A-level Chemistry/OCR/Atoms, Bonds and Groups/Atoms and Reactions/Acids

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Acids and Bases[edit]

An acid is a substance which in an aqueous solution will release H+ ions. Acids are also known as proton donors as they release a proton in the form of a H+ ion. Common laboratory acids include hydrochloric acid (HCl), sulphuric acid (H2SO4), and nitric acid (HNO3).

A base is a proton receiver as it accepts protons. Common bases are metal oxides, metal hydroxides, and ammonia. An alkali is a soluble base, and will release OH- ions in an aqueous solution. Common alkalis include sodium hydroxide, potassium hydroxide, and aqueous ammonia.


A salt is produced during neutralisation: the reaction of an acid with a base, alkali or carbonate. The H+ ion of an acid is replaced by a metal ion or NH4+.

Here are some common reactions:

  • Acid + Base → Salt + Water
  • Acid + Alkali → Salt + Water
  • Acid + Metal → Salt + Hydrogen
  • Acid + Carbonate → Salt + Carbon Dioxide + Water

A base readily accepts H+ ions from an acid: for example, OH- forming H2O and NH3 forming NH4+.

Anhydrous and Hydrated Salts[edit]

Solid salts consist of a lattice of positive and negative ions, sometimes water molecules are incorporated into the lattice.
The water in a lattice is called the water of crystallisation, When a salt contains water the salt is hydrated, if a salt does not contain water of crystallisation the salt is anhydrous.

A mole of a particular hydrated salt always has the same number of moles of water of crystallisation, its formula shows how many water molecules are present for every molecule of salt
eg. copper sulfate, CuSO4, has 5 moles of water for every mole of salt, therefore its hydrated formula is CuSO4.5H2O, (a dot seperates the salts formula from the water of crystallisation).

Many hydrated salts lose their water of crystallisation when heated.