The INCORRECT statement(s) about heavy water is (are):
(A) used as a moderator in nuclear reactor
(B) obtained as a by-product in fertilizer 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:

Let us examine each statement about heavy water ($$D_2O$$) one by one.

Statement (A): Heavy water is used as a moderator in nuclear reactors. This is correct — $$D_2O$$ slows down fast neutrons effectively without absorbing them, making it an excellent moderator in reactors like CANDU.

Statement (B): Heavy water is obtained as a by-product in the fertilizer industry. This is correct — heavy water is produced as a by-product during the electrolysis of water in fertilizer plants that manufacture hydrogen for ammonia synthesis.

Statement (C): Heavy water is used for the study of reaction mechanisms. This is correct — deuterium-labeled compounds (using $$D_2O$$) are widely used as tracers to study reaction mechanisms through kinetic isotope effects.

Statement (D): Heavy water has a higher dielectric constant than water. This is incorrect — the dielectric constant of heavy water ($$D_2O$$) is about 78.06 at 25°C, which is slightly lower than that of ordinary water ($$H_2O$$), which is about 78.36 at 25°C. So heavy water does NOT have a higher dielectric constant than water.

The only incorrect statement is (D), so the answer is option (C): (D) only.