Copper hydroxide precipitate. Copper (II) hydroxide precipitate (Cu(OH)2) formed by adding 0.5 M copper sulphate (CuSO4) solution to a 0.2 M solution of potassium hydroxide (KOH). The reaction is CuSO4 + KOH -> Cu(OH)2 + K2SO4. This is an example of a double replacement reaction.