Identification Of Common Gases

Four stoppered test tubes sit on the bench and not one of them carries a label. Three hold gases you cannot see at all; the fourth has a faint green tint if you catch it against white paper. There is no instrument in the room that will read them out for you. What you do have is a wooden splint, a strip of coloured paper, a bottle of cloudy-looking water and your own sense of smell used very carefully. Within about three minutes, every tube can be named.

That is the quiet promise of qualitative analysis: a handful of cheap, repeatable observations, each one designed so that a single gas gives a result nothing else gives. This lesson works through the four gases OxfordAQA expects you to identify, shows the reaction behind each observation rather than asking you to memorise a list, and deals with the two details that quietly cost marks every year: why the paper has to be damp, and why a limewater test left running too long can tell you the opposite of the truth.