Q1: The latent heat of vaporisation

ANS:D - all (a), (b) & (c).

Certainly! Let's go through each option: a) The latent heat of vaporization decreases with increased temperature: This statement is generally true for most substances. As the temperature of a substance increases, the intermolecular forces holding the liquid molecules together weaken, making it easier for the molecules to escape into the vapor phase. Therefore, less energy is required to overcome these intermolecular forces, resulting in a decrease in the latent heat of vaporization. b) The latent heat of vaporization decreases as pressure increases: This statement is also generally true. Increasing pressure tends to compress the gas phase and condense it into the liquid phase. Consequently, the molecules are brought closer together, and the intermolecular forces become stronger, making it more difficult for the molecules to escape into the vapor phase. Therefore, more energy is required to overcome these stronger intermolecular forces, resulting in an increase in the latent heat of vaporization. c) The latent heat of vaporization becomes zero at the critical point: This statement is specific to the behavior of substances at their critical point. The critical point is the temperature and pressure at which the distinction between the liquid and gas phases disappears. At this point, the density of the liquid and gas phases becomes identical, and the latent heat of vaporization becomes zero. This is because there is no longer any energy required to convert the substance from the liquid phase to the gas phase, as the substance exists as a single phase with properties intermediate between those of a liquid and a gas. So, the correct interpretation is that: all (a), (b), and (c) statements are generally true, with (c) being specifically applicable to the behavior of substances at their critical point.