Question

Define critical temperature and critical pressure. In terms of the kinetic molecular theory, why is it impossible for a substance to exist as a liquid above its critical temperature?

Short answer

Critical temperature (\(T_c\)) is the highest temperature at which a substance can be compressed into a liquid, while critical pressure (\(P_c\)) is the minimum pressure required for liquefaction at the critical temperature. The kinetic molecular theory explains the behavior of gases due to the constant, random motion of particles. When a substance's temperature exceeds its critical temperature, its particles have such high kinetic energy that intermolecular forces cannot keep them together, making it impossible for the substance to exist as a liquid, regardless of the pressure applied.

Step-by-step solution

Define critical temperature

Critical temperature (\(T_c\)) is the temperature above which a substance cannot be compressed into a liquid, regardless of the pressure applied. It represents the highest temperature at which a gas can be liquefied by increasing the pressure.

Define critical pressure

Critical pressure (\(P_c\)) is the minimum pressure required to liquefy a substance at its critical temperature. It is the pressure above which a fluid will always be a gas regardless of its temperature.

Describe the kinetic molecular theory

The kinetic molecular theory is a model used to explain the behavior of gases based on the idea that they consist of a large number of particles (atoms or molecules) in constant, random motion. These particles collide with each other and with the walls of their container. The properties of gases, such as temperature, pressure, and volume, are related to the energy of these collisions and the average kinetic energy of the particles.

Explain why a substance cannot exist as a liquid above its critical temperature

According to the kinetic molecular theory, the particles of a gas move faster with increasing temperature. When the temperature is raised above its critical temperature, the kinetic energy of the particles becomes so high that the intermolecular forces, which are responsible for keeping the particles together in a liquid state, cannot overcome this kinetic energy. This means that no amount of pressure can cause the gas to condense into a liquid. The substance will exist in the gaseous state above its critical temperature, regardless of the pressure applied.