WHAT CHEMISTRY IS.
6. Divisibility of Matter.
Experiment 4.--Examine a few crystals of sugar, and crush them
with the fingers. Grind them as fine as convenient, and examine
with a lens. They are still capable of division. Put 3 g. of
sugar into a t.t., pour over it 5 cc. of water, shake well, boil
for a minute, holding the t.t. obliquely in the flame, using for
the purpose a pair of wooden nippers (Fig. 3). If the sugar does
not disappear, add more water. When cool, touch a drop of the
liquid to the tongue. Evidently the sugar remains, though in a
state too finely divided to be seen. This is called a solution,
the sugar is said to be soluble in water, and water to be a
solvent of sugar.
(Fig 3.)
Now fold a filter paper, as in Figure 4, arrange it in a funnel
(Fig. 5), and pour the solution upon it, catching what passes
through, which is called the filtrate, in another t.t. that rests
in a receiver (Fig. 5). After filtering, notice whether any
residue is left on the filter paper. Taste a drop of the
filtrate. Has sugar gone through the filter? If so, what do you
infer of substances in solution passing through a filter? Save
half the filtrate for Experiment 5, and dilute the other half
with two or three times its own volume of water. Shake well, and
taste.
(Fig 4.)
(Fig 5.) We might have diluted the sugar solution many times
more, and still the sweet taste would have remained. Thus the
small quantity of sugar would be distributed through the whole
mass, and be very finely divided.
By other experiments a much finer subdivision can be made. A
solution of.00000002 g. of the red coloring matter, fuchsine, in
1 cc. of alcohol gives a distinct color.
Such experiments would seem to indicate that there is no limit to
the divisibility of matter. But considerations which we cannot
discuss here lead to the belief that such a limit does exist;
that there are particles of sugar, and of all substances, which
are incapable of further division without entirely changing the
nature of the substance. To these smallest particles the name
molecules is given.
A mass is any portion of a substance larger than a molecule; it
is an aggregation of molecules.
A molecule is the smallest particle of a substance that can exist
alone.
A substance in solution may be in a more finely divided state
than otherwise, but it is not necessarily in its ultimate state
of division.
7. A Chemical Change.--Cannot this smallest particle of sugar,
the molecule, be separated into still smaller particles of
something else? May it not be a compound body, and will not some
force separate it into two or more substances? The next
experiment will answer the question.
Experiment 5.--Take the sugar solution saved from Experiment 4,
and add slowly 4 cc.of strong sulphuric acid. Note any change of
color, also the heat of the t.t. Add more acid if needed.
A substance entirely different in color and properties has been
formed. Now either the sugar, the acid, or the water has
undergone a chemical change. It is, in fact, the sugar. But the
molecule is the smallest particle of sugar possible. The acid
must have either added something to the sugar molecules, or
subtracted something from them. It was the latter. Here, then, is
a force entirely different from the one which tends to reduce
masses to molecules. The molecule has the same properties as the
mass. Only a physical force was used in dissolving the sugar, and
no heat was liberated. The acid has changed the sugar into a
black mass, in fact into charcoal or carbon, and water; and heat
has been produced. A chemical change has been brought about.
From this we see that molecules are not the ultimate divisions of
matter. The smallest sugar particles are made up of still smaller
particles of other things which do not resemble sugar, as a word
is composed of letters which alone do not resemble the word. But
can the charcoal itself be resolved into other substances, and
these into still others, and so on? Carbon is one of the
substances from 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.