2. Particle matters
Droplet size and evaporation P.4 of 13
 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 allow most of the propellant to evaporate quickly - before the spray reaches a surface (like 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.
 Picture 2.5 The temperature of a liquid depends on the average KE of its particles. Heating a liquid increases the average KE.

 What’s in a liquid? A liquid is made from tiny particles moving around randomly. If we heat the liquid, the particles will move faster – they will have more kinetic energy. However, the kinetic energy of the particles is not uniform. Some of them are moving faster than others and therefore have above average kinetic energy. Energy and temperature The temperature of the liquid depends on the average kinetic energy of its particles. When we heat it, the average KE goes up. However, when some of the liquid evaporates, the average kinetic energy goes down and the liquid gets cooler. Let’s see why.
 Picture 2.6 The average height inside drops if only the tall ones escape.
 Selecting the best Imagine a group of prisoners in a walled pen. The prisoners have a range of heights with an average height of 1.7 metres. The wall is over 2.5 metres high but any prisoner who is taller than 1.8 metres can pull himself over. What happens when the guard turns his back? All the tall prisoners escape. The average height of the ones left behind is less than before – because all the tall ones have gone.
 What about evaporation? The particles of a liquid attract each other. So when a particle leaves the surface, there is a force from all the surface particles trying to pull it back. However, if it is travelling really fast, it can escape from this force. So only the particles with a high kinetic energy can escape – or evaporate. The average kinetic energy of the particles left behind is lower than before because all the most energetic ones escape. So the liquid cools.

 Picture 2.7 What happens to the average KE if the high energy particles escape?

Question 4
Look at the particles in picture 2.7. Their kinetic energies are marked on them (in very small units).

 a) What is the maximum kinetic energy? b) What is the average kinetic energy? Any particle with a kinetic energy more than 6 can escape from the surface. c) What is the new average kinetic energy of the particles left behind?

d) Choose the best word for each blank in the sentence below.

The kinetic energy has so the temperature of the liquid will go .

 Summary                                           Close large aerosol droplets feel cool on the skin small droplets allow the propellant to evaporate before landing the temperature of a liquid depends on the average KE of its particles evaporation of a liquid causes cooling because . . . the average kinetic energy of its particles drops