Describe the preparation of soluble and insoluble salts, use solubility rules to choose the method, and carry out crystallisation and precipitation to obtain a pure salt

What this dot point is asking

SEAB wants you to prepare both soluble and insoluble salts, using solubility rules to decide which method to use, and to carry out the practical steps (crystallisation for a soluble salt, precipitation then filtration for an insoluble salt) to obtain a pure, dry product. This is a classic practical-paper topic that also appears as an extended written answer.

The answer

Solubility rules to choose a method

The first decision is whether the salt you want is soluble or insoluble in water. The useful rules:

• All sodium, potassium and ammonium salts are soluble; all nitrates are soluble.
• Most chlorides are soluble (except silver and lead chloride).
• Most sulfates are soluble (except barium, calcium and lead sulfate).
• Most carbonates are insoluble (except sodium, potassium and ammonium).

If the salt is soluble, prepare it in solution and crystallise it. If it is insoluble, make it by precipitation.

Method 1: soluble salt from an acid and an insoluble base, carbonate or metal

For a soluble salt of a metal that is not sodium, potassium or ammonium, react the acid with an excess of an insoluble base, carbonate or (for a reactive enough metal) the metal itself:

1. Add the solid in excess to warm acid and stir until no more reacts (this uses up all the acid).
2. Filter to remove the unreacted excess solid; the filtrate is the salt solution.
3. Evaporate the filtrate to the point of crystallisation, then leave it to cool so crystals form.
4. Filter off the crystals and dry them between filter paper.

Using excess solid is the key idea: it guarantees no acid is left to contaminate the product, and the excess is simply filtered off.

Method 2: soluble salt of sodium, potassium or ammonium (titration)

These salts have soluble starting alkalis, so excess cannot be filtered off. Instead use a titration: find by titration the exact volume of acid that neutralises the alkali, then repeat without the indicator using those volumes, and crystallise the resulting solution. This gives a pure salt uncontaminated by indicator.

Method 3: insoluble salt by precipitation

To make an insoluble salt, mix two soluble solutions that between them supply the right ions. The insoluble salt forms at once as a precipitate:

1. Mix solutions containing the two ions needed (for example barium ions and sulfate ions).
2. Filter to collect the precipitate as the residue.
3. Wash it with distilled water to remove soluble impurities.
4. Dry it in a warm oven.

Examples in context

Example 1. Making copper(II) sulfate for the lab. Blue copper(II) sulfate crystals are prepared by adding excess black copper(II) oxide to warm sulfuric acid, filtering off the excess oxide, then crystallising the blue solution. The colour change from black solid to blue solution shows the reaction proceeding, and the excess oxide guarantees a pure product.

Example 2. Precipitating silver chloride in analysis. Mixing silver nitrate solution with a chloride solution instantly produces a white precipitate of insoluble silver chloride, which is filtered, washed and dried. This precipitation both prepares the insoluble salt and serves as the test for chloride ions, linking salt preparation to qualitative analysis.

Try this

Q1. State the method used to prepare an insoluble salt. [1 mark]

• Cue. Precipitation (mixing two soluble solutions that supply the required ions).

Q2. Explain why excess copper(II) oxide is used when making copper(II) sulfate from sulfuric acid. [2 marks]

• Cue. The excess ensures all the acid is used up, and the unreacted excess can then be filtered off, giving a pure salt with no leftover acid.

Q3. Describe how a pure dry sample of an insoluble salt is obtained once the precipitate has formed. [3 marks]

• Cue. Filter to collect the precipitate, wash it with distilled water to remove soluble impurities, then dry it (in a warm oven or between filter paper).