05 Chemical Bonds
06 Acids, Bases and Salts
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6.11.2 Identifying Anions

Identifying Anions - Carbonate

  1. In form 4 chapter 7, Acids and Bases, you should have learned that the carbonate salts react with acid produce carbon dioxide and water. This chemical property of carbonate is used to test the presence of carbonate in a salt.
    CO32– + 2H+ → H2O + CO2
  2. During the test, some dilute hydrochloric acid / nitric acid /sulphuric acid is added to the carbonate salt.
  3. If the salt contain carbonate, effervescence occurs.
  4. If the gas given off is passed through lime water, the lime water will turns chalky.
  5. This indicates that the gas is carbon dioxide, and hence the salt contain carbonate.

Identifying Anions - Sulphate

  1. We have learned that all salts of sulphate are soluble in water, except lead(II) sulphate, barium sulphate and calcium sulphate.
  2. The insolubility of barium sulphate is used to test for the presence of sulphate in a salt.
  3. 2 cm3 of dilute hydrochloric/nitric acid is added to 2 cm3 of sulphate solution. This is to check whether carbonate is presence in the solution or not because carbonate may give the same result as sulphate.
  4. If not effervescence, then 2 cm3 of barium chloride is added into the solution.
  5. If sulphate ions are presence, a white precipitate will form. The precipitate is barium sulphate.
  6. This is actually the double decomposition reaction that you have learned in preparation of insoluble salt.
    Ba2+ + SO42– → BaSO4

Identifying Anions - Nitrate

Test 1

Add dilute sodium hydroxide and a little aluminium powder. If a nitrate is present, ammonia gas is produced. The gas can be identified as it turns moist red litmus paper blue.

Test 2

  1. About 2cm3 of dilute sulphuric acid is added to the solution that wants to be tested and then followed by 2cm3 iron (II) sulphate solution.
  2. A few drops of concentrated sulphuric acid are carefully dropped through the inclined side of the test tube without shaking the test tube.
  3. A brown ring will form in the middle of the solution.
  4. Explanation: Iron (II) sulphate reduce nitric acid (from the reaction between nitrate ion and concentrated sulphuric acid) to nitrogen monoxide. Afterwards, nitrogen monoxide combines with iron (II) sulphate to form the compound FeSO4.NO which is brown in colour (brown ring).

Identifying Anions - Chloride

  1. You should have learned that all the salts of chloride are soluble in water except lead(II) chloride, silver(I) chloride and mercury chloride.
  2. The insolubility if silver(I) chloride is used in the test of the presence of chloride.
  3. 2 cm³ of dilute nitric acid is added to 2 cm3 solution of chloride ions. This is the check if carbonate ions are presence because carbonate ions may give the same result.
  4. If there is no effervescence, 2 cm³ of silver nitrate solution is then added into the mixture.
  5. A white precipitate will form if chloride is presence in the salt.
  6. The precipitate is silver chloride
    Ag+ + Cl → AgCl