Metal Hydroxides
Chemistry: AQA GCSE Higher, topic 8.3.3
Metal Hydroxides - Calcium, Magnesium and Aluminium
Metal ions can be found in aqueous solutions of metal compounds. These ions can precipitate out of solution when they react with a sodium hydroxide solution (NaOH). This produces insoluble solids (don't dissolve in water) called precipitates.
Calcium hydroxide
- Calcium ions (Ca2+) form the white precipitate calcium hydroxide.
- Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)
Magnesium hydroxide
- Magnesium ions (Mg2+) form the white precipitate magnesium hydroxide.
- Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)
Aluminium hydroxide
- Aluminium ions (Al3+) form the white precipitate aluminium hydroxide.
- Al3+(aq) + 3OH-(aq) → Al(OH)3(s)
- Unlike the other two white precipitates, aluminium hydroxide can be dissolved in excess NaOH. This produces a colourless solution.
Test yourself on Metal Hydroxides
Which metal ion must be present if a white precipitate forms with sodium hydroxide solution and then dissolves when excess sodium hydroxide is added?
What are the correct colours for the following precipitates?
Which equation correctly represents magnesium ions reacting with hydroxide ions to form magnesium hydroxide?
Why does the equation for aluminium hydroxide formation need 3 hydroxide ions, while the equations for calcium and magnesium hydroxide only need 2?
What does the state symbol (s) show about calcium hydroxide in the equation Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)?