Carbonates give off carbon dioxide with dilute acid

1. Dilute acid is added to the solid or to its solution.
2. A carbonate fizzes, because a gas is being released.
3. The reaction releases carbon dioxide: $$carbonate+acid\to salt+water+carbon\; dioxide$$
4. The gas is passed through limewater, which turns milky.
5. Fizzing alone is not enough, since the gas has to be identified as well.

Sulfates give a white precipitate with barium chloride

1. Dilute hydrochloric acid is added to the solution first.
2. Barium chloride solution is then added.
3. A white precipitate of barium sulfate shows a sulfate ion is present.
4. The precipitate forms as: $${Ba}^{2+}(aq)+{SO}_4^{2-}(aq)\to {BaSO}_4(s)$$
5. The acid is added first to remove any carbonate, which would also precipitate.

Halides give coloured precipitates with silver nitrate

1. Dilute nitric acid is added to the solution first.
2. Silver nitrate solution is then added.
3. A chloride gives a white precipitate.
4. A bromide gives a cream precipitate.
5. An iodide gives a yellow precipitate.

Why the acid comes first

1. A carbonate in the sample would give a precipitate with silver or barium ions.
2. That precipitate would be mistaken for a positive halide or sulfate result.
3. Adding acid first destroys any carbonate, releasing it as carbon dioxide.
4. Nitric acid is used before silver nitrate, so that no chloride is introduced.
5. Hydrochloric acid is used before barium chloride, for the same kind of reason.