Represent the chemical change that occurs on reacting copper sulphate and an iron nail.

Complete the crossword puzzle with the help of given clues.
Across
1. A substance formed by mixing metals.
4. A metal that burns with a dazzling white flame.
6. A change in which new substance is formed.
7. A gas that produces a 'pop' sound.
Down
2. A chemical compound represented by the formula $\mathrm{CaO}$.
3. A reddish-brown substance formed on iron articles on exposure to moist air.
5. A yellow colour nonmetal which reacts with iron to form its sulphide.