Contents
Key Stage 3
Meaning
An alkali is a Metal Hydroxide solution that has a pH more than 7.
About Alkalis
- Alkalis can be an Irritant, Harmful or Corrosive and should be handled with care.
- Alkalis can be used to neutralise acids.
- Alkalis can be concentrated or dilute.
- Alkalis can be weak or strong which depends on the type of alkali and the concentration.
- Alkalis turn Litmus paper blue.
- Alkalis turn Universal Indicator blue, indigo and purple depending on the strength of the alkali.
Some common Alkalis you should know:
Key Stage 4
Meaning
An alkali is basic compound which dissolves in water to form OH- ions making a solution with pH greater than 7.
About Alkalis
- Alkalis are bases in solution which form Hydroxide ions.
- When a baseic compound dissolves in water a Hydroxide ion becomes dissociated from either the basic compound or from a water molecule. That Hydroxide ion is free to move separately in the solution.
- The Hydroxide ions in an alkali will donate electrons to Hydrogen ions to form water. This is a neutralisation reaction.
- The strength of an alkali is measured on the pH scale. The more Hydroxide ions in a solution the lower the pH.
Examples
Metal Hydroxides form Hydroxide ions in water.
- NaOH(aq) → Na+(aq)+OH-(aq)
- KOH(aq) → K+(aq)+OH-(aq)
- Mg(OH)2(aq) → Mg+2(aq)+2OH-(aq)
- Ca(OH)2(aq) → Ca+2(aq)+2OH-(aq)
- Al(OH)3(aq) → Al+3(aq)+3OH-(aq)
Metal Oxides react with water to form Metal Hydroxides.
- Na2O(s) + H2O(l) → 2Na+(aq)+2OH-(aq)
- K2O(s) + H2O(l) → 2K+(aq)+2OH-(aq)
- MgO(s) + 2H2O(l) → Mg+2(aq)+2OH-(aq)
- CaO(s) + 2H2O(l) → Ca+2(aq)+2OH-(aq)
- Al2O3(s) + 3H2O(l) → 2Al+3(aq)+6OH-(aq)
Ammonia also reacts with water to produce hydroxide ions:
- NH3O(aq) + H2O(l) → NH4+(aq)+OH-(aq)