The boiling point of heavy water is (a) \(100^{\circ} \mathrm{C}\) (b) \(101.4{ }^{\circ} \mathrm{C}\) (c) \(99^{\circ} \mathrm{C}\) (d) \(110^{\circ} \mathrm{C}\)

Heavy water, chemically referred to as D2O, exhibits fascinating physical properties that distinguish it from ordinary water (H2O). Its defining characteristic is the presence of deuterium, an isotope of hydrogen with one neutron, resulting in a greater molecular mass compared to regular hydrogen atoms which have no neutrons. This difference in mass leads to several notable physical properties.

Heavy water has a higher boiling point, 101.4°C, in contrast to H2O's boiling point of 100°C. This is due to the stronger hydrogen bonds formed between deuterium atoms. Furthermore, heavy water has a higher density, about 11% greater than H2O at 11°C, which means it is heavier per unit volume. Its viscosity, or resistance to flow, is also slightly higher, and its specific heat capacity is lower, suggesting that changes in its temperature require different amounts of energy compared to regular water.

Deuterium is one of the two stable isotopes of hydrogen, symbolized as 'D' or '2H', and its nucleus consists of one proton and one neutron, giving it an atomic mass of approximately 2 u (atomic mass units). The presence of the neutron makes deuterium twice as heavy as the most common hydrogen isotope, protium, which has no neutrons.

This extra neutron significantly influences the chemical and physical behavior of the molecule it is part of. Deuterium forms chemical bonds similar to regular hydrogen but with slight variations in strength and length due to the differences in mass. These subtle changes underpin many of the unique properties of deuterated compounds such as heavy water. Deuterium is not radioactive and occurs naturally in the environment but in much smaller concentrations than protium, about 0.015% of the hydrogen found in ocean water.

While both D2O and H2O are forms of water, the substitution of deuterium atoms in place of regular hydrogen atoms results in several contrasts between the two substances. Because deuterium's bonds are slightly stronger than those of regular hydrogen, heavy water has a higher boiling and freezing point, and these altered phase transition temperatures can be experimentally verified.

In addition to thermal properties, heavy water's biological effects differ from those of H2O; however, in small quantities, D2O is not harmful to most life forms. In larger quantities, it can disrupt normal biological processes because it alters the speed of chemical reactions in a cell. These differences are critical in various scientific domains, including nuclear reactors, where heavy water is used as a neutron moderator, and in spectroscopy, where it helps researchers study different aspects of chemical compounds. Despite these differences, heavy water is colorless, taste, and odor just like regular water, making it visually indistinguishable.