Displacement reaction. Image 1 of 2. Hand dipping strips of magnesium (left) and copper (right) into a solution of the other's sulphate salt. This is being done to show the relative reactivity of the two metals. The more reactive metal will displace the less reactive one, leading to a colour change. In this case, as magnesium (Mg) is more reactive than copper (Cu), the blue copper sulphate (CuSO4) solution will change to colourless magnesium sulphate (MgSO4). The reaction is: Mg + CuSO4 ---> MgSO4 + Cu For an image of the test tubes after the reaction, see image A500/343.