Fig. 2 (a) represents a straight bar made of a strip of brass and a strip of iron riveted together and attached to a handle. Upon heating the compound bar in a gas flame, the brass expands faster than the iron causing the bar to bend toward the latter as in Fig. 2 (b).
If some sugar, however, is heated strongly, say in a test-tube, it is found to blacken, some water is driven off and on cooling some black charcoal is found in the tube instead of the sugar. This action which has resulted in a change in the nature of the substance treated is called a chemical change. To illustrate further, if some magnesium wire is heated strongly in a flame, it burns, giving off an intense light and when it cools one finds it changed to a light powdery substance like ashes. Chemical changes, or those that change the nature of the substance affected, are studied in chemistry. In physics we have to do only with physical changes, that is, with those changes that do not affect the nature of the substance.
Important Topics
1. Physics defined.
2. The three states of matter; solid, liquid, gas.
3. Molecular theory of matter.
4. Physical and chemical changes.
Exercises
Write out in your own words your understanding of:
1. The structure of matter.