Heavy Water: Do you know that water has another form? And what has it been called? Can this water have been used for drinking? Wanted to know more? Here you go. Another form of water is called heavy water, and its formula is \({{\text{D}}_2}{\text{O}}.\) It is slightly heavier than regular water, blue and not drinkable. Heavy water is also known as deuterium oxide. It is the isotope of hydrogen denoted by \({\text{D}}\) or \({{\text{H}}^2}.{{\text{H}}_2}{\text{O}}\) represents ordinary water, and \({{\text{D}}_2}{\text{O}}\) means heavy water.
In this article, we are going to learn about Deuterium oxide (or) heavy water and its types.

Heavy Water: Occurrence

If deuterium atoms replace two hydrogens of ordinary water, then it is called heavy water. Chemically heavy water is deuterium oxide, \({{\text{D}}_2}{\text{O}}{\text{.}}\) The credit for the discovery of heavy water goes to Urey. One part of heavy water is present in about \(6000\) parts of ordinary water. Lewis and Donald were able to isolate a few millilitres of pure heavy water by continuous electrolysis of water containing a tiny amount of alkali. It is found in the remains obtained by the melting of snow in the Himalayas. Minute quantities are also found on the leaves of Banyan trees and in rainwater.

Preparation of Heavy Water

Multi-stage Electrolysis of Heavy Water

When water is electrolyzed, hydrogen liberates much faster than deuterium. As a result, the bonds in \({{\text{H}}_2}{\text{O}}\) broken more readily (\(18\) times more) than the bonds in \({{\text{D}}_2}{\text{O}}.\) Thus, when the electrolysis continued until a small quantity of water is left behind, the residue is pure \({{\text{D}}_2}{\text{O}}.\) When about \(30\) litres of water electrolyzed, we get about \(1\,{\text{ml}}\) of heavy water.
Heavy water prepared by the prolonged and continuous electrolysis of \(0.5\,{\text{M}}\) sodium hydroxide solution in a steel cell. The cell itself acts as a cathode, while a cylindrical perforated sheet of nickel acts as the anode.

Exchange Reactions

Exchange reactions can also prepare heavy water, \({{\text{D}}_2}{\text{O}}.\) For example, when \({{\text{H}}_2}{\text{S}}\) gas passed through hot water, the hydrogen atoms in \({{\text{H}}_2}{\text{S}}\) exchange their places with deuterium atoms from \({{\text{D}}_2}{\text{O}}\) present in water. Thus \({{\text{H}}_2}{\text{S}}\) is enriched with \({{\text{D}}_2}{\text{S}}{\text{.}}\) On passing this \({{\text{H}}_2}{\text{S}}\) enriched with \({{\text{D}}_2}{\text{S}}\) through cold water, the deuterium from \({{\text{D}}_2}{\text{S}}\) and hydrogen from \({{\text{H}}_2}{\text{O}}\) again exchange their places. The process repeats, and the cold water enriched with \({{\text{D}}_2}{\text{O}}\) step-by-step.

Fractional Distillation

Separation of heavy water from ordinary water may be affected by the fractional distillation method. This method takes advantage of the differences in the boiling points of normal water \(\left({{{100}^ \circ }{\text{C}}} \right)\) and heavy water \(\left({{{101.42}^ \circ }{\text{C}}} \right).\) As a result, the lighter fraction distils over first, leaving behind a residue richer in heavy water \(\left({{{\text{D}}_2}{\text{O}}} \right).\)

Physical Properties of Heavy Water

Like ordinary water, heavy water is a colourless, odourless and tasteless mobile liquid. Some physical properties of water and heavy water are listed below. Since the molecular weight of heavy water \(\left({20\,{\text{g}}} \right)\) is greater than that of water \(\left({18\,{\text{g}}} \right),\) some physical constants like boiling point,zing point, specific heat, density, viscosity, the temperature of maximum density and latent heat of vaporization are higher for heavy water than that of water. However, the dielectric constant of heavy water is less than that of water. Hence, ionic compounds are less soluble in heavy water than in water.

Chemical Properties of Heavy Water

Heavy water can participate in all the chemical reactions in which water can participate. But heavy water reacts more slowly than water. The reactivity of heavy water is less than that of water due to the more dissociation energy of the \({\text{O}} – {\text{D}}\) bond than that of the \({\text{O}} – {\text{H}}\) bond.
Deuterolysis: Water brings hydrolysis of certain inorganic salts. Heavy water gives similar reactions, which are termed as salt Deuterolysis.
\({\rm{AlC}}{{\rm{l}}_3} + 3{{\rm{D}}_2}{\rm{O}} \to {\rm{Al}}{\left( {{\rm{OD}}} \right)_3} + 3{\rm{DCl}}\)
\({\rm{BaS}} + 2{{\rm{D}}_2}{\rm{O}} \to {\rm{Ba}}{\left( {{\rm{OD}}} \right)_2} + {{\rm{D}}_2}{\rm{S}}\)
Action on Metals: Reactive metals like sodium and calcium react with heavy water liberating deuterium and forming heavy alkalis.
\(2 {\text{Na}} + 2{{\text{D}}_2}{\text{O}} \to 2{\text{NaOD}} + {{\text{D}}_2}\)
Action on Metal oxides: Basic oxides like sodium monoxide and calcium oxide react with heavy water forming heavy alkalis.
\({\text{N}}{{\text{a}}_2}{\text{O}} + {{\text{D}}_2}{\text{O}} \to 2{\text{NaOD}}\)
Action on non-Metal oxides: Heavy water reacts with acidic oxides of non-metals such as Sulphur trioxide, dinitrogen pentoxide, etc., to form deutero acids.
Action of metal nitrides, phosphides, and carbides: Heavy water liberates heavy ammonia with metal nitrides.
\({\text{M}}{{\text{g}}_3}{{\text{N}}_2} + 6{{\text{D}}_2}{\text{O}} \to 3{\text{Mg}}{\left({{\text{OD}}} \right)_2} + 2{\text{N}}{{\text{D}}_3}\)
With metal phosphides, it liberates deutero phosphine.
\({\rm{C}}{{\rm{a}}_3}{{\rm{P}}_2} + 6{{\rm{D}}_2}{\rm{O}} \to 3{\rm{Ca}}{\left( {{\rm{OD}}} \right)_2} + 2{\rm{P}}{{\rm{D}}_3}\)
With metal carbides, it liberates deuteron acetylene or deuteron methane.
\({\text{Ca}}{{\text{C}}_2} + 2{{\text{D}}_2}{\text{O}} \to 3{\text{Ca}}{\left({{\text{OD}}} \right)_2} + {{\text{C}}_2}{{\text{D}}_2}\)
Formation of salt deuterates: Just as ordinary water forms Salt hydrates (Example: \({\text{CuS}}{{\text{O}}_4}.5{{\text{H}}_2}{\text{O}},\,{\text{MgS}}{{\text{O}}_4}.7{{\text{H}}_2}{\text{O}}\)) heavy water also forms salt deuterates (Example: \({\text{CuS}}{{\text{O}}_4}.5{{\text{D}}_2}{\text{O}},\,{\text{MgS}}{{\text{O}}_4}.7{{\text{D}}_2}{\text{O}}\)). These deuterates are obtained when the salt is crystallized from a solution of it in heavy water.
Exchange Reactions: Heavy water reacts with several compounds containing liable hydrogen atoms wherein \({\rm{H}}\) atoms are partially or wholly replaced by deuterium. The exchange reactions occur more readily if the compounds have ionic (i.e. Polar) hydrogen atoms.
\({\text{NaOH}} + {{\text{D}}_2}{\text{O}} \to {\text{NaOD}} + {\text{HDO}}\)
\({\text{HCl}} + {{\text{D}}_2}{\text{O}} \to {\text{DCl}} + {\text{HDO}}\)
Biological Effects: Heavy water retards the growth of plants and animals. Seeds do not germinate, and small fish die in pure heavy water. In addition, water containing a higher concentration of \({{\text{D}}_2}{\text{O}}\) is poisonous.

Uses of Heavy Water

1. Heavy water used as germicide and bactericide.
2. The exchange reaction property of heavy water used to study the structures and basicity of certain oxyacids.
3. Heavy water used to prepare deuterium either by electrolysis or by reaction with an active metal like sodium.
4. Heavy water is used as a moderator in nuclear reactors. To bring the fission of Uranium atoms, neutrons with slow speed are required. The neutrons are slowed down by passing through heavy water, which acts as a moderator.
5. Heavy water is commonly used as a tracer for studying reaction mechanisms of aromatic electrophilic substitution reactions, the study of metabolic processes, etc.

Frequently Asked Questions on Heavy Water

Q.1. Which isotope of hydrogen is present in heavy water?
Ans: The isotope of hydrogen is deuterium. Heavy water is a compound made up of deuterium and oxygen. It is a stable isotope of hydrogen with double the mass of hydrogen due to an extra neutron in its nucleus.

Q.2. What is heavy water used for?
Ans: Heavy water is used as germicide and bactericide. The exchange reaction property of heavy water is used to study the structures and basicity of certain oxyacids. Heavy water is used to prepare deuterium either by electrolysis or by reaction with an active metal like sodium. Heavy water is used as a moderator in nuclear reactors. To bring the fission of Uranium atoms, neutrons with slow speed are required. The neutrons are slowed down by passing through heavy water, which acts as a moderator. Heavy water is commonly used as a tracer for studying reaction mechanisms of aromatic electrophilic substitution reactions, the study of metabolic processes, etc.

Q.3. Describe the preparation of heavy water?
Ans: When water is electrolyzed, hydrogen is liberated much faster than deuterium. As a result, the bonds in \({{\text{H}}_2}{\text{O}}\) are broken more readily (\(18\) times more) than the bonds in \({{\text{D}}_2}{\text{O}}.\) Thus, when the electrolysis is continued until a small quantity of water is left behind, the residue is pure \({{\text{D}}_2}{\text{O}}.\) Thus, when about \(30\) litres of water is electrolyzed, we get about \(1\,{\text{ml}}\) of heavy water.

Q.4. Give one example of deuterolysis with an equation.
Ans: Water brings hydrolysis of certain inorganic salts. Heavy water gives similar reactions, which are termed salt Deuterolysis.
\({\text{AlC}}{{\text{l}}_3} + 3{{\text{D}}_2}{\text{O}} \to {\text{Al}}{\left({{\text{OD}}} \right)_3} + 3{\text{DCl}}\)
\({\text{BaS}} + 2{{\text{D}}_2}{\text{O}} \to {\text{Ba}}{\left({{\text{OD}}} \right)_2} + {{\text{D}}_2}{\text{S}}\)

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