Illustrate the state changes in water as temperature changes. A state change means that a chemical moves from solid to liquid or liquid to gas, and so on. For water, these changes occur at 32 degrees for solid to liquid and 212 degrees for liquid to gas. To illustrate, bring in an ice cube and a beaker of water. Show the children how the ice melts into the water. Afterward, place the beaker on a hot plate and show the children how steam is released through boiling.
Relate chemistry to one of your pupils' favorite activities -- eating candy -- using a rock candy recipe from Exploratorium. Heat 2 parts water to a boil. Dissolve 1 part sugar in the boiling water until the solution is clear. Transfer the solution to a separate jar for each pupil. Dip a string (one for each pupil) into the solution, then remove it and lay it out to dry for a few days. When the strings are dry, instruct each student to place her string into her jar (using a pencil tied at one end to keep the string from completely submerging). Suspend the string in the jar for at least a week and have pupils monitor the growth of their rock candy.
Create a fingerprint detection kit for your children to play with to explain aspects of chemistry and biology, following instructions from Education.com. Hand out soft paintbrushes and clear plastic sheets to all pupils. Give each a plastic cup of chocolate powder (such as for hot cocoa). They touch a finger against the plastic sheet, then gently brush chocolate powder over the fingerprint. The oils left by the hand bond with the chocolate powder, creating a visible fingerprint. To preserve the fingerprint, put a piece of clear tape over it so that it is stuck between the tape and the plastic.
Make silly putty with your third-graders, using a recipe from the University of Arkansas College of Education and Health Professions. Mix equal parts white glue and liquid starch in a plastic cup. Explain the basics of viscosity (the speed at which liquids move). The children can then create shapes out of their silly putty, which changes from a viscous liquid to a solid when pressure is applied.