Science — CBSE Class 10 board question

(i)

I. Ag (Silver) — does not react with oxygen even at high temperature.
II. Al (Aluminium) — reacts at ordinary temperature and forms a protective Al₂O₃ layer.
III. K (Potassium) — catches fire when kept in the open.
IV. Fe (Iron) — does not burn but hot iron is coated with black Fe₃O₄ layer.

(ii) Amphoteric oxides are metal oxides that react with both acids and bases to form salt and water.

Aluminium oxide (Al₂O₃) is amphoteric:

With acid:
$$\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$$

With base:
$$\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}$$

(iii) Alkalis are bases that dissolve in water to give OH⁻ ions.
Example: Sodium hydroxide (NaOH).

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• For (i), students often confuse Cu and Fe — Cu forms black CuO on heating (not Fe). K is so reactive it ignites in air; Ag is the least reactive metal listed.
• For (ii), the key is to state the definition AND show both reactions (with acid and with base) with balanced equations — examiners expect both.
• For (iii), one line with one example is sufficient for 1 mark. Don't write more.