Experiments can be performed that simply show ordinary water evaporating over time. Do this by leaving an open glass of water in your classroom or home overnight. Make a mark on the glass which shows the level of the water at the start of the experiment or use a measuring jug. The level of the water dips as molecules escape into the air. Another experiment uses two identical containers of water, except one is covered by cling film or foil. The cover keeps the water molecules in the container. The uncovered container will lose water and the covered one will not.
Fill a container with water and then add a few spoonfuls of salt or sugar. Leave the mixture to evaporate, or alternatively heat the water to speed up the process. If the mixture is to be heated, do this yourself for safety. The salt or sugar will be left at the bottom of the container when the liquid has evaporated. Explain to the children that only the water molecules are able to evaporate.
Place one container of water in the classroom at room temperature. Place an identical container of water over heat. Ensure that equal amounts of water are placed into each container. The water in the heated container will evaporate first because the molecules move around faster when they are hot. You can get children to leave one container in the sun to perform this experiment safely. Alternatively, you can use pans or metallic containers and heat a container over an open flame yourself.
Different substances evaporate at different rates. This quality is referred to as their volatility. Use a substance such as propanone, but perform the experiment yourself. Show how quickly propanone and water evaporate by placing equal amounts of substances in separate containers. The propanone will evaporate first. Explain to the children that this is because propanone is more volatile than water.