Reactions of Acids » Making Soluble Salts

Salts you can make from an acid

You already know that acids react with metals, bases and carbonates to make salts. Now you will use those reactions to actually make a salt you can hold in your hand.

This lesson is about soluble salts. These are salts that dissolve in water, so at first they end up as a solution. Zinc sulfate, magnesium nitrate and copper(II) chloride are all soluble salts.

The spec says soluble salts can be made by reacting an acid with a solid insoluble substance. That solid can be:

• a metal, such as zinc or magnesium
• a metal oxide, such as zinc oxide
• a metal hydroxide, such as copper(II) hydroxide
• a metal carbonate, such as magnesium carbonate

The metal part of the salt comes from the solid. The other part comes from the acid.

Not every metal works. A very reactive metal reacts dangerously with acid. A metal below hydrogen in the reactivity series, such as copper, does not react with dilute acid at all.

Key terms:

• Soluble salt: a salt that dissolves in water to form a solution.
• Insoluble: does not dissolve in water.
• Pure, dry sample: a solid salt with nothing else mixed in, and no water or solution left on it.

The three stages

Every soluble salt made this way follows the same three stages. Learn them and you can describe how to make any soluble salt from an insoluble solid.

Stage 1: add the solid until no more reacts

Why keep adding solid until some is left over? Remember, an excess means more than is needed to react. If solid is left, you know all the acid has been used up.

That matters because you want a solution of just the salt in water. If any acid was left, it would end up in your final crystals and the salt would not be pure.

How do you know no more reacts?

• With a metal or a carbonate, the fizzing stops and solid stays at the bottom.
• With an oxide or hydroxide there is no fizzing, so you look for solid that stays undissolved, even after stirring.

Some oxides react slowly in cold acid, so the acid is often warmed first to speed things up.

Stage 2: filter off the excess solid

The spare solid is insoluble, so filtering removes it easily. It stays behind as the residue. What passes through is a solution of the salt.

This is the big reason the solid must be insoluble. Because the leftover solid does not dissolve, you can remove every bit of it just by filtering. That leaves only salt and water.

Stage 3: crystallise the salt

The spec says salt solutions can be crystallised to produce solid salts. To do this:

• Heat the solution gently to evaporate some of the water, so the solution becomes more concentrated.
• Stop heating when crystals start to form at the edge, or on a cold glass rod dipped in and taken out.
• Leave the solution to cool. As it cools, more crystals form.
• Filter off the crystals, then dry them by patting them between filter papers or leaving them somewhere warm.

Do not boil all the water away. Heating to dryness gives a powder rather than crystals, and some salts break down if they get too hot.

The step-by-step practical, with the equipment, hazards and variables, is covered in the next lesson, Required Practical: Making a Pure Dry Salt.

Choosing the reactants

Exam questions often name a salt and ask how you would make it. Work backwards from the name.

Exam-style question

Copper(II) carbonate is an insoluble green solid. Describe how you could make pure, dry crystals of copper(II) chloride from copper(II) carbonate and dilute hydrochloric acid. (6 marks)