Question

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

Short answer

Evaporation and condensation are processes that involve the transformation between liquid and gaseous states. Evaporation occurs when liquid molecules gain enough kinetic energy to escape intermolecular forces, entering the gas phase, while condensation occurs when gaseous molecules lose kinetic energy, coming closer together to form the liquid phase. Evaporation requires an input of energy to overcome the attractive forces between molecules, while condensation releases energy as molecules form intermolecular bonds.

Step-by-step solution

Evaporation

Evaporation is the process through which liquid molecules transform into a gaseous state. This occurs when molecules near the surface of the liquid gain enough kinetic energy to escape the intermolecular forces holding them together. Some of the molecules will have enough kinetic energy to overcome the attractive forces and enter the gas phase, while others will not. During this process, the overall energy of the system stays constant, with the average kinetic energy of the remaining liquid molecules decreasing as a result. This leads to a cooling effect often associated with evaporation.

Condensation

Condensation is the process in which gaseous molecules transform into a liquid state. When a gas is cooled or comes in contact with a colder surface, the kinetic energy of the molecules decreases, causing them to come closer together. At some point, the intermolecular forces become strong enough to overcome the kinetic energy of the molecules, causing them to "stick" together and form the liquid phase. This process releases energy into the surroundings in the form of heat.

Energy input

Evaporation requires an input of energy because the liquid molecules need to gain kinetic energy to overcome the intermolecular forces and enter the gas phase. This energy can come from the surrounding environment, such as when the sun heats up a puddle of water, leading it to evaporate.

Explanation

When a substance converts from a liquid phase to a gas phase through evaporation, it requires energy to break the intermolecular bonds holding the liquid molecules together. This energy requirement is due to the attractive forces between the molecules, which must be overcome in order for the substance to transition into a gaseous state. In contrast, during condensation, energy is released as gaseous molecules lose kinetic energy and form intermolecular bonds, transitioning back into a liquid state. From the above explanations, we can conclude that evaporation requires an input of energy, while condensation releases energy. The input of energy is needed for evaporation because the molecular bonds holding the liquid together must be overcome for the substance to transition into a gaseous state.

Key concepts

Evaporation

Evaporation is a fascinating process where a liquid transforms into a gas. Imagine water in a puddle on a warm, sunny day. As the sun shines, it provides heat to the water molecules. Some molecules gain enough energy to "break free" from the liquid, entering the air as gas, or vapor.
Evaporation happens because molecules within a liquid are constantly moving. Some move faster than others. At any given moment, a few molecules will have sufficient kinetic energy to escape the liquid's surface. This happens despite the liquid's overall energy staying about the same.

• Evaporation occurs on the surface of the liquid.
• Only molecules with enough energy can escape.
• The process causes cooling as it takes energy from the liquid.

This cooling effect is why our skin feels cooler when sweat evaporates off it. As the sweat evaporates, it absorbs heat from our skin, carrying it away.

Condensation

Condensation reverses the evaporation process, changing gas back into a liquid. This occurs when gas molecules lose energy and come closer together, forming a liquid. You might have noticed condensation as droplets on a cold soda can on a hot day.
Here, gas molecules in the surrounding air touch the cold surface of the can. This interaction cools the molecules, slowing them down. Eventually, their reduced movement allows intermolecular forces to bind them into a liquid.

• Condensation occurs when gas molecules lose kinetic energy.
• Molecules pull together due to intermolecular forces.
• Energy is released as heat during this process.

Surprisingly, condensation actually releases energy into the environment. While molecules lose kinetic energy, the energy they release helps warm their surroundings.

Energy Transfer

In these phase changes, energy plays a crucial role. Evaporation and condensation efficiently transfer energy through the processes of absorbing or releasing heat.

**Energy Input for Evaporation:**
Evaporation demands energy input. This energy, often from heat, allows molecules to overcome attractive forces, transitioning them from a liquid to a gas. An example is the sun heating water, enabling its evaporation.

**Energy Release in Condensation:**
Conversely, condensation releases energy because molecules slow down and join together. This exothermic process emits heat, with the released energy warming the immediate surroundings.

• Evaporation absorbs energy (endothermic), causing cooling.
• Condensation releases energy (exothermic), causing warming.

Understanding how energy moves during phase changes helps us appreciate natural processes, like how sweating cools us or how clouds form in the sky.