Man's physical universe

xanabras

THE MATERIALS OF THE UNIVERSE 559

Such changes as the charring of sugar and the combination of iron

and sulfur, in which compounds decompose to form simpler substances

or in which simple substances combine to form compounds, are called

chemical changes. Chemical changes produce changes in chemical

properties, and chemical properties are those properties which cannot be

expressed without referring to a change in the composition of matter.

Every chemical change either stores energy or sets free stored energy.

the next Section we shall learn that chemical changes are not merely

qualitative, i.e., that they not only represent changes in the kind of

materials present, but that they are also quantitative — they always

involve definite weights of reactants and products.

The Occurrence and Classification of the Elements.

Only about two dozen of the ninety-two elements are used to a very

large extent. The following table shows that 99 per cent of the terrestrial

matter is made up of about twelve elements.

Chemical Composition of Terrestrial Matter — Earth, Air,

AND Sea — IN Percentage

Oxygen 50.02 Potassium 2.28

Silicon 25.80 Magnesium 2.08

Aluminum 7.30 Hydrogen 0.95

Iron ....... 4.18 Titanium 0.43

Calcium 3.22 Chlorine 0.20

Sodium 2.36 Carbon 0.18

Some of the elements have never been prepared in a pure state. Others

have no known uses. Still others are so rare that they cannot be used

until more abundant sources can be located.

The elements may be classified as metals, nonmetals, and metalloids.

The distinctive properties of these different classes cannot be given

yet, but metals, with the exception of mercury, are solids under ordinary

conditions and are good conductors of heat and electricity; they

have a metallic luster in a coherent state and are relatively malleable

and ductile. The nonmetallic elements include solids, liquids, and

gases, and in general do not exhibit the properties common to metals.

The metalloids are elements whose properties are intermediate between

those of metals and nonmetals.

How the Elements Were Named.

Inasmuch as the elements have been discovered and named one

after another during a period of several thousand years, it is strange

that any system in naming exists at all; and this system probably

would not exist if it were not for the fact that most of these elements

have been discovered during the past century.

In

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