Edexcel IGCSE Chemistry · Spec 2.39-2.42
Preparing Soluble Salts
How to prepare a soluble salt from an acid and an insoluble base.
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Preparing Soluble Salts
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Explained
Making a soluble salt from an acid and an insoluble base
The problem this method solves is knowing when to stop. An acid and an alkali both dissolve, so mixing them gives you no way of telling by eye when the acid has been used up, and you need an indicator or a titration.
An insoluble base solves it for you. Add too much and the excess simply sits at the bottom undissolved, so you can see that all the acid has reacted, and the excess can be filtered off afterwards. That is the whole logic of the method, and questions about why you add excess are asking for it.
The steps in order
Warm the acid gently, which speeds up the reaction. Take the beaker off the heat before adding anything, because acids are corrosive and you do not want to be adding solids to boiling acid.
Add the insoluble base a little at a time, stirring, until no more dissolves and some solid remains at the bottom. That leftover solid is the signal that the acid is used up.
Filter the mixture to remove the excess base. What passes through is a solution of the salt.
Heat that solution to evaporate about half the water, until crystals just begin to form at the edge. Stop there.
Leave it to cool. Crystals form as the solution cools, because less salt can stay dissolved in cooler water.
Pour off the remaining liquid, or filter the crystals out, and leave them to dry in a warm place or pat them dry between filter papers.
Making copper sulfate
Copper oxide, which is black and insoluble, is added to warm dilute sulfuric acid. The mixture turns blue as copper sulfate forms, and the black excess settles out.
Copper oxide plus sulfuric acid gives copper sulfate plus water. Filtering removes the black excess and leaves a clear blue solution, which is crystallised to give blue crystals of hydrated copper sulfate.
Copper carbonate can be used instead, and then the mixture fizzes as carbon dioxide is released. The fizzing stopping is a second visible sign that the reaction has finished.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme gives one mark for why excess is added, and the credited answer is to ensure all the acid has reacted. It allows acid used up in place of reacted, and allows the phrasing that it has been neutralised.
It then awards five marks for the rest of the method, from six possible points: filter off the excess, heat, heat until crystals first start to form, allow to cool and crystallise, pour off the excess liquid, and leave in a warm place to dry.
Two instructions are attached, and both are worth more than the marking points themselves. Washing the crystals is ignored, so it is neither credited nor penalised. And if the sample is heated to dryness, only the first two marks are available.
That second one is the trap in this practical. Boiling the solution dry does produce a dry solid, so it feels like a reasonable shortcut. But it drives the water of crystallisation out as well, giving a powder rather than crystals, and it can decompose the salt. Three of the five marks depend on cooling slowly instead.
An examiner report on a paper in the same series describes responses losing marks for exactly this, heating the solution but never cooling or filtering, and adds a direct instruction: learn this practical technique, as it is often tested.
Which method for which salt
This method works when the base is insoluble, which covers most metal oxides, hydroxides and carbonates other than the group 1 ones.
For a soluble base, such as sodium hydroxide, the excess trick is unavailable, so a titration is used instead. Find the exact volume of acid needed using an indicator, then repeat with that volume and no indicator, so the crystals are not contaminated by it.
For an insoluble salt, neither applies. Mix two solutions that each contain one of the ions you need, and the salt appears as a precipitate that can simply be filtered, washed and dried.
So the first question in any salt preparation is not what to do but which of the three situations you are in, and the answer comes from the solubility rules.
Spec 2.39-2.42
What you need to know
- Describe the insoluble base method
- Carry out the steps in order
- Apply it to make copper sulfate
Active recall
Quick check
Answer each question before opening the answer.
How do you prepare a soluble salt from an acid and an insoluble base?
Add excess base to the warm acid, filter off the unreacted base, then crystallise the solution.
Why is excess base added?
To make sure all of the acid has reacted.
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