Testing for anions

1. Most salts are made of a metal cation and a non-metal anion, and a chemical test can identify each one.
2. Each anion test uses a reagent that reacts with that anion to give a result you can see: a gas, a precipitate or a colour change.
3. Each test is carried out on a fresh portion of the unknown in a clean test tube, so that the reagents from one test do not interfere with another.
4. Most of the tests are done on a solution, so a solid is first dissolved in distilled water.
   1. The carbonate test can also be carried out directly on the solid.

Carbonate ions

1. To test for carbonate ions, ${CO}_3^{2-}$, add dilute acid, such as dilute hydrochloric acid, to the sample.
2. A positive result is effervescence, because the acid reacts with the carbonate to release carbon dioxide gas.

   1. For example, calcium carbonate reacts with dilute hydrochloric acid:

      $$\mathrm{CaC}{\mathrm{O}}_3(\mathrm{s})+2HCl(\mathrm{aq})\to \mathrm{CaC}{\mathrm{l}}_2(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$$
3. The gas is bubbled through limewater, which turns milky if the gas is carbon dioxide.
4. Fizzing on its own is not enough, because other gases can also bubble off, so the limewater result is needed to confirm carbon dioxide.

Halide ions

1. To test for chloride, bromide and iodide ions, first acidify the solution with dilute nitric acid, then add aqueous silver nitrate.

2. Each halide ion forms a different coloured precipitate of a silver halide.

   1. Chloride, ${Cl}^{-}$, gives a white precipitate of silver chloride.
   2. Bromide, ${Br}^{-}$, gives a cream precipitate of silver bromide.
   3. Iodide, $I^{-}$, gives a yellow precipitate of silver iodide.

3. For example, sodium chloride solution reacts with silver nitrate solution:

   $$\mathrm{NaCl}(\mathrm{aq})+\mathrm{AgN}{\mathrm{O}}_3(\mathrm{aq})\to \mathrm{AgCl}(\mathrm{s})+\mathrm{NaN}{\mathrm{O}}_3(\mathrm{aq})$$

4. The nitric acid removes any carbonate ions, which would otherwise react with silver nitrate to form a precipitate that could be mistaken for a positive result.

5. Nitric acid is used rather than hydrochloric acid, because hydrochloric acid contains chloride ions and would give a white precipitate with every sample.

Sulfate ions

1. To test for sulfate ions, ${SO}_4^{2-}$, acidify the solution with dilute nitric acid, then add aqueous barium nitrate.
2. A white precipitate of barium sulfate shows that sulfate ions are present.

   1. For example, sodium sulfate solution reacts with barium nitrate solution:

      $$\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{aq})+\mathrm{Ba}(\mathrm{N}{\mathrm{O}}_3{)}_2(\mathrm{aq})\to \mathrm{BaS}{\mathrm{O}}_4(\mathrm{s})+2NaN{\mathrm{O}}_3(\mathrm{aq})$$
3. The acid is added first to remove carbonate ions, which would also give a white precipitate, barium carbonate, with barium nitrate.
4. Sulfuric acid is never used to acidify the sample, because it contains sulfate ions itself.

Nitrate ions

1. To test for nitrate ions, ${NO}_3^{-}$, add aqueous sodium hydroxide and then a small piece of aluminium foil, and warm carefully.
2. The aluminium reduces the nitrate ions to ammonia gas.
3. Damp red litmus paper held at the mouth of the test tube turns blue if ammonia is given off.
4. Ammonium ions also give off ammonia when warmed with sodium hydroxide, so the sample is first warmed with sodium hydroxide alone.
   1. If ammonia appears only after the aluminium foil is added, it came from nitrate ions.

Sulfite ions

1. To test for sulfite ions, ${SO}_3^{2-}$, add a small volume of acidified aqueous potassium manganate(VII).
2. The solution changes colour from purple to colourless if sulfite ions are present.
3. Sulfite ions act as a reducing agent, so they react with the purple manganate(VII) ions and remove the colour.