The water turns green because the copper(II)sulfate is breaking apart; the green that you see is the copper. It starts attaching to the magnesium because it starts forming a complex ion with the magnesium and the sulfate; the magnesium and sulfate will bond because they form an insoluble compound, and the copper can bond onto it, forming complex ion. Solution The water turns green because the copper(II)sulfate is breaking apart; the green that you see is the copper. It starts attaching to the magnesium because it starts forming a complex ion with the magnesium and the sulfate; the magnesium and sulfate will bond because they form an insoluble compound, and the copper can bond onto it, forming complex ion..