Let's analyze each statement about heavy water (D₂O) to determine which ones are correct. Heavy water is water in which the hydrogen atoms are replaced by deuterium, an isotope of hydrogen.

Analyzing Statements on Heavy Water Properties

We will examine each statement individually:

1. Statement 1: It is extensively used as a moderator in nuclear reactors.

   A moderator in a nuclear reactor is a substance used to slow down fast neutrons, making them thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235. Heavy water is an excellent moderator because it has a low absorption cross-section for neutrons, meaning it slows neutrons down efficiently without absorbing many of them. This allows natural uranium (which has a low concentration of fissile U-235) to be used as fuel. Reactors like the CANDU reactors in Canada extensively use heavy water as a moderator.

   Therefore, this statement is correct.

2. Statement 2: It cannot be used in an exchange reaction to study the reaction mechanism.

   Deuterium atoms in heavy water can readily exchange with hydrogen atoms in other molecules that have labile (exchangeable) hydrogens, such as those in -OH, -NH₂, or -COOH groups. This property is widely used in chemistry and biochemistry to study reaction mechanisms, track the fate of hydrogen atoms, or label molecules for spectroscopic analysis (like NMR). For example, adding D₂O to an organic molecule containing an -OH group will result in the exchange of the hydrogen on the oxygen with a deuterium from D₂O.

   \( \text{R-OH} + \text{D}_2\text{O} \rightleftharpoons \text{R-OD} + \text{HDO} \)

   This exchange capability makes heavy water valuable for studying reaction mechanisms, not unsuitable.

   Therefore, this statement is incorrect.

3. Statement 3: The viscosity of heavy water is relatively smaller than of ordinary water.

   Heavy water has slightly stronger hydrogen bonds compared to ordinary water (H₂O). This is because deuterium has a smaller zero-point energy than protium, leading to slightly stronger and shorter hydrogen bonds in D₂O. Stronger intermolecular forces generally lead to higher viscosity. At most temperatures, the viscosity of heavy water is greater than that of ordinary water.

   For example, at 25°C, the viscosity of H₂O is approximately 0.89 cP, while the viscosity of D₂O is approximately 1.10 cP.

   Therefore, this statement is incorrect.

4. Statement 4: The dielectric constant of heavy water is smaller than that of ordinary water.

   The dielectric constant (\(\epsilon\)) is a measure of a substance's ability to reduce the electric field strength when it is placed in an electric field. It relates to the polarity of the molecules and their ability to orient themselves in the field. While both heavy water and ordinary water are highly polar, the dielectric constant of heavy water is slightly lower than that of ordinary water at comparable temperatures. For example, at 25°C, the dielectric constant of H₂O is about 78.3, while that of D₂O is about 78.0.

   Therefore, this statement is correct.

Based on the analysis:

• Statement 1 is correct.
• Statement 2 is incorrect.
• Statement 3 is incorrect.
• Statement 4 is correct.

The correct statements are 1 and 4.

Revision Table: Heavy Water Properties vs. Ordinary Water

Additional Information on Heavy Water

Heavy water, or deuterium oxide (D₂O), differs from ordinary water (H₂O) due to the presence of deuterium (\(^2\text{H}\) or D), an isotope of hydrogen with one proton and one neutron, instead of protium (\(^1\text{H}\) or H), which has only a proton. This mass difference leads to subtle but significant differences in physical and chemical properties between H₂O and D₂O.

• Isotopic Effects: The differences in properties are primarily due to isotopic effects, which are variations in reaction rates or physical properties caused by the difference in mass between isotopes. Deuterium forms slightly stronger bonds than protium.
• Nuclear Applications: Heavy water is crucial in certain types of nuclear reactors as a moderator. It is also used as a coolant. Its low neutron absorption allows reactors to operate using natural uranium fuel, which is less enriched than the fuel needed for light water reactors.
• Scientific Research: Besides studying reaction mechanisms via exchange reactions, heavy water is used in Nuclear Magnetic Resonance (NMR) spectroscopy as a solvent when studying proton-containing compounds, as deuterium signals do not interfere with proton signals. It's also used in infrared (IR) spectroscopy to study O-H bond vibrations.
• Biological Effects: While consuming small amounts of heavy water is harmless, high concentrations can be toxic to eukaryotic cells because it interferes with various metabolic processes and cellular functions where hydrogen bonding and kinetic isotopic effects play a role.