Question

When a liquid is evaporated (a) temperature of liquid decreases (b) temperature of the liquid increases (c) liquid molecules become more energetic (d) both (b) and (c)

Short answer

(a) Temperature of liquid decreases and (c) liquid molecules become more energetic.

Step-by-step solution

Understanding Evaporation

Evaporation is the process where liquid molecules at the surface gain enough energy to transform into gas. This process requires energy to convert the molecules from a liquid to a gaseous state.

Relation Between Temperature and Energy

The temperature of a substance is a measure of the average kinetic energy of its molecules. When molecules evaporate, they absorb energy from their surroundings (endothermic process), which is why it typically causes a cooling effect on the remaining liquid.

Effect of Evaporation on Temperature and Molecules

As molecules evaporate, the temperature of the remaining liquid decreases because the more energetic molecules leave, taking their energy with them. At the same time, the molecules that evaporate become more energetic as they transition to the gas phase, indicating an increase in their kinetic energy.

Key concepts

Kinetic Energy

Kinetic energy is the energy that an object possesses due to its motion. In the context of evaporation, this energy plays a crucial role. Imagine each liquid molecule as a tiny, active particle zipping around with a certain speed. This speed relates directly to its kinetic energy. The faster a molecule moves, the more kinetic energy it has.
During evaporation, some molecules in the liquid have more kinetic energy than others. Those with enough kinetic energy can escape the liquid's surface and enter the air as gas molecules. You can think of them as energetic athletes, ready to make the leap from liquid to gas. This transfer of molecules is essential in the process of evaporation and is why understanding kinetic energy is important to grasp evaporation dynamics.

Endothermic Process

Evaporation is an endothermic process, meaning it absorbs energy from its surroundings. When the molecules at the surface of a liquid gain enough energy, they can overcome the forces holding them in the liquid state. This energy typically comes from the heat in the surrounding environment.
In an endothermic process such as this, energy is not randomly added to the liquid. It specifically raises the kinetic energy of the more active molecules at the surface. That's why when you feel a puddle evaporating on a warm day, you might notice that it feels cooler than the surrounding air. It's taking in energy to facilitate the transformation of liquid into gas, demonstrating the endothermic nature of evaporation.

Cooling Effect

The cooling effect during evaporation occurs because as the molecules with the highest kinetic energy escape into the gas phase, they take heat away with them. This lowers the average kinetic energy of the remaining liquid molecules.

• As the more energetic molecules leave, the temperature of the liquid drops.
• This effect is why we feel cool when sweat evaporates from our skin.

Essentially, the removal of heat due to evaporation creates a cooling sensation. This principle is used in cooling technologies and climate adaptation strategies, showcasing how evaporation is not just a simple phase change but an important factor in thermal regulation.

Gas Phase Transition

Transitioning from liquid to gas is a significant change for molecules at the surface of a liquid. During this gas phase transition, molecules shift from being tightly packed together in the liquid to freely moving apart in the gas phase.
This transition requires the surface molecules to gain substantial kinetic energy to overcome intermolecular forces. Once a liquid molecule enters the gas phase, it has more freedom to move. The motion of gas molecules is far more unpredictable and expansive compared to liquid molecules.

• This transition ultimately contributes to the gaseous state having higher entropy than the liquid state.
• The process is essential in natural phenomena such as the water cycle.

The gas phase transition through evaporation is crucial not just to our understanding of thermodynamics but also to ecological and meteorological processes.