Question

What happens when magnesium reacts with copper sulphate solution

Answer 1

I presume that, by "the liquid", you mean copper sulphate solution. 
If so, the liquid will slowly turn from blue to colourless: you'll finish up with a solution of magnesium sulphate and the copper will precipitate out as a brown powder. 

Chemically, what will have happened is that magnesium, being more electropositive than copper, will displace the copper. Another way of putting it is that the magnesium donates two electrons to the copper ions: 
Mg(s) + Cu++(aq) → Mg++(aq) + Cu(s)

Answer 2

When magnesium is placed in copper sulfate solution a redox reaction occurs, causing copper metal to form on the magnesium and the deep blue colour of the solution pales.

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