Who among the following discovered heavy water?

1. Foundations of Atomic Structure (basic)

Welcome to your first step in mastering Atomic and Nuclear Physics! To understand the power of the nucleus or the behavior of isotopes, we must first start with the most fundamental realization in science: matter is particulate. Everything you see—the air you breathe, the water you drink, and the screen you are reading from—is not a continuous, solid mass. Instead, it is composed of unimaginably tiny particles called atoms.

As we observe in everyday life, when you dissolve sugar in water, the sugar crystals seem to disappear. In reality, they are breaking down into their constituent particles and occupying the interparticle spaces between the water particles Science, Class VIII, Particulate Nature of Matter, p.101. This proves two critical things: first, that matter is made of tiny units, and second, that there is empty space between these units. These particles are so minuscule that they cannot be observed even with a high-powered ordinary microscope Science, Class VIII, Particulate Nature of Matter, p.101.

The behavior of these atoms determines the state of matter. In a solid, atoms are held tightly by strong attractive forces; in a liquid, they have more room to move; and in a gas, they possess enough energy to move freely in all directions Science, Class VIII, Particulate Nature of Matter, p.112. Crucially, for a UPSC aspirant, you must remember that in any chemical reaction, atoms are neither created nor destroyed; they simply rearrange themselves. This is why we balance chemical equations—to ensure the number of atoms of each element remains identical on both the reactant and product sides Science, Class X, Chemical Reactions and Equations, p.3.

Sources: Science, Class VIII, Particulate Nature of Matter, p.101; Science, Class VIII, Particulate Nature of Matter, p.112; Science, Class X, Chemical Reactions and Equations, p.3

2. Understanding Isotopes and Isobars (basic)

To understand the building blocks of our universe, we must look at the identity of an atom. Every atom is defined by its Atomic Number (Z), which is the number of protons in its nucleus. However, nature allows for some fascinating variations. Isotopes are atoms of the same element that have the same atomic number but different mass numbers. This happens because while they have the same number of protons, they harbor a different number of neutrons in their core Environment and Ecology, Majid Hussain (3rd ed.), Major Crops and Cropping Patterns in India, p.113.

For example, hydrogen usually has just one proton and no neutrons (Protium). But it can also exist as Deuterium (one proton, one neutron) or Tritium (one proton, two neutrons). Because they have the same number of electrons, isotopes share almost identical chemical properties. However, their physical properties — like density or boiling point — differ. This principle is vital in fields like radioactive isotopic dating, which scientists use to determine the absolute age of rock strata and fossils Environment and Ecology, Majid Hussain (3rd ed.), Major Crops and Cropping Patterns in India, p.111.

On the flip side, we have Isobars. These are atoms of different elements (different atomic numbers) that happen to have the same mass number. Imagine two different people who happen to weigh exactly the same; they are fundamentally different individuals (different elements) but share a specific physical characteristic (total mass). Because their atomic numbers are different, isobars have completely different chemical and physical properties.

Feature	Isotopes	Isobars
Atomic Number (Protons)	Same	Different
Mass Number (Protons + Neutrons)	Different	Same
Chemical Properties	Identical/Similar	Entirely Different
Example	¹²C and ¹⁴C (Carbon)	⁴⁰Ar (Argon) and ⁴⁰Ca (Calcium)

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.111; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.113

3. The Isotopes of Hydrogen (intermediate)

To understand the isotopes of hydrogen, we must start with the fundamental structure of an atom. In chemistry, an element's identity is determined by its atomic number (the number of protons). However, atoms of the same element can have different numbers of neutrons; these variations are called isotopes. Hydrogen is unique because its isotopes are so distinct that they have been given their own names.

Hydrogen was one of the first elements to form in the universe, approximately 300,000 years after the Big Bang (Physical Geography by PMF IAS, The Universe, p.2). While we usually think of hydrogen as a simple non-metal that exists as a diatomic molecule (H₂) (Science Class VIII NCERT, Nature of Matter, p.123), it actually exists in three primary isotopic forms:

1. Protium (¹H): The most common form, consisting of 1 proton and 0 neutrons. It makes up over 99.98% of all natural hydrogen.
2. Deuterium (²H or D): Known as "heavy hydrogen," it contains 1 proton and 1 neutron. Because of this extra neutron, it is roughly twice as heavy as protium.
3. Tritium (³H or T): A rare and radioactive isotope with 1 proton and 2 neutrons. It is often discussed in the context of environmental monitoring as it can be a byproduct of nuclear power plant operations (Environment Shankar IAS, Environment Issues and Health Effects, p.437).

The discovery of Deuterium in 1931 by Harold C. Urey was a landmark event. By using fractional distillation of liquid hydrogen, Urey was able to concentrate and identify this "heavy" version of the element. This discovery paved the way for the production of Heavy Water (D₂O). While chemically similar to regular water (H₂O), heavy water has different physical properties—such as a higher boiling point and greater density—due to the increased atomic mass of the deuterium atoms (Science Class X NCERT, Carbon and its Compounds, p.66).

Isotope	Symbol	Protons	Neutrons	Nature
Protium	¹H	1	0	Stable
Deuterium	²H or D	1	1	Stable (Heavy)
Tritium	³H or T	1	2	Radioactive

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; Science Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.123; Environment Shankar IAS, Environment Issues and Health Effects, p.437; Science Class X NCERT, Carbon and its Compounds, p.66

4. Nuclear Reactors: Moderators and Coolants (intermediate)

In a nuclear reactor, the goal is to maintain a controlled chain reaction. When a Uranium-235 nucleus undergoes fission, it releases 2 to 3 fast neutrons. However, these neutrons are moving too quickly to be easily captured by other U-235 nuclei to keep the reaction going. To solve this, we use a moderator. The moderator is a material—like graphite or heavy water (D₂O)—that slows down these fast neutrons to 'thermal' speeds through collisions, without absorbing them. This process is essential because 'slow' neutrons have a much higher probability of causing further fission. The discovery of deuterium (heavy hydrogen), the crucial isotope that forms heavy water, was achieved by Harold C. Urey in 1931, a feat that fundamentally enabled the development of heavy-water moderated reactors. While the moderator manages the speed of the reaction, the coolant manages the heat. Fission produces an immense amount of thermal energy. If this heat isn't removed, the reactor core would melt down. The coolant circulates through the core, absorbs this heat, and carries it away to a heat exchanger where it typically boils water to create steam for electricity-generating turbines. While ordinary water (H₂O) is the most common coolant, its composition of hydrogen and oxygen Science, Class VIII NCERT (Revised ed 2025), Nature of Matter, p.123 means it can also act as a moderator. Other reactors might use liquid sodium or gases like carbon dioxide as coolants, depending on the specific design and the type of fuel used, such as Uranium-235 or Plutonium-239 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83. Understanding the distinction between these two is vital for reactor safety. A moderator ensures the physics of the reaction works efficiently, while the coolant ensures the engineering of the heat stays within safe limits. In many modern reactors, like the Pressurized Water Reactor (PWR), ordinary water serves both roles simultaneously. However, in specialized designs like the CANDU reactor, heavy water is preferred as a moderator because it is exceptionally efficient at slowing neutrons while absorbing very few of them, allowing the use of natural (unenriched) uranium as fuel.

Sources: Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.123; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.83

5. India’s Nuclear Program and Heavy Water (exam-level)

To understand India’s nuclear journey, we must first look at a unique molecule: Heavy Water (D₂O). While normal water consists of two hydrogen atoms and one oxygen atom, heavy water replaces those hydrogen atoms with deuterium, a heavier isotope of hydrogen discovered by Harold C. Urey in 1931. Urey’s work, which earned him the Nobel Prize, proved that by fractionally distilling liquid hydrogen, one could isolate this heavier isotope. This discovery was the bedrock for the heavy water reactors that would later define India’s nuclear strategy. In a nuclear reactor, heavy water serves two critical functions: as a coolant to carry away heat and, more importantly, as a moderator. A moderator slows down fast-moving neutrons produced during fission so they can effectively split other nuclei, sustaining a chain reaction. India’s choice of Pressurized Heavy Water Reactors (PHWRs) was strategic. Unlike light water reactors that require enriched uranium (which is difficult to produce or import), PHWRs can use natural uranium as fuel. This focus on self-reliance began with the establishment of the Atomic Energy Commission in 1948 and the Bhabha Atomic Research Centre (BARC) in 1967 INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.61. India’s nuclear map expanded rapidly with the commissioning of the first plant at Tarapur in 1969, followed by Rawatbhata in Rajasthan Geography of India, Energy Resources, p.27. However, the program hit a major geopolitical hurdle in 1974 following the 'Smiling Buddha' peaceful nuclear explosion. Because the test utilized plutonium from the CIRUS reactor (supplied by Canada) and heavy water (supplied by the US), international backlash was severe. This event led to the formation of the Nuclear Suppliers Group (NSG) to restrict the trade of nuclear technology and materials, forcing India to develop its own indigenous heavy water production capabilities A Brief History of Modern India, After Nehru..., p.703.

Sources: INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.61; Geography of India, Energy Resources, p.27; A Brief History of Modern India, After Nehru..., p.703

6. Discovery and Chemistry of Heavy Water (D₂O) (exam-level)

To understand Heavy Water (D₂O), we must first look at the fundamental nature of hydrogen. Most hydrogen atoms in the universe consist of a single proton and an electron. However, there exists a rare isotope called Deuterium, which contains both a proton and a neutron in its nucleus. When two atoms of deuterium combine with one atom of oxygen, they form Deuterium Oxide, popularly known as heavy water. While ordinary water (H₂O) and heavy water (D₂O) are both chemical compounds where elements are held together so tightly that they cannot be separated by simple physical means Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.124, the presence of that extra neutron significantly alters the physical behavior of the substance.

The discovery of this heavy isotope was a watershed moment in atomic physics. In 1931, the American chemist Harold C. Urey proved the existence of deuterium. He achieved this by performing the fractional distillation of liquid hydrogen, carefully evaporating the liquid to find the heavier isotopes concentrated in the residue. Urey’s meticulous work earned him the Nobel Prize in Chemistry in 1934. His discovery was not just a laboratory curiosity; it provided the scientific community with a tool to explore nuclear reactions, as heavy water possesses the unique ability to moderate (slow down) neutrons in a nuclear reactor without absorbing them excessively.

Chemically, D₂O behaves very similarly to H₂O because they share the same electron configuration. However, their physical constants differ due to the increased mass. For instance, heavy water is approximately 11% denser than normal water, and it has slightly higher freezing and boiling points. Because it is a compound, it undergoes chemical changes just like regular water, involving reactions that form new substances Science-Class VII, Changes Around Us: Physical and Chemical, p.68, but the "heavy" nature of the hydrogen atoms makes it an invaluable asset in high-tech industrial and scientific applications.

Property	Ordinary Water (H₂O)	Heavy Water (D₂O)
Hydrogen Isotope	Protium (0 neutrons)	Deuterium (1 neutron)
Density	1.000 g/mL	1.106 g/mL
Boiling Point	100°C	101.4°C

Sources: Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.124; Science-Class VII, Changes Around Us: Physical and Chemical, p.68

7. Solving the Original PYQ (exam-level)

Now that you have mastered the concept of isotopes and the molecular structure of deuterium oxide (D2O), this question tests your ability to link those scientific fundamentals to the history of chemical discovery. You have learned that heavy water differs from ordinary water because it contains deuterium—a heavier isotope of hydrogen that includes a neutron in its nucleus. The identification of this specific isotope was the pivotal breakthrough required to isolate and define heavy water itself, moving the concept from theoretical physics into experimental chemistry.

To arrive at the correct answer, remember that the Nobel Prize in Chemistry (1934) was awarded specifically for the discovery of heavy hydrogen. In 1931, H. C. Urey used fractional distillation of liquid hydrogen to prove its existence, as detailed in Deuterium: Preparation, Properties, Environmental Distribution, and Biological Effects. Since heavy water is simply water where the hydrogen atoms are replaced by deuterium, H. C. Urey is the definitive figure associated with its discovery. When you see "Heavy Water" in a UPSC paper, your mind should immediately trigger the logical chain: Heavy Water → Deuterium → Urey.

UPSC often includes "distractor" names from different scientific domains to test the precision of your memory and prevent lucky guessing. For instance, Heinrich Hertz is synonymous with electromagnetic waves and frequency; G. Mendel is the "Father of Genetics" known for his pea plant experiments; and Joseph Priestly is famously credited with the discovery of oxygen. By recognizing these names as pioneers in physics, biology, and basic atmospheric chemistry respectively, you can use the process of elimination to confidently select H. C. Urey as the only scientist whose work aligns with isotopic research.