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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 sample of \(30.0 \mathrm{mL}\) of krypton gas, \(\mathrm{Kr}\), is at \(756 \mathrm{mm} \mathrm{Hg}\) and \(25.0^{\circ} \mathrm{C}\). What is the new volume if the pressure is decreased to \(325 \mathrm{mm}\) Hg and the temperature is decreased to \(-12.5^{\circ} \mathrm{C} ?\)

The density of acetylene gas, \(\mathrm{C}_{2} \mathrm{H}_{2},\) in a \(4-\mathrm{L}\) container at \(0^{\circ} \mathrm{C}\) and 2 atm pressure is \(0.02 \mathrm{g} / \mathrm{mL}\). What would be the density of the gas under identical temperature and pressure if the container were partitioned into two 2 -L compartments?

Ammonia and gaseous hydrogen chloride react to form ammonium chloride according to the following equation: \\[\mathrm{NH}_{3}(g)+\mathrm{HCl}(g) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\\] If \(4.21 \mathrm{L}\) of \(\mathrm{NH}_{3}(g)\) at \(27^{\circ} \mathrm{C}\) and 1.02 atm is combined with \(5.35 \mathrm{L}\) of \(\mathrm{HCl}(g)\) at \(26^{\circ} \mathrm{C}\) and 0.998 atm, what mass of \(\mathrm{NH}_{4} \mathrm{Cl}(s)\) will be generated?

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 \(35^{\circ} \mathrm{C} ?\)

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