Neutralisation of acids and salt production: The particular salt produced in any reaction between an acid and a base or alkali depends on:

The idea

The salt formed in a neutralisation reaction has two parts: the metal or ammonium ion from the base or alkali, and the anion from the acid. The acid determines the anion: hydrochloric acid produces chloride salts, nitric acid produces nitrate salts, and sulfuric acid produces sulfate salts. The base or alkali determines the metal or ammonium ion: sodium hydroxide gives sodium salts, copper(II) oxide gives copper(II) salts, and ammonia gives ammonium salts. For example, sulfuric acid with copper(II) oxide gives copper(II) sulfate and water: H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l). Note that ammonia reacts with acids to form only an ammonium salt, without producing water. The name must show the metal or ammonium ion and the acid-derived anion, and the formula must balance charges, such as Cu²⁺ with SO₄²⁻ giving CuSO₄.

What earns the marks

• The acid used determines the anion in the salt: hydrochloric acid → chloride, nitric acid → nitrate, sulfuric acid → sulfate.
• The base or alkali used determines the metal or ammonium ion in the salt: sodium hydroxide → sodium salts, potassium hydroxide → potassium salts, copper(II) oxide → copper(II) salts, ammonia → ammonium salts.
• Salt names combine the metal or ammonium ion with the acid anion, for example sodium nitrate from sodium hydroxide and nitric acid.
• Formulae must have balanced charges: for example, Mg²⁺ with Cl⁻ gives MgCl₂, and Al³⁺ with SO₄²⁻ gives Al₂(SO₄)₃.
• Roman numerals in names such as copper(II) oxide show the charge on the metal ion and must be reflected in the salt formula.
• Word equations and balanced symbol equations should show the salt and water as products for acid + metal oxide/hydroxide reactions, but only the salt for ammonia + acid reactions.

Common mistakes and how to put them right

• Naming the salt from the base only and ignoring the acid.

  Put it right by combining the metal or ammonium ion with the anion from the acid.

• Writing sulfate as SO₄ without the 2⁻ charge or using S₂O₄.

  Put it right by using the sulfate ion SO₄²⁻.

• Including water as a product when ammonia reacts with an acid.

  Put it right by remembering that ammonia + acid → ammonium salt only.

• Balancing formulae incorrectly, such as NaCl₂.

  Put it right by balancing charges so that Na⁺ and Cl⁻ give NaCl.

Exam tips

• Identify the acid first to fix the anion, then identify the base or alkali to fix the metal or ammonium ion, and combine them to name the salt.
• Check the charge on each ion and use the lowest whole-number ratio when writing a formula, for example Ca²⁺ and NO₃⁻ give Ca(NO₃)₂.
• If a question gives a salt name, work backwards to identify the acid and base that could produce it.
• Include state symbols only when they are requested or when they help show that a solid base has reacted with an aqueous acid.