Preparing Soluble and Insoluble Salts

1Why this matters

A chemist asked to make pure, dry crystals of a salt cannot simply mix any acid with any metal. Some metals react dangerously, some do not react at all, some products dissolve and some do not. Choose the wrong method and you end up with a contaminated mixture, an explosion, or nothing.

The right method depends on one question: is the salt soluble in water? By the end of this lesson you will be able to choose and describe the correct method for preparing a given salt and write the equations.

2Choosing the method

1. Is the salt insoluble? Use precipitation (Method C).
2. Is the salt soluble and a sodium, potassium or ammonium salt? Use titration of an acid with an alkali (Method B).
3. Is it any other soluble salt? React the acid with an excess of an insoluble metal, metal oxide, hydroxide or carbonate, and filter off the excess (Method A).

3Method A: acid + excess insoluble solid

1. Warm some dilute sulfuric acid in a beaker.
2. Add zinc a little at a time, stirring, until no more dissolves and some solid remains. The excess zinc makes sure that all the acid is used up, so the salt is not contaminated with acid.
3. Filter to remove the unreacted zinc. The filtrate is zinc sulfate solution.
4. Heat the filtrate to evaporate some of the water until it is saturated. Test by dipping in a cold glass rod: if crystals form on the rod, stop heating.
5. Leave the solution to cool. Crystals of zinc sulfate form.
6. Filter off the crystals, rinse them with a little cold distilled water, and dry them between filter papers.

Copper metal cannot be used to make copper(II) sulfate, because copper is below hydrogen and does not react with dilute sulfuric acid. Sodium and potassium are never added to acid, because the reaction is explosive.

4Method B: titration (acid + alkali)

Sodium, potassium and ammonium salts are made from an acid and a soluble base (an alkali). Because the alkali and the salt both dissolve, you cannot add an excess and filter it off. Instead you must add exactly the right amount.

1. Measure a known volume of alkali (for example 25.0 cm³ of sodium hydroxide) into a flask using a pipette, and add a few drops of an indicator such as phenolphthalein (pink in alkali, colourless in acid).
2. Add acid from a burette until the indicator just changes colour. Record the volume of acid used.
3. Repeat with the same volumes of acid and alkali but without the indicator, so the salt is pure.
4. Evaporate and crystallise as in Method A.

5Method C: precipitation

An insoluble salt is made by mixing two solutions: one containing its cation and one containing its anion. The insoluble salt forms as a precipitate.

1. Mix barium chloride solution with sodium sulfate solution. A white precipitate forms at once.
2. Filter. The barium sulfate stays on the filter paper as the residue.
3. Wash the residue with distilled water to remove the soluble sodium chloride.
4. Dry it in a warm oven or between filter papers.

6Method D: direct combination

Some salts can be made by joining the elements directly, for example iron(III) chloride from iron and chlorine, or iron(II) sulfide by heating iron with sulfur:

Worked example: planning a preparation

Describe how you would prepare dry crystals of magnesium sulfate.

Solution. Magnesium sulfate is soluble and is not a sodium, potassium or ammonium salt, so use Method A. Add magnesium oxide (or magnesium carbonate) a little at a time to warm dilute sulfuric acid until some remains undissolved. Filter off the excess. Evaporate the filtrate to the point of crystallisation, cool, filter off the crystals and dry them between filter papers.

MgO(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂O(l)

Practice

Try each item before you read the solution.