Question

Heavy water is used for?

Answer 1

It is the water, which contains heavy hydrogen called Deuterium. This Deuterium is used in place of normal regular Hydrogen, in heavy water. Deuterium has one proton and one neutron in the nucleus around which only one electron revolves

Complete answer:
The atomic number is same for both, but the mass number for regular hydrogen is one and that for deuterium it is two. The formula of heavy water is ${D_2}O$ . Heavy water is chemically deuterium oxide .It was discovered in $1932$ by Urey. Heavy water does not occur naturally on earth. However, much smaller quantities than regular water. If we have to give a ratio of its presences, then we can say about one molecule of every twenty million water molecules is heavy water.
It is used for:
$1)$ Moderators: heavy water is used as a moderator in nuclear reactors. The work of the moderators is to slow down the speed of revolving neutrons and hence it helps in controlling nuclear fission.
$2)$ tracer compounds: to study the reaction mechanism, it helps in tracing the compounds.
$3)$ for the preparation of deuterium compounds: it is used as the starting material for preparation of a number of compounds which contain deuterium.

Note:
It is prepared by the exhaustive electrolysis of ordinary water contains very small proportion of heavy water. So when prolonged electrolysis of water is done, then ${H_2}$ is liberated much faster than ${D_2}$ and the remaining water becomes enriched in heavy water.