8.2.1 - Laboratory Activity : Test for anions

Laboratory Activity 8.2.1: Test for anion

Aim: To test for carbonates, nitrates, chlorides and sulphates

Material: » Sodium carbonate » Sodium nitrate » Sodium chloride » Sodium sulphate » Zinc carbonate » Zinc nitrate » Zinc chloride » Zinc sulphate » Copper carbonate » Copper nitrate » Copper chloride » Copper sulphate

Apparatus: » Test tubes » Test tube rack » Test tube holder » Wooden splint » Litmus paper » Bunsen holder

Procedure:

	►	(A) Colour and solubility in water 1. The colour of each salt is observed 2. 10cm3 water is added in test tube. The mixture is shakes and its solubility is observed
	►	(B) Effect of heat 1. The salts are heated separately and the gas/gases which evolved are tested.
	►	(C) Effect of dilute acid 1. Dilute hydrochloric acid is added to the salts. 2. The gas is passed into limewater.
	►	(D) Confirmatory test for anions in solution i. Confirmatory test for nitrates ion. 1. Dilute H2SO4 and FeSO4 is added to NaNO3 and mixed. 2. Then, a few drops of concentrated H2SO4 is added slowly along the test tube side. 3. These steps are repeated with Zn(NO3)2 and Cu(CO3)2. ii. Confirmatory test for chloride ion. 1. Dilute HNO3 is added to NaCl solution, followed by silver nitrate solution. 2. Some Pb(NO3)2 solution is added to NaCl solution. 3. These steps are repeated with ZnCl2 and CuCl2. iii. Confirmatory test for sulphate ion. 1. Dilute HCl is added to Na2SO4 solution followed by PbCl2 solution. 2. Pb(NO3)2 solution is added to Na2SO4 solution. 3. These steps are repeated with ZnSO4 and CuSO4 solutions.

Result:

	►	(A) Colour and solubility in water Salt In water Colour of salt Na2CO3, ZnCO3, CuCO3 All salts are soluble except ZnCO3 and CuCO3. All salts are white except CuCO3 (green). NaNO3, Zn(NO3)2, Cu(NO3)2 All salts are soluble All salts are white except Cu(NO3)2 (blue) . NaCl, ZnCl2, CuCl2 All salts are soluble All salts are white except CuCl2 (blue). Na2SO4, ZnSO4, CuSO4 All salts are soluble All salts are white except CuSO4 (blue).
	►	(B) Effect of heat Salt Observation Inference Na2CO3 Does not decompose − ZnCO3 Gas released turns limewater chalky. Residue is yellow when hot, white when cooled. Blue litmus turns red. Carbon dioxide released. CO32- and Zn2+ ions present CuCO3 Gas released turns limewater chalky. Residue is black. Blue litmus turns red. Carbon dioxide released. CO32- and Cu2+ ions present NaNO3 Gas released rekindles glowing splint. Oxygen released Zn(NO3)2 Brown gas released, Glowing splint rekindled. Residue is yellow when hot, white when cold. Blue litmus turns red. NO2 and O2 released. NO3- and Zn2+ ions present CU(NO3)2 Brown gas released, Glowing splint rekindled. Brown residue. Blue litmus turns red. NO2 and O2 released. NO3- and Cu2+ ions present
	►	(C) Effect of dilute acid Salt Observation Inference Na2CO3, ZnCO3, CuCO3 Effervescence seen. Colourless gas produced turns limewater chalky. CO2 gas released. CO32- ion is confirmed to be present.
	►	(D) Confirmatory test for anions in solution Salt Observation Inference (i) Confirmatory test for nitrates ion. A brown ring is formed. NO3- ion is confirmed to be present. (ii) Confirmatory test for chloride ion. A white precipitate is formed. White precipitate which dissolves when heated, then forms back when cooled is formed. Cl- ion is confirmed to be present. PbCl2 which is insoluble in cold water is formed. (iii) Confirmatory test for sulphate ion. A white precipitate is formed. White precipitate insoluble in nitric acid. SO4- ion is confirmed to be present.

Discussion:

	►	○ All Na+, K+, NH4+ nitrates salts are soluble in water. ○ All chlorides are soluble except for AgCl, HgCl and PbCl2. ○ All sulphates are soluble except for PbSO4, BaSO4 and CaSO4. ○ All carbonates are insoluble except for Na2CO3 and K2CO3.
	►	The blue or green salts and solutions indicate the presence of Cu2+ ions. ○ All carbonates except Na2CO3 and K2CO3 release CO2 when heated. ○ All carbonates release CO2 when a dilute acid is added, confirming the presence of carbonate ions.
	►	○ All nitrates except NaNO3 and KNO3 release NO2 and O2 when heated. NaNO3 and KNO3 release only O2 when heated ○ All nitrates from a brown ring when concentrated H2SO4 is added to a mixture of nitrate, dilute H2SO4 and FeSO4 solution.
	►	All chlorides form a white precipitate with dilute HNO3 and AgNO3 solution. With Pb(NO3)2, a white precipitate soluble in hot water is formed.
	►	All sulphates form a white precipitate with dilute HCl and BaCl2 solution. They also form a white precipitate with Pb(NO3)2 solution.

Conclusion:

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Anions can be tested by ○ observing the colour and solubility of salts ○ observing the effect of heat on salts ○ performing confirmatory tests for specific anions

↶ 8.2 Qualitative analysis of salts

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