Question

Heavy water freezes at __________?

Answer 1

Hydrogen consists of three isotopes namely Protium $\left( {{}_1^1H} \right)$, Deuterium $\left( {{}_1^2H} \right)$ and Tritium $\left( {{}_1^3H} \right)$. The first two isotopes i.e., protium and deuterium are stable isotopes and tritium is radioactive and has a half-life of 12.32 years. Each of these isotopes can form water with oxygen. When protium forms water with oxygen it is normal water $\left( {{H_2}O} \right)$ that we use. When all the protium in the water is replaced by deuterium it forms heavy water or also known as deuterium oxide $\left( {{D_2}O} \right)$.

Complete answer:
Deuterium is one of the stable isotopes of hydrogen which contains one proton and one neutron in its nucleus. The nucleus of deuterium is called deuteron. The name deuterium is taken from the Greek word ‘deuteros’ which means ‘the second’.
When all the hydrogen atoms are replaced with deuterium it forms a different form of water known as heavy water or deuterium oxide. Its formula is ${D_2}O$.
${}^{{}^{{}^{{}^D}} \nwarrow }{O^{{ \nearrow ^{^{^D}}}}}$
The reaction is as follows
$2{H_2}_{\left( g \right)} + {O_{2\left( g \right)}} \to 2{H_2}{O_{\left( g \right)}}$(water)
$2{D_2}_{\left( g \right)} + {O_{2\left( g \right)}} \to 2{D_2}{O_{\left( g \right)}}$(heavy water).
The physical and chemical properties of heavy water are different from normal water because of the increase in mass of heavy water due to the presence of deuterium. It looks like normal water but has a high melting and boiling points and higher density. Heavy water has a lesser value of solvating ability and dielectric constant than normal water due to strong hydrogen bonding in water. The heavy water freezes at ${3.8^0}C$ rather than ${0^0}C$. The boiling point of heavy water is ${101^0}C$.

Note:
The heavy water ice sinks in normal water. This is because each heavy water molecule is slightly more massive than a regular water molecule and heavy water molecules may pack more closely than regular water molecules when they form ice. Since the heavy water is of higher mass, the reaction rate is slow compared to normal water.
Applications of ${D_2}O$:
-Heavy water acts as a neutron moderator to slow down neutrons in nuclear reactors.
-It is used as a tracer compound.
-It is used for the study of mechanisms of chemical and biochemical reactions.