Neutralisation of acids and salt production: Acids are neutralised by alkalis (eg soluble metal hydroxides) and bases (eg insoluble metal hydroxides and metal oxides) to produce salts and water, and by metal carbonates to produce salts, water and carbon dioxide.

The idea

Neutralisation occurs when an acid reacts with a base, removing acidic and basic character. Alkalis are soluble bases, typically soluble metal hydroxides such as sodium hydroxide, NaOH(aq). Insoluble bases include metal oxides such as copper(II) oxide, CuO(s), and insoluble metal hydroxides such as iron(III) hydroxide, Fe(OH)₃(s). Both react with acids to form a salt and water only: acid + base → salt + water. For example, hydrochloric acid with sodium hydroxide gives sodium chloride and water: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). Metal carbonates, such as calcium carbonate, CaCO₃(s), also neutralise acids but produce carbon dioxide as well: acid + metal carbonate → salt + water + carbon dioxide. For example, 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g). The carbon dioxide is observed as effervescence and turns limewater milky.

What earns the marks

• Acid + alkali or base → salt + water; this is neutralisation and no gas is produced in these reactions.
• Alkalis are soluble metal hydroxides, for example NaOH(aq) and KOH(aq); bases include insoluble metal hydroxides and metal oxides such as CuO(s) and Fe(OH)₃(s).
• Acid + metal carbonate → salt + water + carbon dioxide; the gas is identified by bubbling through limewater, which turns milky or cloudy.
• Word equations and balanced symbol equations must use correct formulae, including state symbols where required, for example HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
• The salt name is built from the metal or ammonium ion from the base or carbonate and the acid anion: chloride from hydrochloric acid, nitrate from nitric acid, sulfate from sulfuric acid.
• Neutralisation can be followed by pH change towards pH 7, and in practical work an indicator or pH meter can show when the acid has been neutralised.

Common mistakes and how to put them right

• Writing that acid + alkali produces carbon dioxide.

  Put it right by stating that carbon dioxide is produced only when a metal carbonate reacts with an acid.

• Confusing soluble and insoluble hydroxides.

  Put it right by defining alkalis as soluble metal hydroxides and noting that insoluble metal hydroxides are bases but not alkalis.

• Omitting water from the products of acid + metal carbonate.

  Put it right by writing the full word equation acid + metal carbonate → salt + water + carbon dioxide.

• Using incorrect salt formulae such as CaCl instead of CaCl₂.

  Put it right by balancing charges, since Ca²⁺ needs two Cl⁻ ions.

Exam tips

• Underline the acid and the base or carbonate in the question, then name the salt from the metal or ammonium ion and the acid anion before writing the equation.
• When a gas is produced, state the test and the positive result, for example carbon dioxide turns limewater milky.
• Balance symbol equations by checking each element in turn, and include state symbols if the question asks for them.
• For required practical work, describe adding the solid base or carbonate gradually to the acid until it is in excess, then filtering to remove excess solid.