Air

Air is necessary for the support of life and combustion. It is a mixture, not a compound; that is, the substances of which it is composed are not chemically combined.

It has definite weight. At the sea level the atmosphere exerts a pressure of fifteen pounds on every square inch of surface, but as the pressure is equal in every direction, we are not crushed by this weight.

Air is composed of about one-fifth oxygen and four-fifths nitrogen. There are present also varying amounts of carbon dioxide (C02), watery vapor, ammonia, dust, bacteria, etc. The nitrogen is not in a form in which it can be utilized by either plants or animals. Upon the roots of certain plants, as clover, peas, beans, etc., are found nodules containing bacteria which have the power of taking nitrogen from the air and changing it to such compounds of nitrogen as can be utilized by the plant. From these compounds the plant builds complex nitrogenous compounds which it stores in its cells as protein.

The oxygen in the air supports combustion.

Experiment 1

Light a splint and insert it in a dry, empty test tube. Is the flame extinguished? Why?

Experiment 2

Put a lamp chimney over a lighted candle. Admit air from the bottom. Does the candle burn freely?

Hold a strip of tissue paper near the bottom of the chimney, and also above the chimney. Explain the cause of movement of the paper. Cover the chimney on top and also exclude air from the bottom. Does the candle continue to burn? Give reason.

Experiment 3

(To be performed by the instructor.) Float a cork with a small piece of phosphorus on it in a pan of water. Ignite the phosphorus and cover it quickly with a bell jar. (A fruit jar may be used.)

With what does the phosphorus unite in burning to form the dense white fume?

What becomes of this fume as it disappears?

What part of the jar is filled with water?

What component of the air has been burned out ? What component of the air remains in the jar, preventing the water from filling the entire jar?

What proportion of the air is oxygen? nitrogen ?

Experiment 4

To examine air for dust, bacteria, etc.

Put a drop of glycerine on a clean glass microscope slide. Expose to the dust of the room until the next lesson. Cover the glycerine with a cover glass and examine under a microscope, first with a low power and then with a high power. Make a drawing of what you see.

Oxidation And Combustion

Oxidation, or combustion, is the union of oxygen with any other substance. The process may be slow or rapid, but in either case heat is given off, even though not rapidly enough to be perceptible. Example : the rusting of iron is really the burning of iron, but the process is so slow that we do not detect the heat given off.

Fuels are composed largely of carbon and hydrogen which, in burning, unite with oxygen of the air giving off heat.