點解會no reaction?

其實有反應的, 只不過憑肉眼好難睇得到.....
當Copper(II) chloride react with Magnesium sulphate, Copper(II) sulphate and magnesium chloride are formed, since both of them are soluble in water, so no observable change is detected:

CuCl2 + MgSO4 <--> CuSO4 + MgCl2

Anyway, The reaction will be reversible when two product are all soluble in water,
for example, sodium chloride and potassium nitrate will react to form sodium nitrate and potassium chloride, which are all soluble in water.

When one of the product is insoluble in water, the reaction will flows on only one direction. For example, silver nitrate and sodium chloride will react to form silver chloride and sodium nitrate, since silver chloride is insoluble in water, the reaction can not go back anymore, hence, the reaction is:
AgNO3 + NaCl --> AgCl(s) + NaNO3

你所說的反應不會發生.
兩種化合物的反應是為復分解反應(中和反應是其一),但其中有很多條件才會發生:反應之後有沉澱,或反應之後有氣體生成,或反應之後生成水.復分解反應絕對要以上三個條件其一成立才行.

假設你所說的反應式成立,生成的是MgCl2跟CuSO4.硫酸銅我們很清楚那是可溶的,氯化鎂也是可溶的.反應以後沒沉澱沒氣體沒水生成,反應則無法發生.
中和反應是常見的復分解反應:NaOH+HCl→NaCl+H2O
又例如這個:BaCl2+Na2SO4→BaSO4↓+2NaCl