In the NH4^+ confirmatory test, only the evolved gas should touch the litmus paper. If the liquid touches, the result will be a false positive one...
...Write the relevant chemical equation and explain.
Confirmatory test for NH4+ ion involves heating an ammonium salt with aq. alkali to produce NH3. A piece of blue litmus is then held at the path of the issuing gas. Red colour of litmus is a positive test. Reactions are: NH4X + NaOH gives you NH4OH + NaX; NH4OH breaks down into NH3(gas) and H2O; This NH3(gas) shows alkaline reaction on litmus (turning blue litmus red). Now if the litmus touches the liquid which contains excess of NaOH, a strong alkali, a false positive signal is recorded.