Question

Are the physical properties of \(\mathrm{H}_{2} \mathrm{O}\) different from \(\mathrm{D}_{2} \mathrm{O}\) ? Explain.

Short answer

Yes, the physical properties of \(\mathrm{H}_{2}\mathrm{O}\) (water) and \(\mathrm{D}_{2}\mathrm{O}\) (heavy water) are different due to the difference in mass and strength of hydrogen bonding between their molecules. Heavy water has a higher boiling point (\(101.4^{\circ}\)C) and melting point (\(3.8^{\circ}\)C), a higher density (1.105 g/mL at 4°C), and a higher viscosity (1.25 cP at 20°C) compared to water.

Step-by-step solution

Understand the molecular differences between \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{D}_{2}\mathrm{O}\)

Water consists of two hydrogen atoms (having one proton and one electron each) and one oxygen atom, while heavy water has two deuterium atoms (an isotope of hydrogen with one proton, one neutron, and one electron each) and one oxygen atom. The key component that sets the two molecules apart is the presence of the additional neutron in the deuterium atom, which increases the mass of the heavy water molecule compared to that of the water molecule.

Compare the boiling and melting points

Due to the extra mass of deuterium compared to hydrogen, \(\mathrm{D}_{2}\mathrm{O}\) has stronger hydrogen bonding between its molecules. This results in a higher boiling point and melting point for heavy water compared to water. For \(\mathrm{H}_{2}\mathrm{O}\), the boiling point is \(100^{\circ}\)C (212°F) and the melting point is \(0^{\circ}\)C (32°F). In contrast, the boiling point of \(\mathrm{D}_{2}\mathrm{O}\) is \(101.4^{\circ}\)C (214.5°F) and the melting point is \(3.8^{\circ}\)C (38.8°F).

Compare the densities

The additional mass of the deuterium atoms in heavy water also affects its density. The density of \(\mathrm{H}_{2}\mathrm{O}\) at 4°C is 1.000 g/mL, while the density of \(\mathrm{D}_{2}\mathrm{O}\) at the same temperature is approximately 1.105 g/mL. This means that heavy water is about 10.5% heavier than water.

Compare the viscosities

The viscosity of a liquid is a measure of its resistance to flow. Due to the increased mass and stronger intermolecular forces in heavy water, its viscosity is higher than that of water. At 20°C, the viscosity of \(\mathrm{H}_{2}\mathrm{O}\) is about 1 centipoise (cP), while the viscosity of \(\mathrm{D}_{2}\mathrm{O}\) is about 1.25 cP.

Conclusion

To conclude, the physical properties of \(\mathrm{H}_{2}\mathrm{O}\) (water) and \(\mathrm{D}_{2}\mathrm{O}\) (heavy water) are indeed different due to the difference in mass and strength of hydrogen bonding between their molecules. Heavy water has higher boiling and melting points, a higher density, and a higher viscosity compared to water.

Key concepts

Molecular Differences Between H2O and D2O

Understanding the molecular differences between H2O (water) and D2O (heavy water) is essential when exploring their physical properties. At its most basic level, the distinction lies in the types of hydrogen atoms present in each molecule. Ordinary water molecules are composed of two hydrogen atoms, which have one proton and one electron each, and one oxygen atom. In contrast, heavy water molecules contain two deuterium atoms (an isotope of hydrogen) along with one oxygen atom. Deuterium atoms are unique as they consist of one proton, one neutron, and one electron each, effectively doubling the mass of the hydrogen component of the molecule.

The presence of the extra neutron in deuterium contributes to the various physical differences we observe between H2O and D2O. This additional mass has profound effects on the properties such as boiling and melting points, density, and viscosity, which we will examine in the following sections.

Boiling and Melting Points Comparison

The difference in mass between the hydrogen and deuterium atoms influences the intermolecular forces that act within the substance. Specifically, the heavier deuterium atoms in D2O lead to stronger hydrogen bonding compared to H2O. As a result, more energy is required to overcome these forces, resulting in higher boiling and melting points for heavy water.

For typical H2O, the boiling point is set at 100°C (212°F), and the melting point is at 0°C (32°F), familiar benchmarks we are accustomed to in everyday life. In contrast, D2O exhibits a boiling point of 101.4°C (214.5°F) and a melting point of 3.8°C (38.8°F). While these differences may seem slight, they are significant in scientific contexts and have implications for industrial processes where precision in physical properties is paramount.

Density and Viscosity of Water and Heavy Water

The increased mass of deuterium not only affects the boiling and melting points of D2O but also impacts two other important physical properties: density and viscosity. At 4°C, the typical density of water (H2O) is around 1.000 g/mL. On the other hand, heavy water (D2O) has a density of approximately 1.105 g/mL at the same temperature, meaning that it is about 10.5% denser than ordinary water. This difference in density can even be demonstrated by the fact that ice made from heavy water will sink in regular water, an intriguing phenomenon that goes against the common experience of ice cubes floating in a glass.

As for viscosity, which is essentially a measure of a fluid's resistance to flow, heavy water is again distinct from ordinary water. At 20°C, the viscosity of H2O is about 1 centipoise (cP), indicative of its relatively free-flowing nature. D2O exhibits a higher viscosity of around 1.25 cP, meaning that it flows more sluggishly due to the greater mass of its molecules and the resultant stronger intermolecular forces. This has important implications for industrial and research applications where the precise flow of liquids is critical.

Hydrogen Bonding in Water Molecules

Hydrogen bonding plays a central role in determining the physical properties of water and heavy water. These bonds are a type of attractive interaction between a hydrogen atom which is covalently bonded to a more electronegative atom, such as oxygen, and another electronegative atom in a different molecule. In the case of H2O, hydrogen bonds form between the hydrogen atoms of one water molecule and the oxygen atom of another, creating a network of dynamically forming and breaking connections.

Due to the slight positive charge on the hydrogen atoms and the slight negative charge on the oxygen atoms, these bonds are relatively strong compared to other types of intermolecular forces, thus endowing water with its unique properties, such as its high surface tension and capillary action. In D2O, the extra neutron in the deuterium atoms leads to an even stronger hydrogen bonding interaction, given the increased mass and the resulting higher polarity. This accounts for the elevated boiling and melting points of heavy water and also its greater density and viscosity compared to regular water. Understanding these differences is fundamental in fields ranging from chemistry and physics to environmental science and engineering.