Animated Solution for Chemistry - s and p-Block Elements: The incorrect statement(s) about heavy water is (are) (A) used as a moderator in nuclear reactor. (B) obtained as a by-product in fertiliser industry. (C) used for the study of reaction mechanism. (D) has a higher dielectric constant than water. Choose the correct answer from the options given below :

When we think of water, we immediately picture $H_{2}O$, the life-giving liquid that covers most of our planet. But hidden within the vast oceans is a slightly heavier, more mysterious cousin: Heavy Water, or $D_{2}O$. In this problem, we are tasked with identifying the incorrect statement among a list of common facts about heavy water. Let's dive into the fascinating properties of $D_{2}O$ and see where the trap lies.

Statement A claims that heavy water is used as a moderator in nuclear reactors. Imagine a nuclear reactor as a bustling highway of subatomic particles. When uranium atoms split, they release fast-moving neutrons. If these neutrons are too fast, they won't trigger further fission reactions effectively.

We need something to slow them down—a "moderator." Heavy water is exceptionally good at this. Unlike ordinary water, which tends to absorb neutrons (forming deuterium), $D_{2}O$ already has its fill of neutrons. It simply bounces the fast neutrons around, slowing them down without eating them up. Thus, Statement A is perfectly correct.

Statement B mentions that heavy water is obtained as a by-product in the fertiliser industry. This is also true! When industries produce hydrogen gas for ammonia synthesis (a key ingredient in fertilisers), they often use the electrolysis of water. Because $H_{2}O$ electrolyzes slightly faster than $D_{2}O$, the remaining water becomes progressively enriched with heavy water.

Statement C brings us to the laboratory. Chemists love using $D_{2}O$ to study reaction mechanisms. By swapping a normal hydrogen atom with a deuterium atom (an "exchange reaction"), scientists can track exactly where that specific atom goes during a complex chemical dance. It acts like a molecular tracking device!

Finally, we arrive at Statement D, which asserts that heavy water has a higher dielectric constant than ordinary water. The dielectric constant (${ϵ}_r$) is a measure of a solvent's ability to insulate opposite charges from each other. It's the reason why salt dissolves so well in water—the water molecules surround the ions and weaken the electrostatic attraction between them.

Here is where the subtle physics comes into play. The dielectric constant of ordinary water ($H_{2}O$) is approximately $78.39$. However, the dielectric constant of heavy water ($D_{2}O$) is slightly lower, at $78.06$.

Why? Because the heavier deuterium atoms slightly alter the vibrational dynamics and the effective dipole moment of the molecule in the liquid state. Consequently, $H_{2}O$ is actually a slightly better solvent for ionic compounds than $D_{2}O$.

Therefore, Statement D is the incorrect statement, making option (c) our final answer. Always remember: while heavy water is heavier and has a higher boiling point, its dielectric constant is just a tiny bit lower than that of ordinary water!