Chem 3 - Analysis - Tests for Positive Ions

One of the problems we face in Chemistry is that many substances look similar to the naked eye. Just telling the difference between elements is hard enough. There are lots of shiny grey metals and one colourless, odourless gas looks much like another!

But once elements have been combined to form compounds the possibilities are simply enormous. To identify unknown substances we have a variety of different tests which can help us distinguish one substance from another.

Identifying some of the metals in Groups 1 and 2 of the periodic table is made much easier because most of them produce flames with a characteristic colour. To carry out a flame test you:

- Put a small amount of the compound to be tested in a platinum wire loop which has been dipped in a concentrated Hydrochloric Acid.

- Then hold the loop in the roaring blue flame of a Bunsen Burner

- Then use the colour of the Bunsen flame to identify the metal element in the compound.

Sometimes we can use the reactions of the unknown compounds with Sodium Hydroxide Solution to help us with our identifcation. Aluminium ions, Calcium ions and Magnesium ions all form white precipitates with Sodium Hydroxide solution. So if we add Sodium Hydroxide to an unknown compound and a white precipitate forms we know it contains either Aluminium, Calcium or Magnesium ions. 

If we add more and more Sodium Hydroxide then the precipitate formed with Aluminium ions dissolves - but a precipitate formed with Calcium or Magnesium ions will not. Calcium and Magnesium ions can be distinguished by a flame test. Calcium ions give a brick red flame, but Magnesium ions produce no colour at all.

Some metal ions form coloured precipitates with Sodium Hydroxide.

- If we add Sodium Hydroxide solution to a substance containing Copper ions a light blue precipitate appears

- If the substance contains Iron(II) ions a dirty green precipitate is produced when Sodium Hydroxide solution is added.

- When Sodium Hydroxide solution is added to Iron(III) ions, a reddish brown precipitate is produced.

Sodium Hydroxide solution can also be used to test whether Ammonium ions are present in an unknown substance. Ammonium ions react with Sodium Hydroxide solution to form Ammonia and Water.

To test for Ammonium ions, we add Sodium Hydroxide solution to a solution of the unknown substance. If Ammonium ions are present, Ammonia is formed. When we gently warm the solution, Ammonia is then driven off as a gas. We can detect this using damp red litmus paper. The red litmus paper turns blue because Ammonia is an alkaline gas.