Almost all metals combine with oxygen to form metal oxides. Metal oxides are generally basic in nature. But some metal oxides show both basic as well as acidic behaviour. Different metals show different reactivities towards oxygen. Some react vigorously while some do not react at all.
Generated by claude-sonnet-4-6 · 2026-06-15 06:50 · grounding stimulus
Model Answer
(a) When copper is heated in air, it reacts with oxygen to form copper(II) oxide (a black coating).
$$2Cu_{(s)} + O_{2(g)} \xrightarrow{\Delta} 2CuO_{(s)}$$
(b) Some metal oxides are amphoteric because they react with both acids and bases. Example: Aluminium oxide (Al₂O₃).
(c)
(i)
$$Na_2O_{(s)} + H_2O_{(l)} \rightarrow 2NaOH_{(aq)}$$
(ii)
$$Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$$
(Sodium aluminate + water)
Source: Metals and Non-metals, Reaction of Metals with Oxygen
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Explanation
- (a): The black coating on copper is CuO — mention this observation for full marks.
- (b): "Amphoteric" = reacts with both acid and base. Al₂O₃ is the standard CBSE example; ZnO is also acceptable.
- (c)(i): Na₂O is a basic oxide; it dissolves in water to give NaOH — balance correctly.
- (c)(ii): Al₂O₃ acting as an acid reacting with NaOH gives sodium aluminate (NaAlO₂). Product names earn marks.