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 is the highest temperature at which a substance can exist in the liquid state, while critical pressure is the minimum pressure required to liquefy a substance at its critical temperature. According to the kinetic molecular theory, the phase of a substance depends on the balance between the kinetic energy of the particles and the attractive intermolecular forces. Above the critical temperature, the kinetic energy of the particles becomes so high that the intermolecular forces can no longer hold them together in a liquid state, 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 is the highest temperature at which a substance can exist in the liquid state. Above this temperature, the substance cannot be liquefied regardless of the pressure applied.

Define critical pressure

Critical pressure is the minimum pressure required to liquefy a substance at its critical temperature. If the pressure is lower than the critical pressure, the substance will not liquefy, even if the temperature is at or below the critical temperature.

Understand the kinetic molecular theory

The kinetic molecular theory explains the behavior of gases, liquids, and solids based on the motion of their particles. According to this theory, particles in a substance are in constant motion and have kinetic energy proportional to their temperature. The phase of a substance (gas, liquid, or solid) depends on the balance between the kinetic energy of the particles and the attractive forces between them, known as intermolecular forces.

Explain the behavior of a substance below its critical temperature

When a substance is below its critical temperature and at high enough pressure, the kinetic energy of the particles is not sufficient to overcome the intermolecular forces, allowing the particles to come closer together and form a liquid. The attractive forces between the particles are strong enough to hold them together in a more ordered, dense arrangement, despite the kinetic energy trying to spread them apart.

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

Above the critical temperature, the kinetic energy of the particles in a substance becomes so high that the intermolecular forces can no longer hold them together in a liquid state. The particles move too fast and are too energetic to be confined by the attractive forces. As a result, even if the pressure is increased, the substance cannot exist as a liquid and will remain in the gaseous phase.