3. Do you think that the air which passes out through the stovepipe when there is a fire in the stove contains more, or less, oxygen than when it entered the front of the stove?

Why?

4. If we wish to put out a fire, what must we keep away from it? How can this be done?

5. Is water a combustible? Is it a supporter of combustion? When water is turned into steam, does the steam burn or support combustion? Try this by putting a burning match into the steam that comes from a teakettle in which water is boiling hard. Tell how these facts explain why water puts out a fire.

6. Why does a blanket or rug put out a fire?

7. If your clothes are set on fire, ought you to run outdoors to get help? Why? What ought you to do?

8. Does an old iron stove ever show any signs of being partly burned up?

CHAPTER V

OXYGEN, THE FIRE GAS

30. How Can We Prepare Oxygen?—You would naturally think that the easiest way to get oxygen unmixed with nitrogen would be to remove the nitrogen of the air. But we must remember that the oxygen is the active gas, and that burning substances, like a candle or

phosphorus, always
combine with the oxy-
gen and leave the
nitrogen. So to get
oxygen we must use
chemicals that consist
partly of oxygen and
we must get the oxy-
The
gen out of these.
most common way to

get oxygen is to heat

a mixture of two substances called potassium chlorate and manganese dioxide. The potassium chlorate is a white solid; the manganese dioxide is a black one.

The apparatus used is shown in Fig. 21. To fill three or four 8-ounce (250 cc.) bottles with the gas we use about of a test tube of potassium chlorate and about of a test tube of manganese dioxide. Each should be in the form of a fine powder and the two should be mixed carefully on a clean, smooth sheet of paper. By means of a one-hole stopper

we connect the test tube containing the mixture with a "delivery tube that carries the gas to the water pan.

To heat the mixture we use a very small, smoky flame (a candle flame will do); bubbles of oxygen will soon escape from the end of the delivery tube. If a bottle full of water is placed, mouth downward, over the end of the delivery tube, the oxygen rises into the bottle and pushes out the water. We thus collect the gas over water," as we collected air in § 9, Fig. 4.

66

When a bottle is full of oxygen, we slip under it a piece of cardboard, or of sheet glass, and then set it, right side upward, on the table. We can then fill other bottles with the gas. If the gas is to be kept for a day or two, the bottle should be stoppered tightly. Pint fruit jars with rubber rings and tight covers make excellent storage vessels for gases.

When we have collected enough oxygen, or when all has come off that will, we remove the delivery tube from the water; then, and not until then, should we take away the flame. The reason for this care is that as the gas in the test tube cools, it contracts (cf. § 12); if cold water from the water pan is forced into the hot tube, the tube may break.

We shall want to use the bottles of oxygen in the next section and in Chapter VI.

FIG. 22. When hydrogen peroxide solu

tion is added to manganese dioxide, oxygen is given off.

Another way of getting oxygen is shown in Fig. 22. For this we need only a glass bottle and a loose cover, such as a piece of sheet glass. Into the bottle we put manganese dioxide (or, better, potassium permanganate) and enough water to just cover the solid; then we add some hydrogen peroxide solution. No heating is needed. The mixture bubbles, or foams, as the oxygen rises through the water. The oxygen pushes the air out of the bottle; soon we have the bottle full of oxygen.

31. What is Oxygen Like? We are now ready to learn some of the properties, or qualities, of this interesting gas. We can see, by looking at the bottles of oxygen, especially after they have stood for a few minutes, that oxygen has no more color than air has. Perfectly

pure oxygen has neither odor nor taste.

If we light a pine splinter, and put it for an instant into a jar of oxygen, the wood burns much more vigor

FIG. 23. -a. When burning sulphur is put into oxygen, it burns with a brilliant, purple flame. b. Iron wire burns in oxygen with a brilliant light

and a shower of tiny sparks.

ously than in air. If we blow out the flame, and put the glowing splinter into the bottle, the glow becomes very intense and the splinter bursts into a flame. In this way we can tell a bottle of oxygen from one of air.

A burning candle burns more vigorously when put into oxygen. Try it, if possible. To burn sulphur in oxygen, we hold it in a combustion spoon (cf. § 24). We light the sulphur by holding the bowl of the spoon in a flame, and then put the spoon into a bottle of oxygen (Fig. 23, a). In the air, sulphur burns with a pale, almost colorless flame, but in oxygen the flame is a brilliant, purple one. By smelling cautiously of the gas formed in the bottle we learn that it has the same sharp odor as the gas formed when sulphur burns in air. We call it sulphur dioxide.

We do not see iron burn, ordinarily, in air, but in oxygen it burns (Fig. 23, b) with a brilliant light and a shower of tiny sparks. The best way to show the burning of iron in oxygen is to use a piece of picture cord made up of fine strands of iron wire and to put on one end of the cord a tip of melted sulphur or the head of a match; we then light the sulphur, or match head, and put the wire at once into the jar of oxygen. The shiny, black lump formed on the end of the wire is made up of iron and oxygen; we call it iron oxide.

oxide "

We

(pro

32. What is Oxidation? Are you ready now to learn. the meaning of some important words? We use the word 66 oxidation" so often in science and even in our daily life, that we should get some idea of what it means. see at once that the word comes from nounced ox'id) and that oxide comes from oxygen." Oxidation means uniting with oxygen; the substance that unites with oxygen is said to be oxidized. Thus the iron burning in oxygen is oxidized to iron oxide. When lead is heated in air, it is oxidized to lead oxide. The white smoke formed when phosphorus burns in air (cf. § 25) is called phosphorus oxide. When coal and charcoal, which consist chiefly of carbon, are burned, the oxide formed is called carbon dioxide (cf. § 37). In this, as in sulphur dioxide, the syllable "di," meaning two, is put before "oxide."

33. Why Does Paint Harden? - What has paint to do with oxidation? Perform the following experiment:

Wet the inside of a test tube with linseed oil and then let most of the oil drain out of the tube. Now set the test tube upside down in