Explain the following:

Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate.

Give two examples for the following case:

Physical changes which are reversible.

Give two examples for the following case:

Give two examples for the following case:

Give an example of a chemical reaction for the following situation:

A change in colour is observed.

Give an example of a chemical reaction for the following situation: 

Give an example of a chemical reaction for the following situation:

Sound is produced.

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust. Do you think rust is different from iron?

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.

Can you change rust back into iron by some simple method?