Students should understand that although three phases of matter exist, some objects only exist as solids, while other solids can change to liquid or gas phases. Set a paper towel on a block of wood and center a charcoal briquette on the block. Remove an ice cube from the freezer and place it in a bowl. Ask students to identify the state of both objects. They should agree the briquette and ice cube are solid. Quickly hit the briquette several times with a hammer and ask students if it is still in the solid state. Leave both objects undisturbed for 24 hours. Students will observe the charcoal is still in the solid phase but the ice cube is now in the liquid state. Explain that some changes to matter do not change its state, such as hitting charcoal with a hammer, while other changes, such as temperature, may cause certain solids to change states.
Although matter in the solid state is generally distinguished from the liquid or gas phase by its ability to hold its shape, there are some rare exceptions, including polyethylene. Test this special type of matter by filling one polyethylene bag and one non-polyethylene plastic bag with water and tying both bags off. Set both bags in a sink and stab a pencil through the plastic bag. Stab a pencil through the polyethylene bag and compare results. Although polyethylene is in the solid state, its chemical properties cause it to shrink when torn. Discuss the practical implications of this unusual feature, such as puncture-resistant tires.
An inflated balloon demonstrates the ability of one phase of matter to pass through another form of matter in the solid state. Measure the balloon's circumference by wrapping a string around the largest point and mark on the string where it meets the end. Repeat the measurement and record the circumference twice daily for three days. Students will observe the balloon is shrinking. Explain that molecules are very small particles that make up larger objects, and although the balloon's walls are solid, air molecules are small enough to pass through tiny holes in the solid wall and rejoin the outside air.
Teachers can demonstrate the ability of matter in the solid phase to expand in size by bonding with other molecules of similar composition. Pour out a handful of sugar and roll the sugar around in your hands. Identify the action as a physical change to the sugar and help students understand as long as the sugar remains in the solid phase, the tiny solid pieces of sugar will not merge into one large solid piece of sugar. Fill a measuring cup with 1 cup of sugar. Pour 1 cup of water into a pan and bring it to a boil. Add the sugar, remove from heat and stir. Continue adding sugar until no more will dissolve. Pour the mixture into a glass after it cools. Tie a string around a pencil and rest it on the glass rim so the string hangs down into the glass. Leave the glass undisturbed for one week. Observe and record daily changes for class discussion about the ability of sugar molecules to bond together in the solid state after undergoing a chemical reaction and change of phase such as dissolution in the liquid state.