Introduce the concept of evaporation with a simple saltwater experiment. Put a pot of water on to boil, and add a few tablespoons of regular table salt. Stir the water and salt, so the salt dissolves, and let the water boil vigorously. Allow the kids to see the water evaporate from the pan by watching the water level drop. Tell them it's the same thing that happens in lakes, when the sun heats the water and the water evaporates. Let the water almost completely evaporate, and then turn the heat down very low. As the last of the water evaporates turn off the heat. The kids will see salt crystals form on the base of the pan, because only water evaporates -- even if elements like salt have been dissolved into it. Encourage kids to thin about the differences between the salt granules that they poured into the water and the crystals that are left behind.
Divide the salt crystals in half, and divide them amongst two metal trays. Leave one in a warm place and put the other in the refrigerator. Make a note of the time, and have your child check back on them after an hour. They should notice that the crystals in the refrigerator are smaller than those in the tray in the warm place. This is because crystal growth is affected by temperature. Crystals grow larger when they cool at a slow rate. The temperature in the fridge is cold, so the crystals cooled quickly and stopped forming.
Put 2 tbs. of water into a clear glass. Pour in 2 tbs. of vegetable oil . Get the kids to look through the side of the glass. The oil is floating on the top of the water. This is because oil is less dense than water; it weighs less, so it floats. Get the kids to stir the oil and water. A fork works best. Look through the glass again to see tiny bubbles of oil suspended in the water. Keep watching as slowly the oil rises back to the surface of the water and floats to the top once again.
An emulsion is when two or more liquids that usually don't mix -- like oil and water -- combine together to form a thicker mixture where the different liquids are indiscernible from each other, and reach a point where they stop separating. Crack an egg and put it in the water and oil mixture from the previous experiment. Vigorously whisk the three ingredients. Get the kids to watch as the water turns cloudy and slightly yellow. The oil and water molecules appear to join together because the egg has thickened the solution. They are suspended in the emulsion and can't separate. Although the mixture looks totally combined, if the kids look through a magnifying glass, they will still be able to see very tiny bubbles of oil.