The characteristic test for
(i) Carbon dioxide (CO2) gas turns lime water milky when passed through it due to the formation of insoluble calcium carbonate.
(ii) Sulphur dioxide (SO2) gas when passed through acidic potassium permanganate solution (purple in colour) turns it colourless because SO2 is a strong reducing agent.
(iii) The evolution of oxygen (O2) gas during a reaction can be confirmed by bringing a burning candle near the mouth of the test tube containing the reaction mixture. The intensity of the flame increases because oxygen supports burning.
(iv) Hydrogen (H2) gas burns with a pop sound when a burning candle is brought near it.
