Give the characteristic tests for the following gases:
$(a)$ $CO_2$
$(b)$ $SO_2$

(N/A) Carbon dioxide $(CO_2)$ gas turns lime water milky when passed through it due to the formation of insoluble calcium carbonate.
$Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O$
$(b)$ Sulphur dioxide $(SO_2)$ gas,when passed through an acidic potassium permanganate solution (purple in colour),turns it colourless because $SO_2$ is a strong reducing agent.
$2KMnO_4 + 2H_2O + 5SO_2 \to K_2SO_4 + 2MnSO_4 + 2H_2SO_4$
Alternatively,sulphur dioxide gas,when passed through an acidic potassium dichromate solution (orange in colour),turns it green because sulphur dioxide is a strong reducing agent.