What happens when dilute hydrochloric acid is added to iron filings?

An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions?

The metal used to recover copper from a solution of copper sulphate is

When we heat lead nitrate [P(NO₃)₂] in a boiling tube, we observe the emission of brown fumes. Which one of the following is the brown gas?

A liquid is heated up to a certain temperature. Which one of the following situation would correspond to the boiling of the liquid?

When air is blown from mouth into a test tube containing limewater, the limewater turns milky. This is due to the presence of