Question

amonium chloride heated with an alkali and the evolved tested with red litmus paper

Answer 1

Ammonium chloride decompose like this: 

NH4Cl(s)→NH3(g)+HCl(g)NH4Cl(s)→NH3(g)+HCl(g).

It produces two gases: ammonia and hydrogen chloride. Both dissolves in water. These Two reaction

NH3(g)+H2O(l)→NH+4(aq)+OH−(aq)

HCl(g)+H2O(l)→H3O+(aq)+Cl−(aq)

This is because ammonia is a lighter gas than hydrogen chloride so it will diffuse much faster and reach the paper first.

Answer 2

It will turn red litmus blue as the gas evolved will be ammonia which is a basic gas and bases turns red litmus blue... Hope it helps you...