Bases (Edexcel IGCSE Chemistry: Double Science)

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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

Name of alkali Formula Ions formed in water
Sodium hydroxide NaOH Na+ + OH-
Potassium hydroxide KOH K+ + OH-
Aqueous ammonia NH3 (+ H2O) NH4+ + OH-

Examiner Tip

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

Author: Stewart

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 Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.