Heavy Water - Uses, Properties and Reactions

What is Heavy Water?

Heavy water is a compound that is made up of oxygen and deuterium, a heavier isotope of hydrogen which is denoted by ‘2H’ or ‘D’. Heavy Water is also called deuterium oxide and is denoted by the chemical formula D2O.

Table of Content

It has a greater molar mass than regular water since the atomic mass of deuterium is greater than that of protium. This causes heavy water (D2O) to have slightly different chemical and physical properties when compared to H2O.

Properties of Heavy Water

Heavy Water (Deuterium Oxide) D2O
Molecular Mass 20.02 grams/mole
Density 1.107 g/mL
Dipole moment 1.87 D
Melting Point 3.82oC
Boiling Point 101.4oC

Physical Properties

  • Heavy water has a colorless appearance at STP.
  • At room temperatures, it exists as an odorless liquid.
  • Since the density of D2O is approximately 11% greater than that of H2O, an ice cube made of deuterium oxide will sink in normal water.
  • Heavy water forms a homogeneous mixture when mixed with normal water.

Chemical Properties

  • The isotopes of hydrogen exhibit different chemical behavior, owing to the differences in their atomic masses.
  • The changes to the solvent properties of water due to the presence of high quantities of deuterium can adversely affect biological systems.
  • Normal water dissociates to a further extent when compared to heavy water.
  • At a given temperature, the concentration of D+ ions in a D2O sample is generally lower than the concentration of H+ ions in an H2O sample.

⇒ Also Read: Physical and Chemical Properties of Water

Methods of Preparation

Heavy water is prepared by the prolonged electrolysis of water containing alkali.

  • Electrolyte: Water containing NaOH.
  • Cathode: Steel vessel.
  • Anode: Nickel sheet with holes.

Electrolysis is continued for seven alkalies are neutralized by the passage of CO2 gas. The leftover product is heavy water. Some important features of heavy water are listed below.

  • The nucleus of a deuterium atom holds one proton and one neutron, making it roughly twice as heavy as protium (the isotope of hydrogen present in normal water, denoted by 1H).
  • Approximately 89% of the molecular mass of a water molecule is attributed to oxygen. Therefore, the molecular mass of D2O is not substantially greater than that of H2
  • The density of heavy water is ~11% higher than that of normal water.
  • The deuterium-oxygen bond in D2O is stronger than the protium-oxygen bond in H2
  • D2O is not radioactive in nature because the deuterium atom does not undergo radioactive decay.
  • High doses of heavy water can prove toxic to many organisms.

The first production of deuterium oxide was in the year 1932. It is used as a neutron moderator and coolant in pressurized heavy water reactors (PHWR). The term ‘heavy water’ typically refers to D2O. However, other “heavy” forms of water exist as well.

⇒ Check: Hardness of Water – Types and Treatment

Types of Heavy Water

Semi-Heavy Water (HDO)

  • This form of heavy water is denoted by the formula HDO and is made up of one protium, one deuterium, and one oxygen atom.
  • Water molecules tend to exchange hydrogen atoms between each other. This implies that HDO can be found in samples of water containing both protium and deuterium.
  • A water sample containing an equal ratio of protium and deuterium consists of 50% semiheavy water, 25% normal water, and 25% heavy water.
  • In this sample, there exists a dynamic equilibrium between HDO, D2O, and H2

Heavy-Oxygen Water

  • Water containing heavier isotopes of water such as 17O and 18O is referred to as heavy-oxygen water.
  • Its density is higher than regular water, due to which it is categorized as a form of heavy water.
  • Heavy-oxygen water containing the 18O isotope of oxygen is used in the production of the 18F isotope of fluorine. It is also used in radiotracers and radiopharmaceuticals.

Tritiated Water (T2O)

  • It is a radioactive form of water that contains tritium (denoted by T or 3H) instead of protium.
  • It is commonly called super-heavy water and is denoted by the formula T2
  • Tritiated water can be used to determine the total volume of water in a body.
  • The molar mass of T2O is 22.03 grams per mole and its density is 1.85 g/mL.

Some Important Reactions of Heavy Water

  • Electrolysis: 2O2O → 2O2 (Deuterium)+ O2
  • Reactions with Metals: 2Na + 2D2O → 2NaOD(Sodium Deuteroxide) + D2
  • Reactions with Non Metals: D2O + Cl2 → DCl + DOCl
  • Reactions with Metal Oxide: MgO + D2O → Mg(OD)2
  • Reactions with Non-Metal Oxides: SO3 + D2O → D2SO4 (Deutero sulphuric acid)
  • Reactions with Magnesium Nitride: Mg3N2 + 6D2O → 3Mg(OD)2 + 2ND3 (Deutero Ammonia)
  • Reactions with Calcium Phosphide: Ca3P2+6D2O → 3Ca(OD)2+2PD3 (Deutero Phosphide)
  • Reactions with Calcium Carbide: CaC2 + 2D2O → Ca(OD)2 + C2D2 (Deutero Acetylene)
  • Reactions with Aluminium Carbide: Al4C3 + 12D2O → 4Al(OD)3 + 3CD4 (Deutero methane)

Uses of Heavy Water

As mentioned earlier, deuterium oxide is an integral part of heavy water nuclear reactors, where it is used as a coolant and as a neutron moderator. Some other important applications of heavy water are listed below.

  • Heavy Water is used for the preparation of Deuterium.
  • As a tracer to study the mechanism of respiration, photosynthesis.
  • D2O is used in NMR (nuclear magnetic resonance) spectroscopy, which is used to observe the magnetic fields around the nuclei of atoms.
  • Isotopologues of many organic compounds are prepared with the help of deuterium oxide.
  • Heavy water is often used instead of normal water in IR (infrared) spectroscopy.
  • As a moderator in the nuclear reactor to slow down the neutrons. Since it can slow down the fast-moving neutrons so that they can react with the 235U isotope instead of the 238U isotope.
  • The metabolic rate in humans and animals is tested with the help of a mixture of D2O and heavy-oxygen water.
  • Tritium, the active substance used in controlled nuclear fusion reactions, is formed when the deuterium present in heavy water captures a neutron.

Thus, the physical and chemical properties of heavy water are briefly discussed along with a few uses of this compound. To learn more about deuterium oxide and other important compounds used in nuclear reactors, register with BYJU’S and download the mobile application on your smartphone.

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