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

Explanation

(a) Test for `CO_(2)` gas (lime water test ) When `CO_(2)` gas is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
`underset(underset("(colourless)")("Lime water"))(Ca(OH)_(2)(aq))+underset(underset("dioxide")("Carbon"))(CO_(2))to underset(underset(underset("(white)")("carbonate"))("Carbon"))(CaCO_(3)(s))+underset("Water")(H_(2)O)`
If the gas is in excess, milkiness disappears due to the formation of soluble calcium bicarbonate.
`underset(underset("dioxide")("Carbon"))(CO_(2))+underset("Water")(H_(2)O)+underset(underset("carbonare")("Calcium"))(CaCO_(3))to underset(underset(underset("(soluble)")("bicarbonate"))("Calcium"))(Ca(HCO_(3))_(2))`
(b) Test for `SO_(2)` gas
(i) Sulphur dioxide gas turns moist litmus papar blue to red because `SO_(2)` is acidic in nature.
(ii) Sulphur dioxide gas turns acidified potassium dichromate (vi) solution from orange to green.
`underset(underset("dichromate")("Potassium"))(K_(2)Cr_(2)O_(7))+3SO_(2)+H_(2)SO_(4)to underset(underset("sulphate")("Chromium"))(Cr_(2)(SO_(4))_(3))+K_(2)SO_(4)+H_(2)O`
(c) Test for `O_(2)` gas When a wooden splinter is brought near the mouth of the gas jar containing oxygen gas, it burns brightly as oxygen is the supporter of combustion.
`underset("Wood")(C)+underset("Oxgen")(O_(2))to underset(underset("dioxide")("Carbon"))(CO_(2))+"Heat +Light"`
(d) Test for `H_(2)` gas Hydrogen gas burns in the presence of air or oxygen with a pop sound when a burning candle is brought near it.