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Acids, bases and salts

Neutralisation reactions

Ions are charged particlescharged particle: A particle that carries an electric charge. which are formed when atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ], or groups of atoms, lose or gain electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ]. For the examination, you need to know which ions are produced by acids, and which are produced by alkalis. You will also need to know the ionic equation for neutralisationneutralisation: Neutralisation is the reaction between an acid and a base to form a salt plus water..

State symbols

State symbols are used in symbol equations:

  • (s) means solid
  • (l) means liquid (not the same as dissolved in water - see below)
  • (g) means gas
  • (aq) means aqueous (dissolved in water)


When acids dissolve in water they produce aqueous hydrogen ions, H+(aq). For example, looking at hydrochloric acid:

HCl(aq) → H+(aq) + Cl(aq)


When alkalis dissolve in water they produce aqueous hydroxide ions, OH(aq). For example, looking at sodium hydroxide:

NaOH(aq) → Na+(aq) + OH(aq)

Ammonia is slightly different. This is the equation for ammonia in solution:

NH3(aq) + H2O(l) → NH4+(aq) + OH(aq)

Be careful to write OH and not Oh or oh.

Neutralisation reaction

When the H+(aq) ions from an acid react with the OH(aq) ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction:

H+(aq) + OH(aq) → H2O(l)

For example, hydrochloric acid and sodium hydroxide solution react together to form water and sodium chloride solution. The acid contains H+ ions and Cl ions, and the alkali contains Na+ ions and OH ions. The H+ ions and OH ions produce the water, and the Na+ ions and Cl ions produce the sodium chloride, NaCl(aq).

Back to Acid, bases and salts index

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