Edexcel IGCSE Chemistry · Spec 2.34
Solubility Rules of Ionic Compounds
The solubility rules for common ionic compounds in water.
Chemistry revision video
Solubility Rules of Ionic Compounds
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Explained
Which salts dissolve, and why it decides your state symbols
This is a short list of facts, and it is worth learning properly because it is used constantly. Every state symbol in an ionic equation, every precipitate you are asked to predict, and every preparation method you choose depends on knowing whether a salt dissolves.
What is soluble
- All common sodium, potassium and ammonium salts.
- All nitrates.
- Common chlorides, except silver chloride and lead(II) chloride.
- Common sulfates, except lead(II) sulfate and barium sulfate. Calcium sulfate is only slightly soluble.
What is insoluble
- Most carbonates, except those of sodium, potassium and ammonium.
- Most hydroxides, except those of sodium and potassium. Calcium hydroxide is only slightly soluble, which is why limewater is dilute.
The pattern is easier to hold if you notice that sodium, potassium and ammonium never let you down. Whatever they are joined to, the salt dissolves. Nitrates behave the same way. Everything else needs the exceptions.
Using the rules to predict a precipitate
Mix two solutions and consider the two new pairings that could form. If one of them is insoluble, that is your precipitate.
Barium chloride solution and sodium sulfate solution give barium sulfate and sodium chloride. Sodium chloride is soluble, so it stays in solution. Barium sulfate is insoluble, so a white precipitate forms. That reaction is exactly why barium chloride is the test for a sulfate.
Silver nitrate and a chloride give silver chloride, which is insoluble, and that is the halide test for the same reason.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry return to state symbols repeatedly. In one paper the second mark was most frequently lost because an incorrect state symbol was given for sodium sulfate, and the report advises that candidates would benefit from further practice in applying solubility rules to determine state symbols. A later report notes that many state symbols appeared to be random guesses, with only a small minority of learners gaining that mark.
Sodium sulfate is a sodium salt, so it is soluble, so it is (aq). The rule answers the question directly. Treat state symbols as a mark you can secure by recall rather than a detail added at the end.
Where the rules do the work for you
Choosing a preparation method starts here. A soluble salt is made by reacting an acid with an insoluble base, metal or carbonate, then filtering off the excess and crystallising the solution. An insoluble salt is made by precipitation, mixing two solutions and filtering the solid out. Deciding which route to take is a solubility question before it is anything else.
Spec 2.34
What you need to know
- Recall which common salts are soluble
- Recall the main insoluble ones
- Use the rules to predict precipitates
Active recall
Quick check
Answer each question before opening the answer.
Are all nitrate salts soluble in water?
Yes — all nitrates are soluble.
Give examples of common insoluble salts.
Most carbonates, silver and lead halides (e.g. silver chloride), and barium/lead sulfate are insoluble.
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