An insoluble salt is made by mixing two solutions that each supply one of its ions; the salt forms at once as a precipitate. It is filtered off, washed with distilled water and dried to give the pure salt.
Some salts do not dissolve in water, so they cannot be crystallised from a solution the way soluble salts are. Instead, an insoluble salt is made by precipitation, also called double decomposition: two soluble salts are mixed so that the insoluble product falls out of solution at once. This is a short but frequently examined Form 4 practical. This guide sets out the method for a salt such as lead(II) iodide, the observations to expect, and the Paper 3 skills the practical papers reward.
Aim
To prepare pure, dry crystals of an insoluble salt, for example lead(II) iodide, by mixing two solutions that supply the two ions of the salt (double decomposition).
Apparatus and materials
- Two beakers and a glass stirring rod
- Filter funnel and filter paper
- Wash bottle of distilled water
- Spatula
- A solution supplying the metal ion, for example lead(II) nitrate solution
- A solution supplying the other ion, for example potassium iodide solution
Procedure
- Measure a volume of lead(II) nitrate solution into a beaker.
- Measure a volume of potassium iodide solution into a second beaker.
- Pour the two solutions together and stir with the glass rod; a precipitate of the insoluble salt forms immediately.
- Filter the mixture, so that the precipitate is collected as the residue on the filter paper and the soluble products pass through as the filtrate.
- Wash the residue on the filter paper with distilled water, pouring the water over the solid to rinse away the soluble ions left clinging to it.
- Leave the washed residue to dry, or dry it gently between sheets of filter paper.
Expected observations
When the two colourless (or pale) solutions are mixed, a coloured precipitate forms at once and the mixture turns cloudy, for lead(II) iodide the precipitate is yellow. On filtering, the coloured solid stays on the filter paper and a colourless filtrate passes through. After washing with distilled water and drying, the pure insoluble salt is left as a fine solid.
Inference and conclusion
Mixing the two solutions brings together the metal ion from one solution and the non-metal ion from the other. Because the salt they form is insoluble, it cannot stay in solution and separates out as a precipitate straight away. The soluble ions that remain (here, potassium ions and nitrate ions) stay in the filtrate and are washed away from the solid. The conclusion is that an insoluble salt is best prepared by precipitation, and that washing the residue is essential to remove the soluble ions and leave a pure salt.
Science process skills (Paper 3 style)
Predicting from solubility rules. Use solubility information to predict which product is insoluble and will precipitate, and explain the prediction, for example, that most iodides are soluble but lead(II) iodide is not.
Identifying variables. If the investigation compared the amount of precipitate formed with different volumes of one solution, the manipulated variable is the volume of that solution, the responding variable is the mass of dry precipitate obtained, and the controlled variables are the concentrations of both solutions and the volume of the other solution.
Making an operational definition. Define a precipitate operationally as an insoluble solid that forms and makes the mixture cloudy the moment the two solutions are combined.
Communicating. Write a conclusion that names the insoluble salt formed and states why washing is needed, linking the pure product to the removal of soluble ions.
Safety precautions
- Wear safety goggles, because the salt solutions can irritate the eyes.
- Handle lead(II) compounds with care and wash your hands after the experiment, because lead compounds are toxic.
- Do not pour lead-containing waste down the sink; collect it as directed for safe disposal, so that toxic waste is not released.
- Wipe up spills at once, so that toxic or irritant solution is not left on the bench.
Common errors
- Not washing the residue. Soluble ions left on the solid make the dried salt impure; always rinse the residue with distilled water on the filter paper.
- Washing with tap water instead of distilled water. Tap water contains dissolved ions that would contaminate the salt; use distilled water.
- Using solutions that both give soluble products. If neither product is insoluble, no precipitate forms; choose reactants so that one product is insoluble.
- Filtering too soon or stirring too little. Stir to make sure the reaction is complete before filtering, so the yield is not lost.
- Drying with strong heat. Gentle drying is enough; strong heat is unnecessary and can scatter the fine solid.
How our teachers use this experiment
In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we pair this experiment with the soluble-salt method so students can choose the right route from the salt’s solubility alone. That decision, precipitation for an insoluble salt, the excess-reactant or titration method for a soluble one, is exactly what SPM Chemistry questions test, and preparing salts is a classic Paper 3 practical (Paper 3 is a practical test assessing science process skills). Getting the washing step and its reason right here protects marks that are easy to lose.
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