The maximum number of hydrogen bonds formed by a water molecule in ice is

Identify the reactions in which $H_2O$ is oxidised:
$I$. Photosynthesis
$II$. Reaction of $H_2O$ with fluorine
$III$. Reaction of $H_2O$ with hydrogen
$IV$. Reaction of $H_2O$ with $P_4O_{10}$
The correct answer is

The hardness-producing salt whose solubility in water decreases with a rise in temperature is

Permanent hardness due to $Mg^{2+}$ ions is best removed by

What is "Calgon"? Give its molecular formula.

Some statements about heavy water are given below:
$(i)$ Heavy water is used as a moderator in nuclear reactors.
$(ii)$ Heavy water is more associated than ordinary water.
$(iii)$ Heavy water is more effective solvent than ordinary water.
Which of the above statements are correct?