[Illustration: FIG. 1.--When the bulb is heated, the air within expands forcing down the water in the tube.] =9. Physical and Chemical Changes.= A change of state such as the freezing or boiling of water is called a _physical_ change, for this change has not affected the identity of the substance. It is water even though it has become solid or gaseous. Heating a platinum wire red hot is also a physical change for the wire when on cooling is found to be the same substance as before. Further if salt or sugar be dissolved in water the act of _solution_ is also a physical change since the identical substance (salt or sugar) is in the solution and may be obtained by evaporating the water. [Illustration: 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. 2. Some of the differences between solids, liquids, and gases. 3. How to change solids to liquids and gases and _vice versa_. 4. The reason for the changes of size of a body on heating. 5. Why cooling a gas tends to change it to a liquid or a solid. 6. The actual size of molecules. Which of the following changes are chemical and which physical? Give reasons. 1. Melting of ice. 2. Burning of a candle. 3. Production of steam. 4. Falling of a weight. 5. Drying of clothes. 6. Making an iron casting. 7. Decay of vegetables. 8. Sprouting of seeds. 9. Flying an aeroplane. 10. Growth of a plant. 11. Grinding of grain. 12. Sawing a board. 13. Pulverizing stone. 14. Making toast. 15. Sweetening tea or coffee with sugar. 16. Burning wood or gas. (3) THE METRIC SYSTEM