Preparing Pure, Dry Soluble and Insoluble Salts (Paper 2)

Explained

Finishing a salt preparation, and proving it is dry

Making the salt is the easy half. The marks in this topic are mostly in what happens afterwards: getting the product pure, getting it dry, and being able to say how you know.

Finishing a soluble salt

Add the insoluble base to warm acid until some remains undissolved, so you can see the acid has been used up. Filter to remove the excess.

Heat the filtrate to evaporate about half the water, until crystals just begin to appear at the edge, then stop and leave it to cool. Crystals form as it cools, because less salt stays dissolved at a lower temperature.

Filter or decant the crystals from the remaining liquid and leave them to dry in a warm place, or pat them between filter papers.

Do not boil the solution dry. It looks like the same outcome and it is not. Evaporating everything leaves a powder rather than crystals, drives off the water of crystallisation, and can decompose the salt, and mark schemes for this practical cap the available marks when it is done that way.

Making an insoluble salt by precipitation

Choose two soluble solutions that between them contain the two ions you need. For lead(II) sulfate, use lead(II) nitrate solution and sodium sulfate solution.

Mix them and the insoluble salt appears immediately as a precipitate. The other product, sodium nitrate, stays in solution because both sodium salts and nitrates are soluble.

Filter to collect the precipitate as the residue. Then wash it with distilled water while it is still in the filter paper, and leave it to dry.

Washing is what separates a good answer from a complete one. The precipitate is wet with the solution it formed in, which contains the soluble salt and any unreacted starting materials, so without washing those dry onto the crystals and the product is impure. Use distilled water rather than tap water, since tap water carries dissolved ions of its own.

Choosing the method

The solubility rules decide everything, so check them before planning anything.

Insoluble salt wanted: precipitation, mixing two solutions.

Soluble salt from an insoluble base: add the base in excess, filter, crystallise.

Soluble salt from a soluble base: titration, because the excess trick will not work when everything dissolves. Find the exact volume with an indicator, then repeat with that volume and no indicator so the crystals are not contaminated.

Purity and yield

If a question asks how you would check the salt is pure, one answer is to measure its melting point and compare it with the accepted value. An impurity lowers the melting point and spreads it over a range rather than a single temperature.

For yield, calculate the theoretical mass from the balanced equation, then divide the mass you actually collected by it and multiply by 100.

Losses come from crystals left in the filter paper, solution left in the beaker, and product still dissolved in the liquid poured away. A yield above 100 per cent means the crystals were not fully dry, which brings you back to heating to constant mass.