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An acid is a chemical compound which when dissolved in water gives a pH reading of under 7. This is due to the hydrogen ions which are part of the compound. This means that acids are defined as proton donors.

The Brønsted - Lowry theory of Acids and BasesEdit

This states that acids are proton donors, while bases are proton acceptors which accept a proton (or hydrogen ion) from an acid. It had previously been thought that it was the oxygen in an acid that made it an acid.

Acid reactionsEdit

Acid + MetalEdit

Acids react with a metal to form hydrogen gas and a salt. Bubbles will be given off - the hydrogen.

  • Ionic Equation: 2H+ + 2e- --> 2H2. Note that the 2 electrons come from the metal.
  • Overall Equation: Acid + Metal --> Hydrogen + Salt
  • Example: hydrochloric acid + magnesium --> hydrogen + magnesium chloride
  • Symbol Equation: 2HCl (aq) + Mg (s) --> H2 (g) + MgCl2 (aq)

Acid + CarbonateEdit

Acids react with metal carbonates, forming carbon dioxide, water and a salt. Bubbling is seen (this is the carbon dioxide).

  • Ionic Equation: 2H+ + CO3 --> CO2 + H2O.
  • Overall Equation: Acid + Metal Carbonate --> Water + Carbon Dioxide + Salt
  • Example: hydrochloric acid + magnesium carbonate --> water + magnesium chloride + carbon dioxide
  • Symbol Equation: 2HCl + MgCO3 --> H2O + MgCl2 + CO2

Acid + BaseEdit

Acids react with bases to form a salt and water. This reaction is exothermic.

  • Ionic Equation: H+ + OH- --> H2O
  • Overall Equation: Acid + Base --> Water + Salt
  • Example: hydrochloric acid + magnesium hydroxide --> water + magnesium chloride
  • Symbol Equation: 2HCl + Mg(OH)2 --> 2H2O + MgCl2

Strong and weak acidsEdit

The 'strength' of an acid is whether it completely dissociates (or ionises) when dissolved in water. Acids which totally dissociate are called strong acids, while ones where some molecules remain un-ionised are called weak acids.

Strong acids include hydrochloric acid, sulphuric acid and nitric acid. A weak acid is ethanoic acid.


In GCSE, you will have to study one weak and one strong acid. For most exam boards, this is ethanoic acid and sulphuric acid, however, be sure to check your specifications.

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