Making salts Cambridge IGCSE Chemistry (9–1) revision
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In plain words
A salt is what's left when the hydrogen of an acid is swapped for a metal. Table salt is only one of thousands. How you make one depends on a simple question: does the salt dissolve in water or not?
Three things to know
- Soluble salt: add an excess of an insoluble base, carbonate or metal to the acid, filter off what's left over, then evaporate the solution until crystals form. If both reactants are soluble (acid + alkali), use a titration.
- Insoluble salt: mix two solutions that each contain half of it. The salt appears as a solid (a precipitate). Filter, wash and dry it.
- Solubility rules: all sodium, potassium and ammonium salts and all nitrates dissolve. Chlorides dissolve except silver and lead. Sulfates dissolve except barium, calcium and lead. Most carbonates and hydroxides do not dissolve.
Worked example
How would you make the insoluble salt lead(II) iodide?
- It is insoluble, so use precipitation: one solution with lead ions, one with iodide ions.
- Both must be soluble: lead(II) nitrate (all nitrates dissolve) and potassium iodide (all potassium salts dissolve).
- Mix them, filter off the yellow precipitate, wash it with water and dry it.
Tips and tricks
- Start every salt question by asking "is the salt soluble?" That picks the method.
- Excess solid is added "to make sure all the acid has reacted". Then it is filtered off.
It lands in your notebook with its questions as flashcards.
Making salts: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which acid is used to make chlorides?
Hydrochloric acid, HCl, gives chloride salts.
What kind of salts does sulfuric acid make?
The name of the salt comes from the acid.
Which method is used to make an insoluble salt?
Two solutions are mixed and the salt comes out as a solid.
Which salt is insoluble in water?
Chlorides dissolve, except those of silver and lead.
A soluble salt is made from an acid and an excess of an insoluble base. How is the leftover base removed?
The insoluble solid stays on the filter paper.
Which of these dissolves in water?
All sodium salts are soluble.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Making salts
Cambridge IGCSE Chemistry (9–1) 0971 · 9 marks · papermunch.org
Name ______________________________ Date ______________
Describe how to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.[4]
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Add copper(II) oxide to warm acid until no more reacts (it is in excess). Filter to remove the excess. Heat the filtrate until crystals start to form, then leave it to cool. Filter off the crystals and dry them.
In the method above, explain why the copper(II) oxide is added in excess.[1]
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To make sure that all of the acid has reacted.
Name two solutions that could be mixed to make the insoluble salt barium sulfate.[2]
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Barium chloride (or barium nitrate) and sodium sulfate (or any soluble sulfate, or sulfuric acid).
Name the acid and the alkali used to make potassium nitrate, and the method.[2]
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Nitric acid and potassium hydroxide, by titration.
Answers: Making salts
- 1. Add copper(II) oxide to warm acid until no more reacts (it is in excess). Filter to remove the excess. Heat the filtrate until crystals start to form, then leave it to cool. Filter off the crystals and dry them.
- 2. To make sure that all of the acid has reacted.
- 3. Barium chloride (or barium nitrate) and sodium sulfate (or any soluble sulfate, or sulfuric acid).
- 4. Nitric acid and potassium hydroxide, by titration.