Heat some lead or solder in a spoon till liquid. Let it cool. Do the same with wax.
Heat some water in a flask till it becomes steam. Steam is a gas. Cool the steam and form water again. (See distillation.) Refer to lava (melted rock), moulding iron, melting ice and snow, softening of butter.
All solids may be changed to liquids and even to gases if sufficiently heated. Likewise all gases may be changed into liquids and then to solids.
EXPANSION OF SOLIDS
In winter pupils may find that the ink is frozen. The teacher directs attention to this and inquires why it has occurred. It may be that in a lesson on rocks the teacher will ask the pupils to account for all the little stones. The following experiments will aim at solving the foregoing problems:
1. A brass ball and ring are shown. Pupils handle these and note that both are cold and that the ball just passes through the ring. They are asked to compare the size of the ball with that of the ring.
2. The spirit-lamp is lighted and examined. Pupils hold their hands over the flame to note the heat.
3. The ball is heated in the flame for a short time by one of the pupils, and felt cautiously. An attempt is made to pass it through the ring. How has the ball changed in feeling? In size? How does one know it is larger? What has caused these changes?
4. Cool the ball. Feel it. Try to pass it through the ring now. How has it changed in feeling? In size? What caused these changes? How does heat affect the ball? How does cold affect it?
The teacher may now give the words expand and contract, writing them on the black-board and explaining their use. Pupils may then state their conclusions: A brass ball expands when heated and contracts when cooled.