What is metal activity series?

Name the gas produced when:

(a) steam is passed over hot coke.

(b) chlorine is dissolved in water and exposed to sunlight.

(c) a piece of calcium is added to water.

(d) when fossil fuel is burnt.

Name the compounds responsible for:

(i) temporary hardness

(ii) permanent hardness of water.

Suggest one method for the removal along with the reactions for

(i) temporary hardness

(ii) permanent hardness of water