plate and condenses into water. In winter, steam collects on the window panes, if the room is warm and the air is moist. A pitcher of ice water when standing in a warm room, becomes covered with drops of water. When the moisture in the air comes in contact with the cold surface, it immediately condenses.

When steam condenses into water, heat is released. The amount of heat thus liberated is equal to the amount of heat used in transforming the water into steam. In other words, it requires 536 calories of heat to convert 1 gram of water into steam, and 1 gram of steam gives off 536 calories of heat during its condensation into water.

The liberation of heat by condensation is made use of in the system of steam heating. Water is boiled in a boiler, and a large amount of heat is used to convert the water into steam. The steam passes through pipes which run to the radiators in various parts of the building. There the steam condenses and in so doing liberates large quantities of heat, thereby giving up to the air of the room much of the heat which it had absorbed from the fire.

Change in volume resulting from change in state. A change in the form of a substance, from a solid to a liquid, for example, is invariably accompanied by a change of volume.

With the exception of ice, most solids expand on becoming liquids, and liquids expand when they become gases. The volume of water is greatly

increased when it becomes steam. All of us know of the force exerted by steam. This enormous force is utilized in all the numerous kinds of steam engines.

The return from the gaseous to the liquid state, or from the liquid to the solid state, is always accompanied by a considerable decrease in volume.

Molecular changes resulting from change in state. We already know that if the temperature of a solid is increased, its volume is also increased. We also know that this is because the molecules of the solid have been driven farther apart. By continuous heating, the molecules of a solid may be driven, or forced, far enough apart actually to change the form of the substance from that of a solid to that of a liquid. Similarly, when the molecules of a liquid are forced far enough apart, the liquid becomes a gas.

Anything which interferes with the free movement of the molecules, will increase the amount of heat necessary to change, for example, a liquid to a gas. Consequently, if the air pressure on a liquid is increased, its boiling point is raised. The reverse is also true, that if the pressure upon a liquid is reduced, the molecules can move apart more readily, and consequently, less heat is necessary to convert the liquid into a gas.

We thus see that by decreasing the pressure upon solids or liquids, less heat is necessary to change them into gases.

It is also true that to change a gas to a liquid, or a liquid to a solid, the molecules must be brought closer

together. This can be brought about in two ways: first, by reducing the temperature of the substances, and second, by increasing the pressure upon them.

It is only by the combination of a great increase in pressure and a great decrease in temperature, that it has been possible to convert some gases into liquids, and the resulting liquids into the solid state.

Questions

1. In what three forms may a substance exist? 2. Name two solids which cannot be changed to liquids. Why is it impossible to convert them into liquids?

3. At what temperature does water boil? Freeze? 4. How many calories of heat are required to change one gram of ice to water?

5. What are the sources of heat for melting ice in a refrigerator?

6. Is it warmer or cooler near a body of melting ice than it is at some distance away? Why?

7. Why is salt mixed with chopped ice for use in ice cream freezers?

8. What effect does water which is melting have upon the surrounding temperature?

9. How many calories of heat are given off when 10 grams of water are changed to ice?

10. What is the melting point of ice? Alcohol? Mercury?

11. Why is it impossible to melt iron in a glass dish?

12. What is the boiling point of water? Alcohol? Ether? Mercury? Copper?

13. What is meant by distillation ?

14. State several ways in which the process of distillation is of commercial value.

15. Why is it impossible to cook potatoes, by boiling, on top of Pike's Peak?

16. How many calories of heat are required to change one cubic centimeter of water into steam?

17. Why is it cooler after the streets have been sprinkled?

18. How does perspiration cool the body?

19. What is the method used in the production of artificial ice?

20. Why can you see your breath on a cold winter day?

21. Explain how heat is transferred by means of steam from the coal burning in the furnace to the rooms of a house.

22. What change takes place in the volume of a substance when it changes from a solid to a liquid? A liquid to gas? A gas to a liquid, or a liquid to a solid?

CHAPTER VII

PHYSICAL AND CHEMICAL CHANGES

Physical change. Matter may undergo many changes. One class of these changes is not accompanied by any alteration in the composition of matter. When a piece of glass is broken the small pieces do not differ from the original piece except in size. A piece of iron may be broken, it may be magnetized, it may be heated, it may be melted, and it may be converted into a vapor. In none of these changes, however, has the composition of the iron been affected. The pieces of iron, the magnetized iron, the heated iron, the melted iron, or the iron vapor are just as truly iron as was the original piece. Sugar may be dissolved in water, but neither the sugar nor the water is changed in composition. The resulting liquid has the sweet taste of sugar, but the water can be evaporated by heating and the sugar recovered unchanged. Such changes are called physical changes. Physical changes are those which do not involve a change in the composition of substances. In other words, a physical change is a change in the form but not in the nature of a substance.