Fig. 170. The air openings in the front of a gas stove.
Inference Exercise
Explain the following:
441. Iron tracks are welded together with an electric arc.
442. The cool mirror in a bathroom becomes covered with moisture when you take a hot bath.
443. This prevents you from seeing yourself in the mirror.
444. Carbon dioxid has oxygen in it, yet a burning match dropped into a bottle of it will go out.
445. A ship that sinks to the bottom of the ocean does not decay.
446. When women put their hair in curlers, they usually moisten the hair slightly.
447. To dry a pan after washing it, a person often sets it on the hot stove for a few minutes.
448. When you put a kettle of cold water over a gas flame, drops of water appear on the lower part of the sides of the kettle.
449. Electric power plants are often situated where running water will turn the dynamo. Explain the necessity of turning the dynamo.
450. We make carbon dioxid by burning carbon, but you cannot put different things together to make carbon.
Section 48. Chemical change caused by heat.
Why do you have to strike a match to make it burn?
How does pulling the trigger make a gun go off?
What makes cooked foods taste different from raw ones?
Has it struck you as strange that we do not all burn up, since burning is a combining with oxygen, and we are walking around in oxygen all the time? The only reason we do not burn up is that it usually requires heat to start a chemical change. You already know this in a practical way. You know that you have to rub the head of a match and get it hot before it will begin to burn; that gunpowder does not go off unless you heat it by the sudden blow of the gun hammer which you release when you pull the trigger; that you have to concentrate the sun's rays with a magnifying glass to make it set a piece of paper on fire; and that to change raw food into food that tastes pleasant you have to heat it. If heat did not start chemical change, you could never cook food,—partly because the fire would not burn, and partly because the food would not change its taste even if heated by electricity or concentrated sunlight.
Here is an experiment to show that gas will not burn unless it gets hot enough:
Experiment 97. Hold a wire screen 2 or 3 inches above the mouth of a Bunsen burner. Turn on the gas and light a match, holding the lighted match above the screen. Why, do you suppose, does the gas below the screen not burn? Hold a lighted match to the gas below the screen. Does it burn now?