JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Hydrogen

Heavy water slows fast neutrons without absorbing many of them, which is what a reactor moderator must do, so the first claim is sound. It is recovered industrially from the liquid left after the prolonged electrolysis of water carried out in fertiliser plants, in which ${\mathrm{D}}_2\mathrm{O}$ steadily concentrates, so the second claim is sound as well. Replacing hydrogen by deuterium slows any step in which that bond is broken, so ${\mathrm{D}}_2\mathrm{O}$ and deuterated reagents are standard probes of reaction mechanism, and the third claim is sound too. The dielectric constant of ${\mathrm{D}}_2\mathrm{O}$, however, is a little lower than that of ordinary water, so the only incorrect statement is that heavy water has a higher dielectric constant than water.