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 Contents

• Properties
• Preparation
• Types
• Important Reactions
• Uses of Heavy Water

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

Physical Properties

• Heavy water has a colourless appearance at STP.
• At room temperature, it exists as an odourless 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 behaviour because of 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 that are neutralised 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 H_2.
• 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 H_2.
• 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 pressurised 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 with 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% semi-heavy water, 25% normal water, and 25% heavy water.
• In this sample, there exists a dynamic equilibrium between HDO, D₂O, and H_2.

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 categorised 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 T_2.
• 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)

Some other uses include,

• Heavy water is used in nuclear magnetic resonance spectroscopy.
• It is used as a moderator in certain types of nuclear reactors.
• It is used in the medical field to test the metabolic rate of human beings.

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.
• It is used as a tracer to study the mechanism of respiration and 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.
• It is used as a moderator in the nuclear reactor to slow down the neutrons. Since it can slow down the fast-moving neutrons, 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.

Can You Drink Heavy Water? Is It Dangerous?

Most of the time, when we talk about heavy water, we tend to relate it to nuclear reactors and radioactive materials. However, pure heavy water is not radioactive and not so harmful if ingested by human beings in small quantities. It is only harmful or dangerous if the water is consumed in large quantities or if heavy water is taken for a long duration of time. Poisoning may occur and lead to symptoms like dizziness and loss of blood pressure.

Here, we have learnt about the physical and chemical properties of heavy water along with its uses. To learn more about deuterium oxide and other important compounds used in nuclear reactors, register with BYJU’S and download BYJU’S – The Learning App on your smartphone.

Electrolysis in Molten State