Solubility Rules (Edexcel GCSE Chemistry)
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Solubility Rules
Ionic compounds are generally soluble in water compared to covalent substances, but there are exceptions
A knowledge of the solubility of ionic compounds helps us to determine the most appropriate method for the preparation of salts
The solubility of common ionic compounds is shown below:
Solubility of the common salts
Salts | Soluble | Insoluble |
|---|---|---|
Sodium, potassium and ammonium | All | None |
Nitrates | All | None |
Chlorides | Most are soluble | Silver and lead(II) |
Sulfates | Most are soluble | Barium, calcium and lead(II) |
Carbonates | Carbonates of sodium, potassium and ammonium | Most are insoluble |
Hydroxides | Hydroxides of sodium potassium and ammonium (calcium hydroxide is sparingly soluble) | Most are insoluble |
Note that calcium hydroxide is slightly soluble in water
Examiner Tips and Tricks
Calcium hydroxide solution is more commonly know as limewater and is used to test for carbon dioxide.
Predicting Precipitates
Some salts can be extracted by mining but others need to be prepared in the laboratory
How the salt is made in the laboratory depends on whether the salt being formed is soluble or insoluble in water
To do this the balanced equation is written down to determine the identify of the salt product
Then check the solubility of the salt using the solubility table
If it is soluble in water, then it can be prepared by titration
If it is insoluble then it can be prepared by precipitation
For example a silver nitrate solution is mixed with a sodium chloride solution:
AgNO3 (aq) + NaCl (aq) ⟶ AgCl (s) + NaNO3 (aq)
From the table both AgNO3 and NaCl are water soluble but AgCl, silver chloride, is not and hence forms a precipitate
Examiner Tips and Tricks
The precipitation reaction by combining two soluble salts is also known as a double decomposition or double displacement reaction.
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