Question

An iron knife dipped in a blue copper sulphate solution turns the blue solution light green why ?

Answer 1

When an iron knife is dipped is copper sulphate solution, after some time we'll see it has turned light green (from blue) in colour. this is because as iron is more active than copper [see activity series of metals] it displaces copper from the solution and forms iron sulphate which is light green in colour.
You will also see copper deposited on the dipped portion of the iron knife.

Answer 2

Answer:

iron is more reactive than copper