Man's physical universe

xanabras

UNIT IX

SECTION 3

COMBUSTION FURNISHES THE ENERGY NECESSARY TO

TRANSFORM MANY MINERALS INTO USEFUL MATERIALS

Photosynthesis, to be studied in the next Unit, stores the radiant

energy of the sunHght in the forms of wood, coal, petroleum, and

natural gas. The process by which this stored energy is released, in

which carbon compounds react with oxygen to produce carbon dioxide,

is called combustion. Combustion is not limited to the oxidation

of carbon compounds, however, for any oxidation reaction which produces

heat and light is called burning, or combustion.

The Discovery of Oxygen Revealed the True Nature of Combustion.

In 1771 Carl Wilhelm Scheele (1742-1786) first prepared oxygen by

heating certain substances. Three years later Joseph Priestley in England,

ignorant of Scheele's discovery, prepared oxygen by heating

mercuric oxide by focusing the sun's rays on it by means of a "burning

glass." Although Priestley discovered the clue to the true nature

of combustion, he refused to give up the then current phlogiston

theory, and it remained for the great French chemist, Antoine Laurent

Lavoisier (1743-1794), when he learned of Priestley's experiments, to

realize that combustion is simply the reaction of materials with the

oxygen of the air.

Oxygen Is the Most Abundant and Universally Active Element.

Fully half of the earth's crust is oxygen. It occurs uncombined in

the air to the extent of about one fifth of the total volume, as pointed

out in an earlier section.

The chief chemical characteristic of oxygen is its great readiness to

combine with nearly all of the other elements, including both metals

and nonmetals.

Not All Oxidation Reactions Take Place Rapidly Enough to Produce

Combustion.

Most of the reactions which yield the same products as combustion

produced and the

reactions may take place so slowly that no light is

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