Cooling by evaporation
Cooling by evaporation
- Unit 1: Particles & Pressure
- Unit 2: Forces & Motion
- Unit 3: Energy & Heat
- Unit 4: Electricity
- Unit 5: Magnetism & Electromagnetism
- Unit 6: Waves: Sound & Light
- Unit 7: Scientific Investigations
In this topic you will:
- understand how evaporation causes cooling.
Key words
- porous
- random
Evaporation
Evaporation is the change of state from liquid to gas.
Think of a liquid such as water at room temperature. The particles in the water are moving around in random directions. Some of these particles move faster than others, and the speed of the particles is also random. The word random means unpredictable.
The particles in the water that are moving faster have more energy. Some of these particles have enough energy to escape from the surface of the liquid and become particles of a gas. Some of these are pulled back into the liquid, but others have sufficient energy to leave and not get pulled back. This is summarised in the diagram.
Evaporation and cooling
You will recall from Topic 3.2 that temperature is a measure of the average energy of particles.
Think about the water again. Remember that the particles in the water have different energies.
When the particles with the highest energy escape from the water, this will lower the average energy of the particles that remain. Therefore, the temperature of the water will decrease, so evaporation causes cooling.
You can feel the effect of this when you get out of a swimming pool.
When the water on your skin evaporates, the particles with the highest energy escape causing the water to cool. The thermal energy in the water droplets on your skin comes from your skin. Therefore, the energy provided to change the state of the water from liquid to gas comes from your skin. As your skin loses thermal energy, this causes cooling.
When we are too hot, our skin produces sweat. Many people think that the liquid on the skin alone causes cooling, but since the sweat was produced in the skin, it is at the same temperature as the skin.
When the sweat evaporates, its temperature drops. Thermal energy from the skin is then transferred to the cooler sweat and the process continues to lower the skin temperature.
Some animals that do not sweat very much cover themselves in water to cool down. The evaporation of the water gives the cooling effect.
In very humid conditions, sweat may not be able to evaporate. This can be dangerous, as body temperature can increase without this cooling effect.
The picture shows a type of water cooler.
The water cooler is made from porous clay. The word porous means that water can slowly soak through the clay. When filled with water, some water soaks through the clay to the outside. This water evaporates into surrounding air, cooling the water inside the water cooler.
Some air coolers use evaporation of water to cool air. The picture shows one of these coolers.
The air cooler contains a sponge that is soaked in water. A fan blows warm air from the room through the sponge. The warm air provides the thermal energy to evaporate the water, and so the thermal energy of the air decreases, cooling the air.
Key Term: Porous
A porous material contains small holes or spaces that allow liquids or gases to pass through slowly. Clay is porous and lets water seep through to its surface.
Quick Fact
Evaporative cooling is used in everyday technologies like cooling towers, desert fridges, and air conditioners — all based on the principle that evaporation removes energy from a surface.
Common Mistake
Students often think sweat itself is “cold.” In fact, the cooling only happens as high-energy particles evaporate from it — drawing heat away from the skin.
Evaporation and cooling of other liquids
Different liquids have different forces holding the particles together. Some liquids have forces between particles that are weaker than those in water and some are stronger.
Some liquids, such as perfumes, have weaker forces between the particles. Perfumes are designed to evaporate quickly and easily so you can smell them.
If you put equal volumes of perfume and water at the same temperature on your skin, the perfume will feel colder. Perfume evaporates faster than water, so removes thermal energy faster.
Other liquids, such as liquid soap, have stronger forces between the particles. Liquid soap evaporates much more slowly than water. If you put equal volumes of liquid soap and water at the same temperature on your skin, the water will feel colder. Water evaporates faster than liquid soap, so removes thermal energy faster.
Key Concept: Evaporation Rate
The evaporation rate is the speed at which a liquid changes to gas. Liquids with weaker particle forces evaporate faster and remove heat more quickly.
Quick Fact
Alcohol-based hand sanitizers feel cold because alcohol evaporates very quickly — even faster than perfume — removing heat from your skin rapidly.
Safety Note
Do not put any liquid on your skin apart from water, soap or perfume. Soap should be washed off skin using plenty of water.
Questions
Copy and complete these sentences about water using words from the list. Each word can be used once, more than once or not at all.
least, most, different, the same, evaporate, condense, freeze
The energies of the particles in water at 25 °C are ......................
Show Answer
different
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least
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most
Show Answer
evaporate
A freezing B condensation C evaporation D boiling
Show Answer
C — evaporation
Show Answer
The average energy of the remaining particles decreases because the most energetic particles escape.
Explain your answer.
Show Answer
The temperature decreases because energy is carried away by the escaping particles.
Show Answer
Sweat cools the skin as it evaporates, taking heat energy away from the body.
Show Answer
Sweat cannot remove heat if it does not evaporate, so the body retains more heat and becomes too warm.
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Isopropanol absorbs heat from the skin as it evaporates, making the skin feel cooler.
Think Like a Scientist
In this activity, you will make an air cooler that works by evaporation and compare the temperatures of air at different stages of the setup.
• An old towel
• A frame to hang the towel
• A tray that can hold water and one edge of the towel
• Water
• Desktop fan
• Thermometer
• Stand or clamp for the thermometer
• Keep water away from electric sockets, cables, and the fan.
• Place the fan as far from the wall socket as possible.
• Use dry hands to operate switches and be ready to turn off power if water spills.
Step 2. Measure the temperature of the air in the room.
Step 3. Switch on the fan and measure the air temperature blowing from it.
Step 4. Soak the towel in water and hang it on the frame.
Step 5. Fill the tray with water so the bottom of the towel remains wet.
Step 6. Move the fan about 10 cm from the towel and switch it on to blow air at the towel.
Step 7. Measure the temperature of the air between the fan and the wet towel.
Show Answer
Your hand gives off heat, which could affect the thermometer reading and give an inaccurate air temperature.
Show Answer
The air cooled further when it passed by the wet towel due to evaporative cooling, which absorbed thermal energy from the air.
Show Answer
The air would not cool as much because evaporation would not occur. There would be little or no temperature difference.
Reflect on:
- Did I use the thermometer properly?
- Were the temperatures recorded clearly and at the correct times?
- Did I understand how evaporation caused cooling?