C8.4 – Identification of Ions and Gases

Disclaimer: Due to unforeseen difficulties, we have had to take down the images on this notes page. They will be replaced shortly. We apologise for the inconvenience, but hope that the new images will provide you with an even better learning experience.

1. Use the following tests to identify: aqueous cations:
  • ammonium, copper(II), iron(II), iron(III) and zinc by means of aqueous sodium hydroxide and aqueous ammonia as appropriate (formulae of complex ions are not required)

cation test — When you’re given a cation to identify, add sodium hydroxide to it. The colour of the precipitate formed will help you identify the cation. If there is no precipitate, gas may have formed. Use the litmus test on this gas to confirm that it is ammonia.

anions:
• carbonate by means of dilute acid and then limewater
• chloride by means of aqueous silver nitrate under acidic conditions
• nitrate by reduction with aluminium
• sulfate by means of aqueous barium ions under acidic conditions

anion test — When you are given an anion to identify, separate four portions of it into four test-tubes and conduct the four tests above- one on each test-tube.

It may also be useful to note the following: The test for any of the halides (i.e. chloride, bromide or iodide) are the same as the above-listed test for chlorides (acidify with dilute nitric acid, then add a few drops of aqueous silver nitrate). The only difference is in the colour of the formed precipitate – chloride ions result in a white precipitate, bromide ions result in a pale cream precipitate, and bromide ions result in a pale yellow precipitate.

gases:
• ammonia by means of damp red litmus paper
• carbon dioxide by means of limewater
• chlorine by means of damp litmus paper
• hydrogen by means of a lighted splint
• oxygen by means of a glowing splint.