2. Particle matters
Droplet size and evaporation P.4 of 13 
Keeping cool
When a liquid evaporates, it cools down.

This can be useful. For example sweating helps your body stay cool when you run around. Your skin produces sweat, which then evaporates off the surface, making it cooler.

However, you wouldn't want a body spray to feel cold on your skin. We can stop it feeling cold by making sure it breaks up into small particles when it leaves the can. Let's see why small particles stop a spray feeling cold.

Particle size and surface area
Picture 2.4 Small droplets have a larger total surface area than the same amount of liquid in a big droplet. This means that more propellant can evaporate.
Why do large droplets feel cooler?
The small droplets that make up the aerosol spray are a mixture of the propellant and the product. The propellant is chosen so that it will evaporate, leaving behind only the product.

The small droplets have a larger total surface area than the same amount of liquid in a big droplet. This allows most of the propellant to evaporate quickly – before the spray reaches your skin.

However, the large droplets retain the liquid propellant until they reach the skin. As the propellant evaporates on the skin, so it feels cool on the surface.

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Question 4
Look at these statements about evaporation. In each case, decide whether it is true or false. Then make your choice using the menu.
a) Sweat helps you cool because it reacts with the salt in your skin.
b) It's best to dry yourself quickly after swimming in the sea because if the water is allowed to evaporate you will feel cold.
c) A refrigerator uses an evaporating liquid to cool it down.
d) Large aerosol droplets are warmer than small ones because there's more liquid in them.
e) Large aerosol droplets feel colder than small ones because the propellant evaporates after they have landed on your skin.

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