INVISIBLE OBJECTS
94. Very Small Objects. We saw in Section 84 that gases have a
tendency to expand, but that they can be compressed by the application
of force. This observation has led scientists to suppose that
substances are composed of very minute particles called molecules,
separated by small spaces called pores; and that when a gas is
condensed, the pores become smaller, and that when a gas expands, the
pores become larger.
The fact that certain substances are soluble, like sugar in water,
shows that the molecules of sugar find a lodging place in the spaces
or pores between the molecules of water, in much the same way that
pebbles find lodgment in the chinks of the coal in a coal scuttle. An
indefinite quantity of sugar cannot be dissolved in a given quantity
of liquid, because after a certain amount of sugar has been dissolved
all the pores become filled, and there is no available molecular
space. The remainder of the sugar settles at the bottom of the vessel,
and cannot be dissolved by any amount of stirring.
If a piece of potassium permanganate about the size of a grain of sand
is put into a quart of water, the solid disappears and the water
becomes a deep rich red. The solid evidently has dissolved and has
broken up into minute particles which are too small to be seen, but
which have scattered themselves and lodged in the pores of the water,
thus giving the water its rich color.
There is no visible proof of the existence of molecules and molecular
spaces, because not only are our eyes unable to see them directly, but
even the most powerful microscope cannot make them visible to us. They
are so small that if one thousand of them were laid side by side, they
would make a speck too small to be seen by the eye and too small to be
visible under the most powerful microscope.
We cannot see molecules or molecular pores, but the phenomena of
compression and expansion, solubility and other equally convincing
facts, have led us to conclude that all substances are composed of
very minute particles or molecules separated by spaces called pores.
95. Journeys Made by Molecules. If a gas jet is turned on and not
lighted, an odor of gas soon becomes perceptible, not only throughout
the room, but in adjacent halls and even in distant rooms. An uncorked
bottle of cologne scents an entire room, the odor of a rose or violet
permeates the atmosphere near and far. These simple everyday
occurrences seem to show that the molecules of a gas must be in a
state of continual and rapid motion. In the case of the cologne, some
molecules must have escaped from the liquid by the process of
evaporation and traveled through the air to the nose. We know that the
molecules of a liquid are in motion and are continually passing into
the air because in time the vessel becomes empty. The only way in
which this could happen would be for the molecules of the liquid to
pass from the liquid into the surrounding medium; but this is really
saying that the molecules are in motion.
From these phenomena and others it is reasonably clear that substances
are composed of molecules, and that molecules are not inert, quiet
particles, but that they are in incessant motion, moving rapidly
hither and thither, sometimes traveling far, sometimes near. Even the
log of wood which lies heavy and motionless on our woodpile is made
up of countless billions of molecules each in rapid incessant motion.
The molecules of solid bodies cannot escape so readily as those of
liquids and gases, and do not travel far. The log lies year after year
in an apparently motionless condition, but if one's eyes were keen
enough, the molecules would be seen moving among themselves, even
though they cannot escape into the surrounding medium and make long
journeys as do the molecules of liquids and gases.
96. The Companions of Molecules. Common sense tells us that a
molecule of water is not the same as a molecule of vinegar; the
molecules of each are extremely small and in rapid motion, but they
differ essentially, otherwise one substance would be like every other
substance. What is it that makes a molecule of water differ from a
molecule of vinegar, and each differ from all other molecules? Strange
to say, a molecule is not a simple object, but is quite complex, being
composed of one or more smaller particles, called atoms, and the
number and kind of atoms in a molecule determine the type of the
molecule, and the type of the molecule determines the substance. For
example, a glass of water is composed of untold millions of molecules,
and each molecule is a company of three still smaller particles, one
of which is called the oxygen atom and two of which are alike in every
particular and are called hydrogen atoms.
97. Simple Molecules. Generally molecules are composed of atoms
which are different in kind. For example, the molecule of water has
two different atoms, the oxygen atom and the hydrogen atoms; alcohol
has three different kinds of atoms, oxygen, hydrogen, and carbon.
Sometimes, however, molecules are composed of a group of atoms all of
which are alike. Now there are but seventy or eighty different kinds
of atoms, and hence there can be but seventy or eighty different
substances whose molecules are composed of atoms which are alike. When
the atoms comprising a molecule are all alike, the substance is called
an element, and is said to be a simple substance. Throughout the
length and breadth of this vast world of ours there are only about
eighty known elements. An element is the simplest substance
conceivable, because it has not been separated into anything simpler.
Water is a compound substance. It can be separated into oxygen and
hydrogen.
Gold, silver, and lead are examples of elements, and water, alcohol,
cider, sand, and marble are complex substances, or compounds, as we
are apt to call them. Everything, no matter what its size or shape or
character, is formed from the various combinations into molecules of a
few simple atoms, of which there exist about eighty known different
kinds. But few of the eighty known elements play an important part in
our everyday life. The elements in which we are most interested are
given in the following table, and the symbols by which they are known
are placed in columns to the right:
|Oxygen |O |Copper |Cu |Phosphorus |P |
|Hydrogen |H |Iodine |I |Potassium |K |
|Carbon |C |Iron |Fe |Silver |Ag |
|Aluminium Al |Lead |Pb |Sodium |Na | |
|Calcium |Ca |Nickel |Ni |Sulphur |S |
|Chlorine |Cl |Nitrogen |N |Tin |Sn |
We have seen in an earlier experiment that twice as much hydrogen as
oxygen can be obtained from water. Two atoms of the element hydrogen
unite with one atom of the element oxygen to make one molecule of
water. In symbols we express this H_2O. A group of symbols, such as
this, expressing a molecule of a compound is called a _formula_. NaCl
is the formula for sodium chloride, which is the chemical name of
common salt.