The following data present the temperatures at which certain vapor pressures are achieved for dichloromethane (CH2Cl2) and methyl iodide (CH3I): (c) The order of volatility of these two substances changes as the temperature is increased. What quantity must be different for the two substances for this phenom- enon to occur?

Key Concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of molecules to escape from the liquid phase into the gas phase. As temperature increases, the kinetic energy of the molecules also increases, leading to higher vapor pressures. Understanding vapor pressure is crucial for analyzing the volatility of substances.

Volatility

Volatility refers to the tendency of a substance to vaporize. A more volatile substance has a higher vapor pressure at a given temperature, meaning it evaporates more readily. The order of volatility between substances can change with temperature due to differences in intermolecular forces, such as hydrogen bonding or van der Waals forces, which affect how easily molecules can escape into the vapor phase.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. These forces, including hydrogen bonds, dipole-dipole interactions, and London dispersion forces, significantly influence the physical properties of substances, including boiling points and vapor pressures. For the order of volatility to change with temperature, the strength of these intermolecular forces must differ between the two substances, affecting their vapor pressures at elevated temperatures.