Following are the importance of heavy water

(i) It is extensively used as a moderator in nuclear reactors.

(ii) It is used as a tracer compound in the study of reaction mechanism.

(iii) It is used for the preparation of other deuterium compounds such as $\mathrm{CD}_4, \mathrm{D}_2 \mathrm{SO}_4$, etc.

The Lewis structure of hydrogen peroxide is

Following are the chemical equation of $\mathrm{H}_2 \mathrm{O}_2$ in which it behaves as an oxidising as well as reducing agent

(i) $\mathrm{H}_2 \mathrm{O}_2$ oxidises acidified KI to iodine.

$$2 \mathrm{KI}+\mathrm{H}_2 \mathrm{O}_2+\mathrm{H}_2 \mathrm{SO}_4 \longrightarrow \mathrm{I}_2+\mathrm{K}_2 \mathrm{SO}_4+2 \mathrm{H}_2 \mathrm{O}$$

(ii) $\mathrm{H}_2 \mathrm{O}_2$ reduces $\mathrm{KMnO}_4$ to $\mathrm{MnO}_2$ in alkaline medium.

$$2 \mathrm{KMnO}_4+3 \mathrm{H}_2 \mathrm{O}_2 \longrightarrow 2 \mathrm{MnO}_2+2 \mathrm{KOH}+3 \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O}$$

The bleaching action of hydrogen peroxide is due to the nascent oxygen which it liberates on decomposition.

$$\mathrm{H}_2 \mathrm{O}_2 \longrightarrow \mathrm{H}_2 \mathrm{O}+[\mathrm{O}]$$

The nascent oxygen combines with colouring matter, in turn, gets oxidised. Thus, the bleaching action of $\mathrm{H}_2 \mathrm{O}_2$ is due to the oxidation of colouring matter by nascent oxygen. It is used for the bleaching of delicate materials like ivory, feathers, silk, wool etc.

Colouring matter $+[\mathrm{O}] \longrightarrow$ Colourless matter

Oxygen is more electronegative $(\mathrm{EN}=3.5)$ than hydrogen $(\mathrm{EN}=2.1)$ hence, $\mathrm{O}-\mathrm{H}$ bond is polar. In the water molecule, two polar $\mathrm{O}-\mathrm{H}$ bonds are present which are held together at an angle of $104.5^{\circ}$. Due to the resultant of these two dipoles, water molecule is polar and has an dipole moment of 1.84 Debye.