Edexcel IGCSE Chemistry · Spec 2.40C-2.43C
Salt Preparation: Titration & Insoluble Salts
Making salts by titration and preparing insoluble salts by precipitation.
Chemistry revision video
Salt Preparation: Titration & Insoluble Salts
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Explained
Choosing a method to make a salt
There are three ways to make a salt, and the first decision in any question is which one applies. That decision is made for you by solubility. Work out whether the reactants dissolve and whether the salt you want dissolves, and the method follows without any guesswork.
When the base is insoluble
If you have an acid and an insoluble base, such as copper oxide, you can add the base in excess. Warm the acid, keep adding the solid until no more reacts, then filter off what is left over. The excess base is the point of the method: you can see when the reaction has finished because solid stops disappearing, and you can remove the surplus because it does not dissolve.
When both reactants are soluble
Sodium chloride is the awkward case. Sodium hydroxide is soluble and hydrochloric acid is soluble, so there is no leftover solid to filter and nothing to tell you when the reaction is complete. Adding excess would simply contaminate the product with alkali.
This is why titration is used. You find the exact volume of acid that neutralises a known volume of alkali, using an indicator to show the end point. Then you repeat the experiment with those same volumes and no indicator, so the crystals are not coloured. Evaporate some of the water, leave the solution to crystallise, and dry the crystals between filter papers.
When the salt itself is insoluble
If the salt you want will not dissolve, you can make it by precipitation. Mix two solutions that each contain one of the ions you need, and the insoluble salt appears as a solid. Filter it, wash it with distilled water, and dry it.
The washing step is not optional. Without it, the crystals stay contaminated with the soluble salt left behind in the solution.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry note answers lost for giving an incorrect colour and for leaving out the key term precipitate. In this topic the vocabulary is the mark. A solid appearing when two solutions are mixed is a precipitate, and saying so matters.
Reports also record candidates suggesting sensible apparatus improvements, such as using a burette or pipette instead of a measuring cylinder, but then failing to explain why the change helps. Mark schemes for this practical reward the reasoning: repeat the titration so that concordant results are obtained, and use a white tile so the colour change can easily be seen.
Deciding quickly under exam conditions
Two questions get you to the right method every time. Is the salt soluble? If not, precipitate it. If it is, is the base soluble? If the base is insoluble, add it in excess and filter. If everything is soluble, titrate.
Learn the solubility rules well enough to answer the first question with confidence, because everything else depends on it.
Spec 2.40C-2.43C
What you need to know
- Make a soluble salt by titration
- Make an insoluble salt by precipitation
- Choose the right method for a given salt
Active recall
Quick check
Answer each question before opening the answer.
How do you make an insoluble salt?
By precipitation — mix two soluble salt solutions, then filter, wash and dry the precipitate.
How do you make a soluble salt from an acid and a soluble alkali?
Titrate to find the volumes that react, then repeat without indicator and crystallise the solution.
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