A copper sulphate solution changes its colour because a nail dipped in the solution is more reactive than copper sulphate and it displaces the molecules. The reaction occurs when the copper gets deposited over the nail as the copper sulphate solution becomes iron sulphate solution and colour changes from blue to green.
The reactivity series is a list of metals, in order of reactivity from highest to lowest. This is used to determine single displacement reactions, whereby metal A is a part of a solution if it will replace another metal B. Metals react with each other due to the instability of their electron configurations. As they are all metals, they form positive ions when reacting with each other. Metal atoms with a greater total number of electrons tend to be more reactive as their outermost electrons exist further from the positive nucleus and therefore they are held less strongly.
Metals from potassium to calcium turn into ions when they react with water. Metals from magnesium to lead can form ions in acids. Metals from copper to platinum turn into ions only if they are forced by things like extreme heat and pressure.
If a more reactive metal is placed in the salt solution of a less reactive metal, then the more reactive will displace the less reactive metal from its salt solution. These types of reactions are known as displacement reactions.
When an iron nail is placed in copper sulphate (CuSO4) solution for some time, then the blue color of copper sulphate solution fades and a red-Brown coating of copper metal is deposited on the iron nail.
CuSO4 (Blue solution) + Fe (grey) → FeSO4 (greenish) + Cu (red−brown)