Test for anions and cations

Question 1

Test for carbonates

Answer 1

1. In a test tube, add dilute nitric acid to the solid or solution to be tested.
2. If you see bubbles, the unknown compound could be a carbonate
3. To prove that the gas is carbon dioxide, use the test for carbon dioxde

Question 2

Test for carbon dioxide

Answer 2

• Bubble the gas through lime water - a saturated aqueous solution of calcium hydroxide, Ca(OH)2.
• Carbon dioxide reacts to form a fine white precipitate of calcium carbonate, which turns the lime water cloudy (milky).

CO2 (g) + Ca(OH)2 (aq) —> Caco3(s) + H2O(l)

Question 3

Tests for Sulphates

Answer 3

• To the solution left from the carbonate test, add an excess of Ba(NO3)2 (aq).
  Any sulphate ions present will precipitate out as barium sulphate
• Filter the solution to remove the barium sulphate
  Ba2+ (aq) SO42- (aq) —> BaSO4 (s)

Don’t use BaCl2 as the Cl2 will show up in the halide test

Question 4

Test for Halides

Answer 4

1. Add aqueous silver nitrate, AgNO3, and Ethanol to hydrolyse the halides giving X- , to an aqueous solution of a halide. Place in water bath 50°C
2. The silver halide precipitates are different colours - silver chloride is white, silver bromide is cream-coloured, and silver iodide is yellow.
3. Add aqueous ammonia to test the solubility of the precipitate.

Ag+ (aq) + X- (aq) —> AgX (s)

Question 5

Colour of precipitate of Ag+ (aq) + Cl- (aq) —> AgCl (s), and solubility in ammonia

Answer 5

• White

- Soluble in dilute NH3 (aq)

Question 6

Colour of precipitate of Ag+ (aq) + Br- (aq) —> AgBr (s), and solubility in ammonia

Answer 6

• Cream

- soluble in concentrated NH3 (aq)

Question 7

Test for ammonium ion, NH4+

Answer 7

1. Aqueous sodium hydroxide, NaOH, is added to a solution of an ammonium ion.
2. Ammonia gas is produced. You are unlikely to see gas bubbles as ammonia is very soluble in water
3. The mixture is warmed and ammonia gas is released
4. You may be able to smell the ammonia, but it is easy to test the gas with moist pH indicator paper. Ammonia is alkaline and its presence will turn the paper blue
   NH4+ (aq) + OH- (aq) — NH3 (g) + H2O (l)