A metal oxide is a base that reacts with a dilute acid to give a salt and water only, for example CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l); the ionic equation is CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l).
A metal oxide is a base, and like every base it neutralises an acid. Because most metal oxides are insoluble, this reaction is the standard way to prepare a soluble salt using the “acid + insoluble base” method, and it is examined for its equation, its colour change and its method in 4541.
The balanced equation
The general word equation is: metal oxide + acid → salt + water. With correct formulae and state symbols, common examples are:
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
ZnO(s) + 2HNO3(aq) → Zn(NO3)2(aq) + H2O(l)
Only a salt and water are formed, there is no gas, which distinguishes this reaction from acid with a metal or with a carbonate. The ionic equation removes the spectator acid anion and shows the oxide accepting hydrogen ions:
CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l)
The same pattern gives MgO(s) + 2H+(aq) → Mg2+(aq) + H2O(l) for magnesium oxide.
Conditions required
The dilute acid is usually warmed gently to speed up the reaction, and an excess of the insoluble metal oxide is added and stirred until no more dissolves. Warming is helpful but not violent; no catalyst is needed. Because the oxide is insoluble, the excess is filtered off afterwards, and the salt is obtained from the filtrate by crystallisation.
What you observe
As the oxide reacts, the solid dissolves and disappears when the acid is warm, and no gas is given off (an important negative observation). Colour is the memorable clue: black copper(II) oxide dissolves to give a blue copper(II) sulfate solution; white magnesium oxide and white zinc oxide dissolve to give colourless solutions. If excess oxide remains, it settles as undissolved solid that is then filtered out. The mixture warms slightly because neutralisation is exothermic.
Where it appears in the SPM exam
In 4541/2 this is a core salt-preparation question: describe how to prepare a named soluble salt from an insoluble oxide, including warming, using excess, filtering, and crystallising. It also appears as equation writing, including the ionic equation, and as the blue colour of copper(II) salts. In 4541/1 it is tested as recognising a base and its products, and in the practical paper 4541/3 the preparation of copper(II) sulfate from copper(II) oxide and sulfuric acid is a familiar procedure.
How we teach it
We connect the equation to the practical steps: warm the acid, add oxide in excess, filter off the excess, then crystallise. Students most often lose marks by forgetting that no gas is produced, by not balancing the acid for a metal that forms a 2+ ion, or by describing the wrong colour. Learning copper(II) oxide as the model case fixes the whole idea, and it carries directly into salt-preparation planning questions.
Quick summary
The essentials are short: a metal oxide is an insoluble base, it neutralises acid to give salt and water with no gas, and the method is warm-excess-filter-crystallise. Keep the copper example, black solid to blue solution, as your anchor, write the balanced full and ionic equations, and you have a reliable answer for both the theory and the practical versions of this question.
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