How can $D_2O$ be prepared from water? Mention the physical properties in which $D_2O$ differs from $H_2O$. Give at least three reactions of $D_2O$ showing the exchange of hydrogen with deuterium.

(N/A) $D_2O$ (heavy water) is prepared by the prolonged electrolysis of water,as $H_2O$ is electrolyzed faster than $D_2O$.
Physical properties in which $D_2O$ differs from $H_2O$:
$(i)$ Density: The maximum density of $D_2O$ is $1.1073 \ g \ mL^{-1}$ at $11.6^{\circ}C$,whereas for $H_2O$ it is $1.0000 \ g \ mL^{-1}$ at $4^{\circ}C$.
$(ii)$ Viscosity: $D_2O$ has higher viscosity than $H_2O$.
$(iii)$ Solubility: Salts are generally less soluble in $D_2O$ compared to $H_2O$.
Exchange reactions of hydrogen with deuterium:
$NaOH + D_2O \longrightarrow NaOD + HOD$
$HCl + D_2O \longrightarrow DCl + HOD$
$NH_4Cl + 4D_2O \longrightarrow ND_4Cl + 4HOD$