Problem 20 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

Yes, the physical properties of H₂O and D₂O are different due to their differences in molecular composition, mass, and bond strength. These differences lead to variations in boiling point, melting point, density, and viscosity. D₂O has a higher boiling point (101.4°C) and melting point (-3.82°C) than H₂O (100°C and 0°C, respectively), as well as a greater density (1.107 g/mL) and slightly higher viscosity. This is due to the presence of deuterium, a heavier isotope of hydrogen, in the D-O bonds of D₂O, which makes these bonds stronger compared to the H-O bonds in H₂O.

Step 1: Understand the Molecular Composition of H₂O and D₂O

H₂O, also known as water, is composed of two hydrogen atoms (H) and one oxygen atom (O). D₂O, or heavy water, is composed of two deuterium atoms (D) and one oxygen atom (O). Deuterium is an isotope of hydrogen and has one extra neutron in its nucleus, making it heavier than hydrogen.

Step 2: Differences in Mass

Due to the extra neutron in deuterium, D₂O has a larger molecular mass compared to H₂O. This directly affects the physical properties of the two substances. The molecular mass of H₂O is 18 g/mol, while the molecular mass of D₂O is 20 g/mol.

Step 3: Differences in Bond Strength

Because of the additional mass in the deuterium nucleus, the D-O bond in D₂O is stronger than the H-O bond in H₂O. This results in differences in bond strength and vibrational frequencies between the two substances.

Step 4: Differences in Physical Properties

The differences in mass and bond strength lead to various physical property differences between H₂O and D₂O: 1. Boiling Point: D₂O has a higher boiling point (101.4°C) compared to H₂O (100°C), as more energy is required to break the stronger D-O bonds. 2. Melting Point: D₂O has a higher melting point (-3.82°C) than H₂O (0°C), for the same reason as the boiling point. 3. Density: The density of D₂O (1.107 g/mL) is greater than that of H₂O (1.000 g/mL) due to its larger molecular mass. 4. Viscosity: D₂O has a slightly higher viscosity than H₂O because of the stronger D-O bonds and greater mass. These differences in physical properties show that H₂O and D₂O are indeed different.