreases, making survival of aquatic organisms difficult, as they can survive only in a narrow pH range.

(Bonus: Our body functions within a pH range of 7.0 to 7.8; bee stings inject acid and are relieved by applying a mild base like baking soda.)

Source: Chapter 2, Section 2.3.1

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• The question asks for any THREE examples, so choose three and write each as a clear, labelled point.
• Examiners expect: the example name → the pH-related problem → how pH knowledge helps solve/explain it.
• Common mistakes: writing only what happens without linking it to pH; forgetting the remedy or consequence.
• The 5-mark allocation means ~3 marks for three examples (1 mark each) + 1–2 marks for introduction/accuracy, so keep each point substantive but concise.
• "Acid rain," "digestion/antacids," and "tooth decay" are the three explicitly detailed examples in section 2.3.1—always prefer these.

Significance of H₃O⁺ and OH⁻ ions:

Acids do not show acidic properties in the absence of water. When dissolved in water, acids produce hydronium ions (H₃O⁺), as shown below:

$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$

H⁺ ions cannot exist alone; they combine with water to form H₃O⁺. The acidic nature of a substance is due to the formation of H⁺(aq)/H₃O⁺ ions in solution.

Similarly, bases dissolved in water release hydroxide ions:

$$\text{NaOH(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}$$

The basic nature of a substance is due to the formation of OH⁻(aq) ions.

Neutralisation at the ionic level:

Since every acid supplies H⁺(aq) and every base supplies OH⁻(aq), the neutralisation reaction can be written as:

$$\text{HX} + \text{MOH} \rightarrow \text{MX} + \text{H}_2\text{O}$$

At the ionic level, this reduces to:

$$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$$

The H⁺ and OH⁻ ions combine to form neutral water, which is why acids and bases cancel each other's effect. The remaining ions (e.g., Na⁺ and Cl⁻) form the salt.

Source: Chapter 2, Section 2.2 / 2.2.1

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• Examiners expect the key point that H⁺ cannot exist alone — it must be written as H⁺(aq) or H₃O⁺.
• The ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) is the core of this answer and must be written.
• Show at least one equation for acid dissociation and one for base dissociation to earn full marks.
• The significance is that all neutralisations are essentially the same reaction at the ionic level regardless of which specific acid or base is used.

A strong acid completely ionises in water, producing a large number of H⁺ ions, while a weak acid partially ionises, producing fewer H⁺ ions.

Distinction: Using a universal indicator or pH paper, a strong acid shows a lower pH value than a weak acid of the same concentration.

Example: If we take 1 molar HCl (strong acid) and 1 molar acetic acid (CH₃COOH, weak acid), HCl produces more H⁺ ions and shows a lower pH, whereas acetic acid produces fewer H⁺ ions and shows a comparatively higher pH, even though both concentrations are the same.

Source: Chapter 2, Section 2.3; Chapter 4, Section 4.4.2

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• Examiners expect the definition of strong vs. weak acid in terms of degree of ionisation / number of H⁺ ions produced.
• The HCl vs. acetic acid example is directly from the textbook (Section 2.3) and must be included for full marks.
• Mentioning the pH/universal indicator as the method of distinction earns the "how to distinguish" mark.
• Do not simply say "strong acid has more H⁺" without linking it to pH or ionisation — that costs a mark.

Dissolving a concentrated acid or base in water must be done carefully because it is a highly exothermic process — it releases a large amount of heat. The acid/base must always be added slowly to water (never water to acid), as the sudden heat released can cause the mixture to splash or boil, causing serious burns.

It is an exothermic process.

Source: Chapter 2, What you have learnt

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• The examiner expects two things: why (large heat is released / exothermic) and what type (exothermic process). Both are needed for full 2 marks.
• A common mistake is saying "water is added to acid" — it is always acid added to water (though the textbook simply calls it exothermic; the safety rule reinforces the answer).
• Keep it concise; don't over-explain.

Lactic acid is listed as an acid because it produces H⁺ ions in solution, which is the property that defines an acid. Milk has a pH of 6, which is already slightly acidic (below 7).

When milk turns into curd, lactic acid is produced by bacterial action. This increases the concentration of H⁺ ions in the solution. Since higher H⁺ ion concentration means lower pH, the pH of curd will be less than 6 — making it more acidic than fresh milk.

Source: Chapter 2, Section 2.3 (Table 2.3 and pH scale discussion)

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• The key link is: acid → H⁺ ions → lower pH. Examiners expect you to connect lactic acid's acidic nature (H⁺ production) to the pH drop.
• Milk at pH 6 is already slightly acidic. Curd, having more acid, must have a lower pH than 6.
• Common mistake: saying curd is basic or neutral. Remember — pH < 7 = acidic, and more acid means pH goes further down, not up.
• This question is directly linked to Exercise Q.11 from Chapter 2.

NaCl and Na₂SO₄ belong to the family of sodium salts (same positive radical). NaCl and KCl belong to the family of chloride salts (same negative radical).

Source: Chapter 2, Section 2.4.1 Family of Salts

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The key concept is that salts sharing the same positive radical (cation) form one family, and those sharing the same negative radical (anion) form another. Examiners expect both families to be correctly named in one answer. The textbook uses these exact examples, so reproduce the terminology precisely.

The atmosphere of Venus is made up of thick clouds of sulphuric acid (H₂SO₄). The chapter mentions this to show that water is essential to make acids acidic — H⁺ ions are produced only in the presence of water.

Source: Chapter 2, Section 2.2.1

The textbook uses the example of Venus in the context of explaining that HCl (or acids in general) show acidic properties only in aqueous solution. Since Venus has no liquid water, the sulphuric acid clouds there cannot dissociate to give H⁺ ions, reinforcing the concept that water is necessary for acidic behaviour. Examiners expect both parts: the name of the acid and the reason it is mentioned.

Calcium sulphate hemihydrate (CaSO₄·½H₂O) is called Plaster of Paris because it was historically manufactured from the large gypsum deposits found in Paris, France.

This is a factual 1-mark question. The examiner simply wants the historical/geographical reason for the name. No chemical explanation is required here. Remember: Plaster of Paris is made by heating gypsum (CaSO₄·2H₂O) at 373 K, and the name comes from the gypsum mines near Paris.

In reality, half a water molecule does not exist independently. The formula CaSO₄·½H₂O means that two formula units of CaSO₄ share one water molecule. In other words, for every 2 units of calcium sulphate, only 1 molecule of water is present as water of crystallisation. The formula is written this way as a simplified representation.

Source: Chapter 2, Section 2.4.4 – Are the Crystals of Salts really Dry?

• Examiners expect two key points: (1) half a water molecule cannot exist alone, and (2) the actual meaning — 2 CaSO₄ units share 1 H₂O molecule.
• Quoting directly from the textbook explanation is safest here.
• Do not write a long paragraph; two clear sentences covering both points are enough for 2 marks.

CO₂ produced during the reaction of baking powder causes the bread/dough to rise by making it porous and soft, giving it a light, fluffy texture.

Baking powder contains sodium hydrogen carbonate (NaHCO₃) which, on heating with a mild acid, releases CO₂ gas. This gas gets trapped in the dough, causing it to expand and become spongy. Examiners expect students to mention "rise/porous/fluffy" as the key function of CO₂. Note: The source passages do not directly cover this topic, so the answer draws on standard CBSE Chapter 2 (Acids, Bases and Salts) content on baking powder.

Washing soda (sodium carbonate, Na₂CO₃) is used to remove permanent hardness of water, as it reacts with dissolved calcium and magnesium salts to form insoluble carbonates.

The question is from the topic of salts and their uses (Chapter 2). Examiners expect the chemical name sodium carbonate and its common name washing soda. Permanent hardness is caused by soluble chlorides and sulphates of Ca²⁺/Mg²⁺, which washing soda removes by precipitation. Note: this topic is referenced under uses of washing soda in the CBSE syllabus, so knowing both the name and the reason is ideal even for a 1-mark answer.

Activity: Testing whether zinc reacts with dilute sulphuric acid to produce hydrogen gas.

Apparatus/Procedure:
Take about 5 mL of dilute sulphuric acid in a test tube. Add a few pieces of zinc granules to it. Pass the gas evolved through soap solution using a delivery tube. Bring a burning candle near a soap bubble filled with the gas.

Labelled Diagram:

```
[Test tube]
Zn granules + H₂SO₄
|
[Delivery tube]
|
[Soap solution]
(bubbles form)
↑
[Burning candle → brought near bubble]
```
(Label: zinc granules, dilute H₂SO₄, delivery tube, soap solution, gas bubble)

Observations:

1. Brisk effervescence is seen on the surface of zinc granules.
2. Bubbles form in the soap solution, showing gas is being produced.
3. When a burning candle is brought near the gas-filled soap bubble, it burns with a pop sound.

Chemical Equation:
$$\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$$

Conclusion: The gas produced is hydrogen. Zinc displaces hydrogen from dilute sulphuric acid, producing zinc sulphate and hydrogen gas.

Source: Chapter 2, Section 2.1.2, Activity 2.3

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• The examiner expects: setup description, a neat labelled diagram, two clear observations (effervescence + pop sound with candle), the balanced equation, and a conclusion. Missing the "pop" sound or the equation loses marks.
• The diagram need not be artistic — labels are what matter. Include: test tube, zinc granules, H₂SO₄, delivery tube, soap solution.
• "Pop sound" is the standard test for hydrogen — always mention it explicitly.
• The balanced equation is already balanced as written (1:1:1:1 ratio), so no coefficients need adjusting.

Yes, I agree. Both the assertion and reason are correct, and the reason correctly explains the assertion.

A solution with pH = 12 is far above 7, which indicates it is strongly basic. According to the pH scale, as the pH value increases from 7 to 14, it represents an increase in OH⁻ ion concentration in the solution, meaning greater alkaline strength. Therefore, pH = 12 corresponds to a high concentration of OH⁻ ions, making it strongly basic. The reason directly and correctly explains the assertion.

Source: Chapter 2, Section 2.3 – How Strong Are Acid or Base Solutions?

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• CBSE Assertion-Reason questions at this level ask you to confirm the relationship, not just label it.
• Key textbook line to quote/paraphrase: "As the pH value increases from 7 to 14, it represents an increase in OH⁻ ion concentration."
• Always link the reason back to the assertion explicitly — examiners check that you explain how the reason justifies the assertion, not just that both are true.
• pH = 12 is strongly (not weakly) basic because it is close to 14, far above 7.

A nettle sting causes a burning sensation because the nettle injects formic acid (methanoic acid) into the skin — an acidic substance. The dock plant leaf is basic (alkaline) in nature.

When rubbed on the affected area, the basic compounds in the dock leaf neutralise the acid injected by the nettle. This acid-base neutralisation reaction reduces the irritation and provides relief from the burning sensation.

This is similar to how acids and bases cancel each other's effects, as bases are known to neutralise acids.

Source: Chapter 2, Introduction / Acids, Bases and Salts

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• The key concept tested is neutralisation: an acid is neutralised by a base.
• Examiners expect you to: (1) identify the nettle sting as acidic, (2) conclude the dock plant is basic/alkaline, and (3) state that neutralisation provides relief.
• The word "suggest" means you must use reasoning — don't just state facts; link the remedy to the chemical nature logically.
• Do not overthink the biology; this is purely a Chemistry (acid-base) application question worth 3 marks, so one point per mark is ideal.