Describe how to prepare a pure, dry soluble salt from an acid and an insoluble base or carbonate, and how to make an insoluble salt by precipitation.

What this topic is asking

WJEC topic 2.2 wants you to describe two preparation methods: making a pure, dry soluble salt from an acid and an insoluble base or carbonate, and making an insoluble salt by precipitation. These are standard required practicals and very common exam questions.

Preparing a soluble salt

The aim is a pure, dry sample of a soluble salt such as copper sulfate. You react an acid with an insoluble base or carbonate, using an excess of the solid so that all the acid is used up (this guarantees no leftover acid contaminates the salt).

The slow crystallisation gives larger, purer crystals than fast evaporation. The excess solid is what makes the salt pure: because it is insoluble, it is easily filtered out, and because it is in excess, no acid is left behind.

Why use an insoluble base?

Using an insoluble base or carbonate is convenient because you can simply add it until no more dissolves and then filter the rest out. If you used a soluble alkali instead, you could not see when the reaction was complete, so you would need a different method (a titration) to get the exact amounts right.

Preparing an insoluble salt by precipitation

Some salts are insoluble (they do not dissolve in water). These are made by precipitation: mixing two soluble solutions whose ions react to form the insoluble salt as a solid.

For example, barium chloride solution plus sodium sulfate solution gives a precipitate of insoluble barium sulfate: $\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{NaCl}$.

Knowing which salts are soluble

To choose the right method you need to know which salts are soluble. As a general guide for GCSE: all common sodium, potassium and ammonium salts are soluble, all nitrates are soluble, and most common chlorides and sulfates are soluble (with a few exceptions, such as silver chloride and barium sulfate, which are insoluble). Most carbonates and hydroxides are insoluble, apart from those of sodium, potassium and ammonium. So if a question asks for a soluble salt you use the acid-plus-excess-solid method, but if the salt is one of the insoluble exceptions you must use precipitation instead. Recognising the solubility from these rules is often the first step in a salt-preparation question.