Using the apparatus shown, chlorine was passed through the tube. After a short time, coloured substances were seen at P and Q

What were these coloured substances?

Chlorine gas when comes in contact with KBr, bromine is displaced by chlorine.

At ‘P’ Bromine – Red brown vapour.

The reaction taking place is

${\mathrm{Cl}}_2+2\mathrm{KBr}\to 2\mathrm{KCl}+\underset{\mathrm{Red} \mathrm{brown} \mathrm{vapour}}{ {\mathrm{Br}}_2}\uparrow$

The bromine gas thus formed reacts with potassium iodide displacing iodine from KI.

At ‘Q’ Iodine - Violet vapour

The reaction taking place is

${\mathrm{Br}}_2+2\mathrm{KI}\stackrel{\mathrm{Heat}}{\to }2\mathrm{KBr}+\underset{\mathrm{Violet} \mathrm{vapour}}{ {\mathrm{I}}_2}\uparrow$