Chem 3 - Analysis - Tests for Negative ions

If we add a dilute acid (such as Hydrochloric Acid) to a carbonate it fizzes and produces Carbon Dioxide gas. This is a good test if an unknown substance is a carbonate.

2 Particular metal carbonates have very distinctive colour changes when we heat them. This means that if we have indentified an unknown substance as a carbonate, and then heat some of that substance, we may be able to complete the identification. For example, Copper Carbonate is a green substance which decomposes when we heat it to give black Copper Oxide and Carbon Dioxide.

Similarly, Zinc Carbonate is white substance which decomposes to give Zinc Oxide when heated. Zinc Oxide is also white, but when it is hot it is a lemon yellow colour, turning back to white as it cools down. It is the only substance to show such a colour change.

A very simple test shows whether Chloride, Bromide or Iodide ions are present in a compound. If we add dilute Nitric Acid and Silver Nitrate solution to an unknown solution, the appearance of a precipitate tells us that one of the Halide ions is present. The colour of this precipitate tells us which Halide ion it is:

- Chloride ions give a white precipitate

- Bromide ions give a cream precipitate

- Iodide ions give a pale yellow precipitate

Sulfates:

Sulfate ions in solution produce a white precipitate when we add Hyrochloric Acid to them followed by Barium Chloride solution. The white precipitate is the insoluble salt, Barium Sulfate.

Nitrates:

To detect the presence of Nitrate ions in an unknown compound we make use of the test for Ammonia.

Once again, we add Sodium Hydroxide solution to a solution of the unknown substance and gently warm it. If no Ammonia is detected, we add a little Aluminium powder. The Aluminium powder reduces the Nitrate ions to Ammonium ions. These then react with the Sodium Hydroxide solution to form Ammonia gas, which is given off. This is detected using damp red litmus paper which turns blue.