Physics. Willis E. Tower
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.
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
10. The Metric System.—In order to study the three states of matter with sufficient exactness it is necessary to employ a system of measurement. The system universally employed by scientists is called The Metric System. In many respects it is the most convenient for all purposes. Every student should therefore become familiar with it and learn to use it. At the present time, not only do scientists everywhere use it, but many countries have adopted it and use it in common measurements. It was legalized in the United States in 1866. The metric system was originated by the French Academy of Sciences during the latter part of the 18th century. There were so many different systems of weights and measures in use, each country having a system of its own, that commerce was much hindered. It was therefore decided to make a system based upon scientific principles. The length of the earth's quadrant passing from the equator to the pole was determined by surveying and computation. One-ten-millionth of this distance was selected as the unit of length and called a meter. Accurate copies of this meter were made and preserved as standards.
Later surveys have shown that the original determination of the earth's quadrant was not strictly accurate; so that after all the meter is not exactly one-ten-millionth of the earth's quadrant.
11. The Standard Meter.—The standard unit of length in the metric system is the meter. It is the distance, at the temperature of melting ice, between two transverse parallel lines ruled on a bar of platinum (see Fig. 3), which is kept in the Palace of the Archives in Paris. Accurate copies of this and other metric standards are also kept at the Bureau of Standards at Washington, D. C. Fig. 4 shows the relation between the inch and the centimeter (one-hundredth of a meter).
12. Units and Tables in the Metric System.—The metric unit of area commonly used in physics is the square centimeter.
The standard unit of volume or capacity is the liter. It is a cube one-tenth of a meter on each edge. It is equal to 1.057 quarts. It corresponds, therefore, to the quart in English measure.
The standard unit of mass is the kilogram. It is the mass of 1 liter of pure water at the temperature of its greatest density, 4°C. or 39.2°F.
The three principal units of the metric system, the meter, the liter, and the kilogram, are related to one another in a simple manner, since the liter is a cube one-tenth of a meter in each dimension and the kilogram is the mass of a liter of water. (See Fig. 5.)
The metric system is a decimal system that is, one unit is related to another unit in the ratio of ten or of some power of ten. This is indicated by the following tables:
| Metric Table of Length | |
| 10 millimeters (mm.) | equal 1 centimeter. |
| 10 centimeters (cm.) | equal 1 decimeter. |
| 10 decimeters (dm.) | equal 1 meter. |
| 10 meters (m.) | equal 1 dekameter. |
| 10 dekameters (Dm.) | equal 1 hectometer. |
| 10 hectometers (hm.) | equal 1 kilometer. |
| 10 kilometers (km.) | equal 1 myriameter. |
The measures commonly used are the centimeter, meter and kilometer.