Question

A gas which gives dense white fumes with chlorine and its aqueous solution gives a blue colour with copper (2) hydroxide

Answer 1

Ammonia gas gives dense white fumes with chlorine due to the formation of ammonium chloride and ammonium hydroxide reacts with copper hydroxide to give tetrammine copper hydroxide which is deep blue in colour
NH3+H2O=NH4OH
Cu (OH) 2+4NH4OH= [Cu (NH3) 4] (OH) 2+4H2O