6.5 - Making Insoluble Salts

The solubility rules, and predicting a precipitate

• A substance that dissolves in water is soluble; one that doesn't is insoluble.
• An insoluble salt made in water appears as a solid precipitate.

The solubility rules

Nitrates, sodium, potassium, and ammonium compounds have no exceptions - they're always soluble.

What happens when two solutions are mixed

• In solution, a salt's positive and negative ions are separate and free to move through the water.
• Mixing two solutions puts four kinds of ion into one liquid, creating two new pairings of positive and negative ions.
• Check each new pairing against the solubility rules:
  • If one pairing is insoluble, its ions join together to form a solid called a precipitate, and the reaction is a precipitation reaction.
  • If both pairings are soluble, all four ions stay dissolved and no precipitate forms.

Predicting what a mixture will do

1. Write down the ions each solution supplies.
2. Pair them the other way round to find the two new possibilities.
3. Check each possibility against the solubility rules.

Worked example - Predicting a precipitate

Copper(II) sulfate solution is mixed with sodium carbonate solution. Does a precipitate form, and if so what is it?

Making an insoluble salt by precipitation

• An insoluble salt can't be crystallised out of a solution, because it doesn't dissolve.
• It's made by mixing solutions containing its two ions, causing the insoluble salt to form as a solid precipitate.

Choosing the right two starting solutions

Between them the starting solutions must supply the two ions needed:

• Both starting compounds must be soluble:
  • Every nitrate dissolves, making nitrates a reliable source of the metal ion.
  • Every sodium, potassium, and ammonium compound dissolves, making them a reliable source of the negative ion.
• The other pairing must be soluble too, so it stays dissolved in the water and can be washed away from the solid.

Example - Choosing solutions for lead(II) sulfate

• To make lead(II) sulfate (insoluble), lead(II) ions and sulfate ions have to meet in solution.
• Lead(II) nitrate is soluble (all nitrates are), so it supplies the lead(II) ions.
• Sodium sulfate is soluble (all sodium salts are), so it supplies the sulfate ions.
• Mixing them leaves sodium nitrate in the solution.
• Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)

Method

1. Dissolve about a spatula of lead(II) nitrate in deionised water in one test tube, and a spatula of sodium sulfate in deionised water in another, until fully dissolved. Deionised water is used so no stray ions are introduced.
2. Pour the two solutions into a small beaker and stir. Insoluble lead(II) sulfate precipitates immediately.
3. Filter the mixture using filter paper and a funnel. The precipitate is collected as the residue; the solution drains through as the filtrate.
4. Rinse the beaker with a little deionised water and pour that through too, so no product is left behind.
5. Wash the solid on the filter paper with deionised water to remove any dissolved sodium nitrate.
6. Leave the solid to dry on fresh filter paper, in a warm oven or in a desiccator (a sealed container holding a drying agent).