Science — CBSE Class 10 board question

Double displacement reaction: A reaction in which two different ions are exchanged between the reactants to form two new compounds.

(i) Neutralization reaction (also double displacement):

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

Justification: Na⁺ and H⁺ ions exchange partners. It is a neutralization reaction because an acid (HCl) reacts with a base (NaOH) to form salt and water.

(ii) Precipitation reaction (also double displacement):

$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$

Justification: SO₄²⁻ and Cl⁻ ions exchange partners. It is a precipitation reaction because insoluble BaSO₄ (white precipitate) is formed as a product.

Source: Chapter 1 – Chemical Reactions and Equations, Section 1.2.4

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• Definition: Always mention "exchange of ions" — that is the key phrase examiners look for.
• Neutralization: Any acid + base → salt + water equation works; NaOH + HCl is the standard textbook example.
• Precipitation: The BaSO₄ example is directly from the textbook (Activity 1.10); always write the state symbol (s) for the precipitate.
• Justification: For each, briefly state why it qualifies — this earns the explanation mark. Do not skip it.