A liquid cools itself down as it evaporates
Definition
Cooling by evaporation
The fall in temperature of a liquid caused by its most energetic particles escaping, which lowers the average kinetic energy of the particles left behind.
- Evaporation removes particles from the surface, but not just any particles: the ones that escape are the fastest-moving ones, because only they have enough energy to break away.
- Taking the fastest particles out of a group lowers the average kinetic energy of everything that remains, in the same way that the average height of a class drops when the tallest students leave the room.
- Temperature is a measure of that average kinetic energy, so the temperature of the liquid falls.
- No energy has to be taken away by anything else for this to work: the liquid cools because of what leaves it, which is why a drink left out on a warm day can end up cooler than the air around it.
- Anything that speeds up evaporation therefore speeds up the cooling, so a wide surface and a draught both make the effect stronger.
- You can feel it directly by putting a little water on the back of your hand and blowing across it, because the skin underneath goes noticeably cold.
An object in contact with an evaporating liquid is cooled too
- Start with the liquid cooling itself, so the film of liquid sitting on the object is now at a lower temperature than the object.
- Whenever two things at different temperatures touch, thermal energy is transferred from the hotter to the cooler, so energy flows out of the object and into the liquid.
- Losing that energy lowers the average kinetic energy of the object's own particles, so the object cools.
- The energy arriving from the object is immediately used up by more particles escaping from the liquid, so the liquid stays cool and the process keeps going.
- The cooling only lasts while there is liquid left, so once the surface has dried the object warms back up to the temperature of its surroundings.
- This is exactly how sweating keeps you cool: the sweat evaporates from your skin and draws energy out of your body, which is also why a humid day feels so much worse, since the air is already full of water particles and fewer can escape.
- The same physics runs a refrigerator, where a liquid is made to evaporate in pipes inside the cabinet and draws energy out of the food.
Example
- Climbing out of a swimming pool on a warm day leaves you shivering until you reach for a towel.
- The film of water on your skin is evaporating, and its most energetic particles are leaving.
- The remaining water cools, so energy is transferred out of your skin into that water.
- Drying yourself removes the water, the evaporation stops, and you stop feeling cold.
Common Mistake
- Do not write that evaporation cools the liquid because "the cold particles are left", because the particles left behind are simply the slower ones and that is what lowers the average.
- Do not say the air cools the object, because the cooling comes from energy being transferred into the evaporating liquid.
- Do not stop your answer at "the liquid cools" when the question asks about the object, because the mark for the object needs the transfer from object to liquid.
- Do not confuse this with boiling, because nothing here reaches a boiling point and no bubbles form.
Exam technique
- Build the answer as a chain and write every link: most energetic particles escape, average kinetic energy of the rest falls, temperature of the liquid falls.
- For the object, add the extra link about energy transferred from the object to the cooler liquid, which is the mark that separates a full answer from a half one.
- Use the word average every time you mention kinetic energy here, because that is where the whole explanation turns.
- If the question gives a context such as sweating or a refrigerator, refer back to it by name in your final sentence.
Self review
- Why does a liquid cool down as it evaporates?
- Which particles leave the liquid, and what does that do to the average kinetic energy of the rest?
- Explain why your skin feels cold when water on it evaporates.
- Why does the cooling stop once the surface has dried?
- Name one device that uses cooling by evaporation.
