Salts: how they form, and why some are acidic or basic
A salt forms, along with water, when an acid neutralises a base. Its solution is neutral, acidic or basic, depending on the strength of the parent acid and base.
Ways to make a salt
| Method | Word equation | Balanced example |
|---|---|---|
| Acid + base (neutralisation) | Base + acid → salt + water | KOH + HNO₃ → KNO₃ + H₂O |
| Acid + metal | Metal + acid → salt + hydrogen | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| Acid + metal carbonate or hydrogen carbonate | → salt + water + carbon dioxide | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ |
| Acid + metal oxide | → salt + water | CaO + 2HCl → CaCl₂ + H₂O |
| Base + non-metal oxide | → salt + water | 2NaOH + CO₂ → Na₂CO₃ + H₂O |
Acid radical and basic radical
In every salt, the cation comes from the base. It is called the basic radical. The anion comes from the acid. It is called the acid radical. In NaCl, Na⁺ comes from NaOH and Cl⁻ comes from HCl. In K₂SO₄, the basic radical is K⁺ (from KOH) and the acid radical is SO₄²⁻ (from H₂SO₄). So from its radicals you can name the acid and the base used to make any salt.
Nature of a salt's solution
| Parent acid | Parent base | Salt solution | pH at 25 °C | Examples |
|---|---|---|---|---|
| Strong | Strong | Neutral | 7 | NaCl (from HCl and NaOH), K₂SO₄ |
| Weak | Strong | Basic | Above 7 | Na₂CO₃ (from H₂CO₃ and NaOH), CH₃COONa |
| Strong | Weak | Acidic | Below 7 | NH₄Cl (from HCl and NH₄OH), CuSO₄ |
| Weak | Weak | More information needed | Cannot say simply | CH₃COONH₄ |
Activity: nature of salt solutions (also an online lab)
- 1Dissolve a little of each salt in water in separate test tubes: sodium chloride, sodium carbonate, ammonium chloride and copper sulphate.
- 2Test each with blue and red litmus paper, or with pH paper.
- 3★Record what you see.NaCl: no change, pH about 7. Na₂CO₃: red litmus turns blue, pH above 7. NH₄Cl and CuSO₄: blue litmus turns red, pH below 7
- 4Explain each result from the strength of its parent acid and base.
★ key step
Are all salts neutral?
Learn the four rows of the table above: the stronger parent decides the nature of the salt.
Washing soda solution feels slippery and turns red litmus blue. It is the salt of a weak acid and a strong base, and this basic nature helps it cut grease on kitchen vessels.
2 marks: “Name the acid and the base that react to form ammonium sulphate, (NH₄)₂SO₄. Is its solution acidic, basic or neutral?” Acid: sulphuric acid, H₂SO₄ (gives SO₄²⁻). Base: ammonium hydroxide, NH₄OH (gives NH₄⁺). A strong acid and a weak base, so the solution is acidic (pH below 7).
2 marks: “What is a salt? Write the general equation for neutralisation.” An ionic compound with a cation other than H⁺ and an anion other than OH⁻, formed with water when an acid reacts with a base. MOH + HX → MX + H₂O.
1 marks: “Name the acid radical in sodium nitrate, NaNO₃.” NO₃⁻, the nitrate ion, which comes from nitric acid.
The stronger parent wins: a strong acid gives an acidic salt, a strong base gives a basic salt, and two strong parents give a neutral salt.