CHEMISTRY · Stage 9 · Chapter 12: Chemical Reactions 3 and Rates

Chemistry 12.7 More about salts — Making Salts from Insoluble Metal Oxides

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12.7 More about salts — Making Salts from Insoluble Metal Oxides — Alhussan International Academy — Chemistry (Stages 7–9) notes: Choose reactants to make different salts and prepare a salt from an insoluble metal oxide and an acid.Salt = a compound made when a metal ion replaces…

Acid determines the type of salt
Other reactant
Other products
Separation
Availability and cost
Zinc metal

Part 1 of 6Choosing reactants to make salts

1Salt1 point

= a compound made when a metal ion replaces the hydrogen ion in an acid.

Salts exist naturally, such as those dissolved in the water of Lake Qinghai in China, and can also be made in reactions.

2Acid determines the type of salt3 points

— different acids make different salts.

Hydrochloric acid, HCl, makes chlorides.
Sulfuric acid, H₂SO₄, makes sulfates.
Nitric acid, HNO₃, makes nitrates.

3Other reactant

— it must contain atoms of the metal in the salt: a metal, a metal oxide or a metal carbonate.

Metal-containing reactant	Equation (with sulfuric acid)	Other products
Zinc	Zn + H₂SO₄ → ZnSO₄ + H₂	Hydrogen
Zinc oxide	ZnO + H₂SO₄ → ZnSO₄ + H₂O	Water
Zinc carbonate	ZnCO₃ + H₂SO₄ → ZnSO₄ + CO₂ + H₂O	Carbon dioxide and water

Part 2 of 6Factors when choosing the reactant

Other productsconsider what they are.
Separationconsider whether it is easy to separate the salt from the other products.
Availability and costconsider whether the reactant is easily available and how much it costs.
Zinc metalreaction 1 has two disadvantages, because zinc is more expensive than zinc oxide and zinc carbonate and hydrogen is flammable.
Examplessalts can be made from nickel chloride (hydrochloric acid and nickel carbonate), cobalt sulfate (sulfuric acid) and sodium nitrate (nitric acid).

Part 3 of 6Why use an insoluble metal oxide

Unreactive metals — some metals, such as copper, do not react with dilute acids, but their oxides do.
Bases — a base is a substance that neutralises an acid, and an insoluble metal oxide such as copper oxide or magnesium oxide can make a salt in a neutralisation reaction.
Equation — metal oxide + acid → salt + water.
- copper oxide + sulfuric acid → copper sulfate + water, in which black copper oxide dissolves to give a blue solution.
Safer route — the metal route gives off hydrogen, whereas the oxide route produces only water.

Oxide	Acid	Salt
Copper oxide	Sulfuric	Copper sulfate
Magnesium oxide	Hydrochloric	Magnesium chloride
Zinc oxide	Nitric	Zinc nitrate

Part 4 of 6Method with an insoluble oxide

Warm dilute sulfuric acid in a beaker, but do not boil it.
Add a spatula of black copper oxide powder at a time and stir, so the powder dissolves and the solution turns blue.
Continue until no more dissolves and a little black powder remains, which shows all the acid has been neutralised.
Filter to remove the excess copper oxide.
Heat the blue filtrate gently to evaporate some water, then leave it to cool and crystallise.
Filter and dry the blue crystals of copper sulfate.

Part 5 of 6Explaining the reaction

Neutralisationoxide ions in the base react with hydrogen ions in the acid to form water: O^2- + 2H⁺ → H₂O.
Spectatorsthe metal ion and the acid anion stay in solution.
Excess basebecause the base is insoluble, any excess is removed by filtration, so the solution contains only the salt.
Symbol equationCuO + H₂SO₄ → CuSO₄ + H₂O.
Soluble baseswith a soluble base such as sodium hydroxide, the exact amounts would need to be found by titration.

Part 6 of 6Fault-finding

Black powder in the productfiltration was incomplete or the filter paper tore.
Few crystalstoo much water remains or the solution was not allowed to cool.
Solution still acidictoo little oxide was added, so add more until some stays undissolved.
Powdery crystalsheating too strongly makes hydrated crystals lose water.

Last updated: 13 October 2026