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Chapter 5: Problem 81

Why does the temperature of a liquid drop as a result of evaporation?

Short Answer

Expert verified
The temperature drops because high-energy molecules escape, reducing the average kinetic energy.

Step by step solution

01

Understanding Evaporation

Evaporation is the process by which molecules at the surface of a liquid gain enough energy to transition into a vapor phase. This typically occurs at temperatures below the boiling point of the liquid.
02

Energy Transfer in Evaporation

During evaporation, only the molecules with the highest kinetic energy escape from the liquid's surface. These molecules take away energy from the liquid when they evaporate.
03

Effect of Energy Loss

The removal of high energy molecules reduces the average kinetic energy of the remaining molecules in the liquid. Since temperature is a measure of the average kinetic energy, this reduction lowers the temperature of the liquid.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Temperature Change
When a liquid undergoes evaporation, its temperature experiences a notable change. This change occurs because evaporation involves the transition of molecules from the liquid phase to the vapor phase. As some molecules gain enough energy to escape the liquid, they take away heat energy from the remaining molecules.

This loss of energy results in a decrease in the average kinetic energy of the liquid鈥檚 molecules. Since temperature is a direct measure of this average kinetic energy, the temperature of the liquid consequently drops.
  • Evaporation removes high energy molecules, lowering kinetic energy.
  • Lower kinetic energy means a cooler liquid temperature.
This is why, after you sweat, the evaporation of your sweat can make you feel cooler even if the surrounding air is warm.
Kinetic Energy
Kinetic energy is the energy that an object possesses due to its motion. In the context of evaporation, this concept is critical. Molecules are constantly moving, and the speed of this movement is what we refer to as kinetic energy.

Not all molecules in a liquid have the same amount of kinetic energy. The molecules at the surface that have the highest kinetic energy can overcome the forces holding them in the liquid. These molecules escape into the air above, transitioning into a gaseous state.
  • Molecules with higher kinetic energy can evaporate.
  • Evaporation removes the most energetic molecules.
By losing these high-energy molecules, the remaining molecules in the liquid have lower kinetic energy on average, which impacts the liquid鈥檚 temperature as discussed earlier.
Phase Transition
Phase transition is the process where a substance changes from one state of matter to another. In evaporation, the transition is from liquid to gas. This phase change is vital to understanding why evaporation causes a cooling effect.

Unlike boiling, which happens at a specific temperature, evaporation can occur at various temperatures because only the most energetic molecules transition phases.
  • Phase transitions involve moving molecules from liquid to gas.
  • Evaporation is a type of phase transition.
During this transition, energy is used, which is why the remaining liquid cools down. Understanding phase transitions helps explain natural phenomena, such as evaporative cooling, on a molecular level.

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Most popular questions from this chapter

A balloon filled with \(1.2 \mathrm{L}\) of helium at \(25^{\circ} \mathrm{C}\) and 0.98 atm pressure is submerged in liquid nitrogen at \(-196^{\circ} \mathrm{C}\). Calculate the final volume of the helium in the balloon.

Ethanol, \(C_{2} H_{5} O H,\) and carbon dioxide, \(C O_{2},\) have approximately the same molecular weight, yet carbon dioxide is a gas at STP and ethanol is a liquid. How do you account for this difference in physical property?

Name three ways by which the volume, of a gas can be decreased.

What is the volume in liters occupied by \(1.21 \mathrm{g}\) of Freon-12 gas, \(\mathrm{CCl}_{2} \mathrm{F}_{2},\) at 0.980 atm and \(35^{\circ} \mathrm{C} ?\)

Answer true or false. (a) According to the kinetic molecular theory, gas particles have mass but no volume. (b) According to the kinetic molecular theory, the average kinetic energy of gas particles is proportional to the temperature in degrees Celsius. (c) According to the kinetic molecular theory, when gas particles collide, they bounce off each other with no change in total kinetic energy. (d) According to the kinetic molecular theory, there are only weak intramolecular forces of attraction between gas particles. (e) According to the kinetic molecular theory, the pressure of a gas in a container is the result of collisions of gas particles on the walls of the container. (f) Warming a gas results in an increase in the average kinetic energy of its particles. (g) When a gas is compressed, the increase in its pressure is the result of an increase in the number of collisions of its particles on the walls of the container. (h) The kinetic molecular theory describes the behavior of ideal gases, of which there are only a few. (i) As the temperature and volume of a gas increase, the behavior of the gas becomes more like the behavior predicted by the ideal gas law. (j) If the assumptions of the kinetic molecular theory of gases are correct, then there is no combination of temperature and pressure at which a gas would become liquid.
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