Analysis of Group 1,2 cations

In the qualitative analysis of group 3 cations blood red colouration is a test for

H_(2)S is passed through the solution in an acididc medium to precipitate the sulphides of group II cations, but in an alkaline medium to precipitate the sulphides of group IV cations because:

The role of NH_(4)Cl in the precipitation of the hydroxides of group III cations is to: