Common Science

by Carleton Washburne

Fig. 167. The water rises in the bottle after the burning candle uses up the oxygen.

Carbon and hydrogen the chief elements in fuel. Carbon and hydrogen make up the larger part of almost every substance that is used for fuel, including gas, gasoline, wood, and soft coal; alcohol, crude oil, kerosene, paper, peat, and the acetylene used in automobile and bicycle lamps. Hard coal, coke, and charcoal are, however, chiefly plain carbon. Since burning is simply the combining of things with oxygen, it is plain that when the carbon of fuel joins oxygen we shall get carbon dioxid (CO₂). When the hydrogen in the fuel joins oxygen, what must we get?

When things do not burn up completely, the carbon may be left behind as charcoal. That is what happens when food "burns" on the stove. But if anything burns up entirely, the carbon or charcoal burns too, passing off as the invisible gas, carbon dioxid, just as the hydrogen burns to form steam or water.

It is because almost every fuel forms water when it burns, that we find drops of water gathering on the outside of a cold kettle or cold flatiron if either is put directly over a flame. The hydrogen in the fuel combines with the oxygen of the air to form steam. As the steam strikes the cold kettle or iron, it condenses and forms drops of water.

Nothing ever destroyed. One important result of the discovery that burning is only a combining of oxygen with the fuel was that people began to see that nothing is ever destroyed. There is exactly as much carbon in the carbon dioxid that floats off from a fire as there was in the wood that was burned up; and there is exactly as much hydrogen in the water vapor that floats off from the fire as there was in the wood. Chemists have caught all the carbon dioxid and the water vapor and weighed them and added their weight to the weight of the ashes; and they have found them to weigh even more than the original piece of wood, because of the presence of the oxygen that combined with them in the burning.

If everything in the world were to burn up, using the oxygen that is already here, the world would not weigh one ounce more or less than it does now. All the elements that were here before would still be here; but they would be combined in different compounds. Instead of wood and coal and oxygen we should have water and carbon dioxid; instead of diamonds, we should have just carbon dioxid; and so on with everything that can burn.

Why water puts out a fire. Water puts out a fire because it will not let enough free oxygen get to the wood, or whatever is burning, to combine with it. The oxygen that is locked up in a compound, like water, you remember, has lost its ability to combine with other things. Sand puts out a fire in the same way that water does. Most fire extinguishers make a foam of carbon dioxid (CO₂) which covers the burning material and keeps the free oxygen in the air from coming near enough to combine with it.

Water will not put out burning oil, however, as the oil floats up on top of the water and still combines with the oxygen in the air.

Why electric lamps are usually vacuums. Electric lamps usually have vacuums inside because the filament gets so hot that it would burn up if there were any oxygen to combine with it. But in a globe containing no oxygen the filament may be made ever so hot and it cannot possibly burn.