Demonstrate what happens when baking soda reacts with vinegar, forming a gas called carbon dioxide. Remove the cap from an empty 2-liter soda bottle and set it on a baking pan. Pile dirt around the bottle to form a mountain, adding a bit of water to the dirt so that it sticks to the bottle. Encourage children to help by adding a few drops of red food coloring to a cup of vinegar to create red lava. Preschoolers can help pour the baking soda into the bottle and then add the red vinegar mixture to the bottle. Red foam will spray out of the bottle and flow down the mountain, creating a simulated volcano eruption.
Mixing paint allows preschoolers to discover what happens when primary colors are mixed together to form other colors. Provide each child with a small amount of red and blue paint and paint stir sticks. Encourage the preschoolers to mix the colors on a paper plate and observe the results. Next, repeat with red and yellow paint and then again with blue and yellow paint. Ask questions such as "What happened when you mixed yellow and blue?" and "What happened when you mixed red paint with blue paint?" Together, create a chart to record the preschoolers' observations.
Making slime will show preschoolers that liquids can combine to make a solid. Children can help by pouring and adding ingredients to create the slime. Add 1/2 cup white glue to 1/2 cup of water in a bowl. Add 5 drops of food coloring. Stir well and set aside. Fill a large measuring cup with a cup of distilled water and add a teaspoon of borax. Stir well. Allow the preschoolers to pour the borax solution into the bowl that contains the glue mixture and stir with a fork. Have them help knead the mixture to create a mound of slime. Ask the preschoolers what happened when the two mixtures were combined. Discuss the texture and shape of the slime.
Introduce preschoolers to the concept of chemical reactions by cleaning pennies. Show children dirty pennies and ask them how you might get pennies to go from dull to shiny. Combine one teaspoon of salt and 1/4 cup of white vinegar in a shallow non-metal bowl. Reserve a few of the dirty pennies and add the rest of them to the solution in the bowl. Encourage the preschoolers to observe the pennies as they become clean. Rinse them in clean water and then ask the children to compare them to the dirty pennies.