Question

why does a moist blue Litmus Paper turns red when it is brought near the mouth of the test tube in which ferrous sulphate crystal are being heated

Answer 1

During the heating of ferrous sulphate solution to gases namely SO2 and SO3 are released which are acidic in nature and turns the moist blue Litmus Paper to red.

I hope this will help

Answer 2

ANSWER :

When blue coloured litmus paper is brought close to the opening of the apparatus containing heated ferrous sulphate, the blue litmus paper becomes red coloured because -

Heating of crystals of ferrous sulphate releases gases like SO2 and SO3. These are the acidic oxides of sulphur.

And we know that, in the existence of acid, the blue litmus becomes red colour. So, since SO2 and SO3 are acidic, so the moist blue litmus becomes red.