Question

Describe, on a microscopic basis, the processes of evaporation and condensation. Which process requires the input of energy?

Short answer

Evaporation is the process of a liquid turning into a gas when molecules gain enough kinetic energy to overcome attractive forces, requiring an input of energy, usually heat. Condensation is the process where a gas turns back into a liquid when gas molecules lose kinetic energy, slow down, and their attractive forces pull them together, releasing energy in the process. Among the two processes, evaporation requires an input of energy.

Step-by-step solution

Understanding Evaporation

Evaporation is the process in which a liquid turns into a gas or vapor. On a microscopic level, this occurs when the molecules within the liquid gain enough kinetic energy to overcome the attractive forces between them. As a result, these molecules escape the liquid surface and move freely as a gas. This process requires an input of energy (usually in the form of heat) to provide the necessary kinetic energy for the molecules to break away from the liquid.

Understanding Condensation

Condensation is the process in which a gas or vapor turns back into a liquid. On a microscopic level, this occurs when gas molecules lose kinetic energy and slow down, causing their attractive forces to pull them closer together. This results in the formation of liquid droplets. In this process, energy (heat) is released as the gas molecules lose energy and transition back into a liquid state.

Identifying the Process Requiring an Input of Energy

In the processes of evaporation and condensation, only evaporation requires an input of energy, such as heat. This additional energy is necessary to provide the liquid molecules with enough kinetic energy to overcome the attractive forces holding them together and allowing them to move freely as a gas. In contrast, condensation involves a release of energy as gas molecules slow down, and their attractive forces pull them closer together, forming a liquid.

Key concepts

Evaporation

Evaporation is a key phase transition where a liquid turns into a gas. This might happen naturally, like water evaporating from a puddle on a sunny day, or be induced, like boiling water. But what really happens on a microscopic level during evaporation? In a liquid, molecules are constantly moving and bumping into each other. These molecules possess kinetic energy, which is related to their speed.

• Some molecules move faster than others, gaining more kinetic energy.
• When a molecule has enough kinetic energy, it can break free from the liquid's surface.
• This is because it overcomes the attractive forces that hold the liquid molecules together.
• The molecule then escapes into the air as a gas or vapor.

To enhance evaporation, energy input is required, like sunlight or heat from a stove. This energy boosts the molecules' kinetic energy, helping them break away from the liquid surface.

Condensation

Condensation is another important phase transition. It refers to the process where gas or vapor turns back into a liquid. Think of the droplets forming on the outside of a cold glass. On the microscopic level, gas molecules are in constant motion with kinetic energy governing their speed.

• As gas molecules move, they sometimes bump into objects or other molecules.
• When gas molecules lose kinetic energy, they slow down.
• The decrease in kinetic energy increases their attraction to other molecules.
• This leads them to cluster together and form liquid droplets.

During condensation, energy in the form of heat is released. This release happens because the gas molecules' kinetic energy is decreased as they move closer, turning back into a liquid state.

Kinetic Energy

Kinetic energy is the energy of motion. It plays a vital role in both evaporation and condensation processes. In general, all molecules possess some level of kinetic energy. It determines how fast molecules move and is directly related to temperature.

• High kinetic energy: Molecules move quickly and are more likely to break away, as seen in evaporation.
• Low kinetic energy: Molecules slow down and are more likely to join together, as observed in condensation.
• Kinetic energy depends on temperature, with higher temperatures providing more energy to molecules.

Understanding how kinetic energy affects molecular motion aids in grasping why energy input is needed for evaporation. The input energy increases the molecules' speed, enabling them to overcome the liquid's attractive forces.