Displacement reaction, also known as replacement reaction, is a type of chemical reaction where a more reactive element replaces a less reactive element in a compound. In the case of an iron nail being put into a copper sulphate solution, the following displacement reaction takes place:

Fe (Iron) + CuSO₄ (Copper Sulphate) → FeSO₄ (Iron Sulphate) + Cu (Copper)

Here's what this reaction means:

1. Iron is more reactive than copper, according to the reactivity series of metals. This means that iron atoms are more likely to lose electrons and become positively charged ions (Fe²⁺) compared to copper atoms.

2. When the iron nail is immersed in the copper sulphate solution, iron atoms from the nail react with the copper sulphate compound. Iron atoms replace copper atoms in the compound, forming iron sulphate (FeSO₄) and liberating copper atoms.

3. Copper atoms, being displaced from the compound, are deposited onto the surface of the iron nail. This deposition of copper on the iron nail is visually observed as a reddish-brown coating or layer on the nail's surface.

4. As the reaction progresses, more and more iron atoms from the nail dissolve into the solution as Fe²⁺ ions, while copper atoms continue to be deposited onto the nail. This exchange of atoms results in the gradual consumption of the iron nail and the formation of copper metal.

5. The displacement reaction continues until all the iron atoms in the nail are consumed, or until the copper sulphate solution is depleted.

In summary, when an iron nail is put into a copper sulphate solution, a displacement reaction occurs where iron atoms replace copper atoms in copper sulphate, leading to the formation of iron sulphate and the deposition of copper metal onto the iron nail. This reaction highlights the relative reactivity of iron and copper, with iron being more reactive and readily displacing copper from its compound.