An Introduction to Chemical Science | Page 8

which nothing else has
been obtained. There are about seventy others which have not been
resolved. These are called elements; and out of them are built all the
compounds-- mineral, vegetable, and animal--which we know.
8. An element is a chemically indivisible substance, or one from which
nothing else can be extracted.
A compound is a substance which is made up of elements united in
exact proportions by a force called chemism, or chemical affinity.
A mixture is composed of two or more elements or compounds blended
together, but not held by any chemical attraction.
To which of these three classes does sugar belong? Carbon? The
solution of sugar in water?

Carbon is an element; we call its smallest particle an atom.
An atom is the smallest particle of an element that can enter into
combination. Atoms are indivisible and usually do not exist alone. Both
elements and compounds have molecules.
The molecule of an element usually contains two atoms; that of a
compound may have two, or it may have hundreds. For a given
compound the number is always definite.
Chemism is the force that binds atoms together to form molecules. The
sugar molecule contains atoms, forty-five in all, of three different
elements: carbon, hydrogen, and oxygen. That of salt has two atoms:
one of sodium, one of chlorine. Should we say "an atom of sugar"?
Why? Of what is a mass of sugar made up? A molecule? A mass of
carbon? A molecule? Did the chemical affinity of the acid break up
masses or molecules? In this respect it is a type of all chemical action.
The distinction between physics and chemistry is here well shown. The
molecule is the unit of the physicist, the atom that of the chemist.
However large the masses changed by chemical action, that action is
always on the individual molecule, the atoms of which are separated. If
the molecule were an indivisible particle, no science of chemistry
would be possible. The physicist finds the properties of masses of
matter and resolves them into molecules, the chemist breaks up the
molecule and from its atoms builds up other compounds.
Analysis is the separation of compounds into their elements.
Synthesis is the building up of compounds from their elements.
Of which is the sugar experiment an example? Metathesis is an
exchange of atoms in two different compounds; it gives rise to still
other compounds.
A chemical change may add something to a substance, or subtract
something from it, or it may both subtract and add, making a new
substance with entirely different properties. Sulphur and carbon are two
stable solids. The chemical union of the two forms a volatile liquid. A
substance may be at one time a solid, at another a liquid, at another a
gas, and yet not undergo any chemical change, because in each case the
chemical composition is identical.
State which of these are chemical changes: rusting of iron, falling of
rain, radiation of heat, souring of milk, evaporation of water, decay of
vegetation, burning of wood, breaking of iron, bleaching of cloth. Give

any other illustrations that occur to you.
Chemistry treats of matter in its simplest forms, and of the various
combinations of those simplest forms.



CHAPTER III
.
MOLECULES AND ATOMS.
9. Molecules are Extremely Small.--It has been estimated that a liter of
any gas at 0 degrees and 760 mm. pressure contains 10^24 molecules,
i.e. one with twenty-four ciphers.
Thomson estimates that if a drop of water were magnified to the size of
the earth, and its molecules increased in the same proportion, they
would be larger than fine shot, but not so large as cricket balls.
A German has recently obtained a deposit of silver two-millionths of a
millimeter thick, and visible to the naked eye. The computed diameter
of the molecule is only one and a half millionths of a millimeter.
By a law of chemistry there is the same number of molecules in a given
volume of every gas, if the temperature and pressure are the same.
Hence, all gaseous molecules are of the same size, including, of course,
the surrounding space. They are in rapid motion, and the lighter the gas
the more rapid the motion. This gives rise to diffusion. See page 114.
10. We Know Nothing Definite of the Form of Molecules.--In this
book they will always be represented as of the same size, that of two
squares. A molecule is itself composed of atoms,--from two to several
hundred. The size of the atom of most elements we represent by one
square.11. Atoms.--If the gaseous molecules be of the same size, it is
clear that either the atoms themselves must be condensed, or the spaces
between them must be smaller than before. We suppose the latter to be
the