While many preschoolers may know that water freezes into ice, they will be interested to learn how ice turns back into water with the application of heat. Give each preschooler a small bowl or cup with an ice cube inside and invite them to find a place in their room. Challenge some students to find a place where the ice cube will melt the fastest, while others should look for a spot for it to melt the slowest. Let preschoolers observe the effects of sun or shade on their ice cube and note the melting process.
Introduce preschoolers to the water cycle and explain what happens to all the rain puddles all in one easy science project. Fill a glass with water and stir in a spoonful or two of sugar or salt. Let preschoolers taste the mixture to prove that the salt or sugar is still in the cup even though it is no longer visible. Use a marker or piece of tape to mark the water level and the time on the glass. Set the mixture in a warm, sunny spot in the classroom. Have a student check the glass every hour or so to mark the new water level and the time. Do this until all that is left is the dry sugar or salt at the bottom of the glass. This may take at least one night to occur, so make this "magic" trick an ongoing project as you explain the process of evaporation.
Amaze students by creating an invisible gas out of two common kitchen substances. Pour some baking soda into a clear glass or plastic bottle. Add vinegar and then very quickly put a deflated balloon over the mouth of the bottle. Note preschoolers' reactions when the gas formed by combining vinegar and baking soda inflates the balloon right before their eyes.
When explaining the various states of matter to preschoolers, keep it interesting by letting students get their hands dirty in this kitchen mixture of solid and liquid. Pour cornstarch into a tray large enough for several students to share. Add an equal amount of water and let students mix the solid and liquid together to create a strange combination of the two that gets hard when you press on it, but oozes like a liquid when you pour it.
Science is magic with this basic chemistry lesson. Invite students to dip paintbrushes into lemon juice to paint a picture on white paper, watching as their brushstrokes disappear when the lemon juice dries. Trade papers with a friend and carefully paint the papers with iodine to make the "secret message" appear again as the iodine interacts with the acidic lemon juice.