Heavy water $(D_2O)$ is used as:

Polyphosphates are used to soften water because they:

Oxygen is slightly denser than air,so it is collected over:

Exhausted permutit is

The process used for the removal of hardness of water is

An inorganic salt $(A)$ is decomposed on heating to give two products $(B)$ and $(C)$. Compound $(C)$ is a liquid at room temperature and is neutral to litmus,while the compound $(B)$ is a colourless neutral gas. Compounds $(A)$,$(B)$ and $(C)$ are: