During the qualitative analysis of $SO_{3}^{2-}$ using dilute $H_{2}SO_{4}$,$SO_{2}$ gas is evolved which turns $K_{2}Cr_{2}O_{7}$ solution (acidified with dilute $H_{2}SO_{4}$):

The salt which responds to dilute and concentrated $H_2SO_4$ is

The gas evolved when sodium sulfide is heated with dilute $HCl$ is:

The phosphate radical reacts with ammonium molybdate to form a precipitate of which color?

$A$ brown ring appears in the test for

How do you account for the following observations?
$(a)$ Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants,yet in the manufacture of benzoic acid from toluene,we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox equation for the reaction.
$(b)$ When concentrated sulphuric acid is added to an inorganic mixture containing chloride,we get a colourless pungent-smelling gas $HCl$,but if the mixture contains bromide,then we get red vapours of bromine. Why?