Bases (Edexcel IGCSE Science (Double Award))
Revision Note
Written by: Stewart Hird
Reviewed by: Lucy Kirkham
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 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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