Give the characteristic tests for the following gases : (a) `CO_(2)` (b) `SO_(2)` (c) `O_(2)` (d) `H_(2)`

Explanation

(a) `CO_(2)` gas : It is a colourless and odourless gas. When passed through lime water, it will initially become milky.
`Ca(OH)_(2)(aq) + CO_(2)(g) + H_(2)O(aq) to underset("(Colourless)")(Ca(HCO_(3))_(2)(aq))`
(b) `SO_(2)` gas : It is a colourless but a pungent smelling gas. The gas when passed through an acidified solution of potassium permanganate (pink) will decolourise it.
`underset("(Pink)")(2KMnO_(4)) + 5SO_(2) + 2H_(2)O to K_(2)SO_(4) + 2MnSO_(4) + 2H_(2)SO_(4)`
Similarly, the gas when passed through an acidified solution of potassium dichromate (orange) will make it light green.
`underset("(Orange)")(K_(2)Cr_(2)O_(7)) + 3SO_(2) + H_(2)SO_(4) to K_(2)SO_(4) + underset("(Light green)")(Cr_(2)(SO_(4))_(3) +) H_(2)O`
(c) Oxygen gas : The gas is a supporter of combustion. It can be tested by bringing a burning or smouldering match-stick near the mouth of the test tube from which the gas escapes. The match-stick will burn more brightly.
(d) Hydrogen gas : The gas itself is combustible. It will burn with a pop sound when a burning candle or match-stick is brought near it.
`2H_(2)(g) + O_(2) (g) to 2H_(2)O(g)`