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[link] shows some of the general rules about the solubility of different salts based on a number of investigations:
Salt | Solubility |
Nitrates | All are soluble |
Potassium, sodium and ammonium salts | All are soluble |
Chlorides | All are soluble except silver chloride, lead(II) chloride and mercury(II) chloride |
Sulphates | All are soluble except lead(II) sulphate, barium sulphate and calcium sulphate |
Carbonates | All are insoluble except those of potassium, sodium and ammonium |
It is also possible to carry out tests to determine which ions are present in a solution.
Prepare a solution of the unknown salt using distilled water and add a small amount of silver nitrate solution. If a white precipitate forms, the salt is either a chloride or a carbonate.
Cl + Ag + + NO 3 AgCl + NO 3 (AgCl is white precipitate)
CO 3 2 + 2Ag + + 2NO 3 Ag 2 CO 3 + 2NO 3 (Ag 2 CO 3 is white precipitate)
The next step is to treat the precipitate with a small amount of concentrated nitric acid . If the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed, and the precipitate disappears, the salt is a carbonate.
AgCl + HNO 3 (no reaction; precipitate is unchanged)
(precipitate disappears)
Add a small amount of barium chloride solution to a solution of the test salt. If a white precipitate forms, the salt is either a sulphate or a carbonate.
(BaSO is a white precipitate)
(BaCO is a white precipitate)
If the precipitate is treated with nitric acid, it is possible to distinguish whether the salt is a sulphate or a carbonate (as in the test for a chloride).
(no reaction; precipitate is unchanged)
(precipitate disappears)
If a sample of the dry salt is treated with a small amount of acid, the production of carbon dioxide is a positive test for a carbonate.
Acid + CO 3 2 CO 2
If the gas is passed through limewater and the solution becomes milky, the gas is carbon dioxide.
Ca(OH) 2 + CO 2 CaCO 3 + H 2 O (It is the insoluble CaCO 3 precipitate that makes the limewater go milky)
As was the case with the chlorides, the bromides and iodides also form precipitates when they are reacted with silver nitrate. Silver chloride is a white precipitate, but the silver bromide and silver iodide precipitates are both pale yellow. To determine whether the precipitate is a bromide or an iodide, we use chlorine water and carbon tetrachloride (CCl ).
Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a reddish brown.
Chlorine water frees iodine gas from an iodide and colours the carbon tetrachloride purple.
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