Question

why does the blue colour of copper sulphate solution dissappear when some aluminium powder is added to it?

Answer 1

Blue colour of copper sulphate solution disappears when aluminium powder is added to it because aluminium being more reactive than copper displaces copper from its sulphate solution which leads to the formation of aluminium sulphate plus copper and the solution of aluminium sulphate is different in colour

Answer 2

Since aluminium is higher in the reactivity series, it displaces copper from the copper sulphate solution and forms aluminium sulphate and copper. Since aluminium sulphate is a different color than copper sulphate, the blue color of copper sulhpate dissapears.