Some of the major uses of heavy water are given below. Which one is not correct? (a) It is used as a moderator in nuclear reactors. (b) It is used as a tracer compound for studying reaction mechanism. (c) High concentration of heavy water accelerates the growth of plants. (d) It is used in preparing deuterium.

Heavy water, or deuterium oxide (D2O), plays a critical role in the functioning of certain nuclear reactors. Its most significant application is as a neutron moderator. Neutrons are particles within an atomic nucleus that bring about the critical chain reactions for nuclear energy production.

For a nuclear reaction to be sustained and controlled, these neutrons often need to be slowed down, or moderated. Heavy water is efficient at this because its deuterium atoms have a greater mass compared to regular hydrogen. This higher mass means that when neutrons collide with the deuterium atoms, they lose speed more effectively without being captured or absorbed, which is essential for maintaining a sustained nuclear chain reaction.

This distinctive property allows heavy water reactors to use natural uranium, which is less processed compared to the enriched uranium required by other reactors, making them a strategic choice for certain countries and applications. Furthermore, the minimal neutron absorption by D2O helps preserve the reactor's neutron economy, crucial for its long-term fuel efficiency.

A tracer compound is used in chemical research to track the progress of a reaction or to elucidate complex reaction mechanisms. Heavy water becomes an invaluable tool in this aspect due to its deuterium content. Since deuterium has a different atomic mass from hydrogen, heavy water can be detected using mass spectrometry or nuclear magnetic resonance (NMR), which makes it possible to trace the path of hydrogen atoms in chemical reactions.

When heavy water is used in experiments involving compounds that contain hydrogen, it can replace the hydrogen atoms. Researchers can then track how these atoms move and transform during a reaction, giving valuable insight into the reaction pathways and intermediate states of the molecules involved. This substitution does not typically alter the course of the reaction appreciably, allowing scientists to study the mechanism as it naturally occurs but with the added advantage of being able to 'see' the normally invisible hydrogen atoms.

Contrary to what option (c) from the exercise suggests, heavy water (D2O) does not enhance plant growth. It is actually toxic to plants, as well as most eukaryotic life forms, when present in high concentrations. This is due to the fact that deuterium atoms, which replace the hydrogen in water, form stronger chemical bonds with other atoms.

The increased bond strength alters the rates of the chemical reactions inside a living organism, because these reactions usually involve breaking and forming hydrogen bonds. In plants, such alterations can disrupt essential biological processes, including photosynthesis and respiration, ultimately leading to reduced growth rates or even death of the plant if exposed to enough heavy water. It is also worth noting that the toxicity of heavy water is dosage-dependent, with lower concentrations having negligible effects, but higher concentrations being progressively more harmful. Understanding the toxicity threshold of heavy water is important not only for plant health but also for environmental safety in areas surrounding heavy water reactors.