A ............... solution is observed after placing Magnesium metal in a solution of Copper sulphate for half an hour.

Colourless

Reason — According to the reactivity series of metals, magnesium is more reactive compared to copper. Hence, it replaces copper from CuSO₄ and the blue colour fades as colourless magnesium sulphate is formed and brown bits of copper metal form a precipitate:

Mg + CuSO₄ ⟶ MgSO₄ + Cu