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+
