Evaporation Causes Cooling

Introduction

Step out of a swimming pool on a breezy day and you shiver, even when the air is warm. A dab of hand sanitiser feels cold on your skin. Your body sweats when you exercise. All of these have the same explanation: evaporation causes cooling . This page uses the particle model to explain why losing the fastest particles lowers the temperature of what is left behind.

Core explanation

Particles have a range of energies. In a liquid, particles move at many different speeds. The temperature of the liquid depends on their average kinetic energy.

Only the fastest particles escape. During evaporation, the particles that leave the surface are those with the most kinetic energy — they need it to overcome the attractions of their neighbours. The slower particles cannot escape and stay behind.

The average falls. When the most energetic particles leave, the particles remaining in the liquid have, on average, less kinetic energy than before. A lower average kinetic energy means a lower temperature . So the liquid cools down as it evaporates.

Energy from the surroundings. A cooler liquid is now colder than its surroundings, so energy flows into it from whatever it touches — your skin, the air, a container. This energy tops up the particles, some of which gain enough to escape, and evaporation continues. The overall effect is that energy is taken from the surroundings and carried away by the vapour. That is why a surface covered in an evaporating liquid feels cold.

How sweating works. When you are hot, glands in your skin release sweat, which is mostly water. As sweat evaporates, it takes energy from your skin, cooling it and the blood flowing just beneath it. This is one of the main ways the body keeps its core temperature close to 37 °C.

When it works well. Evaporative cooling is strongest when evaporation is fast, so the factors that speed up evaporation — warm, dry, moving air and a large wet surface — all increase the cooling effect. In very humid weather, sweat evaporates slowly, so it drips off instead and you feel hot and sticky.

Step-by-step reasoning

To explain why a wet arm feels cold:

1. Water particles on the skin move at a range of speeds. 2. The fastest particles escape from the surface. 3. The remaining particles have a lower average kinetic energy. 4. So the water becomes cooler. 5. Energy flows from the skin into the cooler water, so the skin feels cold.

Visual explanation

Picture a group of particles on the skin, drawn as dots with arrows of different lengths showing their speeds. The dots with the longest arrows fly off the top. The dots left behind all have short arrows, and a thermometer beside them shows a falling reading.

Real-world analogy

Imagine a class whose average test score is 60. If the five highest scorers leave to join another class, the average of those left falls, even though no remaining student's score has changed. Losing the fastest particles lowers the average energy in the same way.

Real-world example

Unglazed clay pots have long been used to keep water cool in hot, dry climates. Water seeps slowly through the porous clay and evaporates from the outer surface, drawing energy from the pot and the water inside, which stays noticeably cooler than the surrounding air.

Why?

Why does a breeze feel colder on wet skin than on dry skin? The moving air carries water vapour away, speeding up evaporation. Faster evaporation removes energetic particles more quickly, so more energy is taken from the skin each second, and the cooling effect is much stronger.

Common misconception

"Evaporation makes the liquid cold because the liquid gives out cold." There is no such thing as "cold" flowing. The liquid cools because it loses its most energetic particles, and then energy flows into it from the warmer surroundings.

Worked example

Question: Two thermometers are placed in a warm room. The bulb of one is wrapped in dry cotton wool; the other is wrapped in cotton wool soaked in water. After ten minutes, which reads lower, and why?

Reasoning: Water evaporates from the wet cotton wool. The fastest particles escape, lowering the average energy of the remaining water, and energy is drawn from the thermometer bulb.

Answer: The wet thermometer reads lower, because evaporation cools the water and takes energy from the bulb.

Quick check

1. Which particles leave a liquid first during evaporation? Answer: The particles with the most kinetic energy, the fastest ones.

Exam focus

Examiners want the full chain of reasoning: fastest or most energetic particles escape, so the average kinetic energy of the remaining particles decreases, so the temperature falls. Use the word "average". For sweating, add that energy is transferred from the skin to the sweat.

Advanced insight

The energy needed to turn a liquid into a gas is its specific latent heat of vaporisation. For water it is very large, about 2.26 million joules per kilogram at 100 °C and even more at skin temperature. This large value makes sweating an extremely effective cooling system: evaporating just 100 g of sweat removes roughly 240 kJ of energy from the body.

Summary

Evaporation causes cooling because the most energetic particles escape from the surface, lowering the average kinetic energy, and so the temperature, of the liquid left behind. Energy then flows into the liquid from its surroundings, cooling them. Sweating uses this to keep the body cool, and it works best in warm, dry, moving air.

Practice questions

1. Explain, in terms of particles, why a liquid cools as it evaporates. Answer: The fastest particles escape, so the remaining particles have a lower average kinetic energy, which means a lower temperature. 2. Why does sweating help to cool the body? Answer: As sweat evaporates from the skin it takes energy from the skin, lowering its temperature. 3. Why do you feel hotter on a humid day even when sweating? Answer: Humid air slows evaporation, so sweat evaporates slowly and less energy is removed from the skin. 4. Why does a swimmer feel cold when stepping out of the pool on a windy day? Answer: The wind speeds up evaporation of the water on the skin, removing energy from the skin more quickly.