Identify the substances that are oxidised and the substances that are reduced in the following reactions.
4Na(s) + O2(g) → 2Na2O(s)
Sodium (Na) is oxidised because it gains oxygen to form sodium oxide (Na₂O). Oxygen (O₂) is reduced.
CuO(s) + H2(g) → Cu(s) + H2O(l)
Copper oxide (CuO) is reduced because it loses oxygen to form copper (Cu). Hydrogen (H₂) is oxidised because it gains oxygen to form water (H₂O).
Explanation
The textbook provides clear definitions: if a substance gains oxygen during a reaction, it is oxidised; if a substance loses oxygen during a reaction, it is reduced. In reaction (i), sodium gains oxygen to form Na₂O, so it is oxidised. In reaction (ii), the textbook explicitly states that 'the copper(II) oxide is losing oxygen and is being reduced' and 'the hydrogen is gaining oxygen and is being oxidised.'