Testing Aqueous Cations 1
Chemistry: Cambridge International GCSE Supplement, topic 12.5.1
Metal Hydroxides
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.
Zinc hydroxide
- Zinc ions (Zn2+) form the white precipitate zinc hydroxide.
- The white precipitate dissolves in excess NaOH to give a colourless solution.
Test yourself on Testing Aqueous Cations 1
Which colour of precipitate is formed by aluminium, zinc, calcium and magnesium?
What are the correct colours for the following precipitates?
Which equation correctly represents magnesium ions reacting with hydroxide ions to form magnesium hydroxide?