Neutralisation and salts

Acids react with bases, alkalis, carbonates and reactive metals to make salts. In neutralisation, hydrogen ions from the acid and hydroxide ions from the alkali join to make water. A salt’s name comes from the metal and the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.

Key facts

  • acid + base → salt + water. Bases are metal oxides and hydroxides; alkalis are bases that dissolve in water.
  • acid + metal → salt + hydrogen, and acid + carbonate → salt + water + carbon dioxide.
  • Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
  • The ionic equation for neutralisation is H⁺(aq) + OH⁻(aq) → H2O(l).
  • To make a soluble salt, add an insoluble base or carbonate to the acid until it is in excess, filter, then crystallise. With an alkali, use a titration to find the exact amounts.
  • Insoluble salts are made by mixing two solutions to form a precipitate, which is filtered, washed and dried.

Worked examples

Example 1

Name the salt formed when zinc oxide reacts with sulfuric acid, and write the word equation.

  1. Zinc oxide is a base, so the products are a salt and water.
  2. The metal part of the name comes from zinc oxide: zinc.
  3. Sulfuric acid makes sulfates, so the salt is zinc sulfate.
  4. zinc oxide + sulfuric acid → zinc sulfate + water

Answer: zinc sulfate; zinc oxide + sulfuric acid → zinc sulfate + water

Example 2

Describe how to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.

  1. Warm the acid and add copper(II) oxide a little at a time, stirring, until no more dissolves. The base is now in excess, so all the acid has reacted.
  2. Filter to remove the unreacted copper(II) oxide. The blue filtrate is copper(II) sulfate solution.
  3. Heat the filtrate to evaporate some of the water, then leave it to cool so crystals form.
  4. Filter off the crystals and dry them between filter papers.

Answer: react with excess copper(II) oxide, filter, evaporate some water, crystallise, dry

Example 3

Write the formula of calcium nitrate and a balanced equation for calcium carbonate reacting with nitric acid.

  1. Calcium forms Ca²⁺ and the nitrate ion is NO3⁻, so you need two nitrates: Ca(NO3)2.
  2. acid + carbonate → salt + water + carbon dioxide.
  3. Two nitrates in the salt need two HNO3: CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2.

Answer: Ca(NO3)2; CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2