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GCSEChemistryEdexcelRevision Notes3. Chemical changesAcidsBases

Bases (Edexcel GCSE Chemistry)

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Stewart Hird

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Bases

What makes a base act like a base?

  • Bases are substances which can neutralise an acid, forming a salt and water

  • The term base and alkali are not the same

  • A base which is water-soluble is referred to as an alkali

    • So, all alkalis are bases, but not all bases are alkalis

  • Alkalis have pH values of above 7

  • In basic (alkaline) conditions red litmus paper turns blue

  • Bases are usually oxides, hydroxides or carbonates of metals

  • The presence of the OH- ions is what makes the aqueous solution an alkali

  • One unusual base is ammonia solution

    • When ammonia reacts with water it produces hydroxide ions

Some Common Alkalis and the Ions they Contain

Equilibria Table 1_Common Alkalis, downloadable AS & A Level Chemistry revision notes

Examiner Tips and Tricks

Aqueous ammonia and ammonium hydroxide are the same thing. When ammonia gas dissolves in water it forms ammonium hydroxide. Be careful to use the correct terminology: ammonia is the gas, NH3, ammonium is the ion present in ammonium compounds, NH4+

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    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.