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List Of Contents | Contents of An Introduction to Chemical Science
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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.

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 case, and that they
do not touch one another, the same thing being true of molecules.
Atoms composing sugar must be crowded nearer together than those
of salt. These atoms are probably in constant motion in the
molecule, as the latter is in the mass. If we regard this square
as a mass of matter, the dots may represent molecules; if we call
it a molecule, the dots may be called atoms, though many
molecules have no more than two or three atoms.

The following experiments illustrate the union of atoms to form
molecules, and of elements to form compounds.

12. Union of Atoms.

Experiment 6.--Mix, on a paper, 5 g. of iron turnings, and the
same bulk of powdered sulphur, and transfer them to an ignition
tube, a tube of hard glass for withstanding high temperatures.
Hold the tube in the flame of a burner till the contents have
become red-hot. After a minute break it by holding it under a jet
of water. Put the contents into an evaporating-dish, and look for
any uncombined iron or sulphur. Both iron and sulphur are
elements. Is this an example of synthesis or of analysis? Why? Is
the chemical union between masses of iron and sulphur, or between
molecules, or between atoms? Is the product a compound, an
element, or a mixture?

Experiment 7.--Try the same experiment, using copper instead of
iron. The full explanation of these experiments is given on page
13.

CHAPTER IV.

ELEMENTS AND BINARIES.

13. About Seventy Different Elements are now recognized, half of
which have been discovered within little more than a century.
These differ from one another in (1) atomic weight, (2) physical
and chemical properties, (3) mode of occurrence, etc. Page 12
contains the most important elements.

The symbol of an element is usually the initial letter or letters
of its Latin name, and stands for one atom of the element. C is
the symbol for carbon, and represents one atom of it. O means one
atom of oxygen.[The symbols of elements will also be used in this
book to stand for an indefinite quantity of them; e.g. O will be
used for oxygen in general as well as for one atom. The text will
readily decide when symbols have a definite meaning, and when
they are used in place of words.] Write, explain, and memorize
the symbols of the elements in heavy type.

14. The Atomic Weight of an element is the weight of its atom
compared with that of hydrogen. H is taken as the standard
because it has the least atomic weight. The atomic weight of O is
16, which means that its atom weighs 16 times as much as the H
atom. Every symbol, then, stands for a definite weight of the
element, i.e. its atomic weight, as well as for its atom.

How much bromine by weight does Br stand for? What do these
symbols mean--As, Na, N, P? If O represents one atom, how much
does O2 or 2 O stand for? How much by weight? Most elements have
two atoms in the molecule. How many molecules in 6 H? 10 N? S8?
I20?

The symbol of a compound is formed by writing in succession the
symbols of the elements of which it is composed. How many atoms
in the following molecules, and how many of each element: C2H60?
HNO3? PbSO4? MgCl2? (Hg2(NO3)2?)

15. The Simplest Compounds are Binaries.--A binary is a substance
composed of two elements; e.g. common salt, which is a compound
of sodium and chlorine. Its symbol is NaCl, its chemical name
sodium chloride. The ending ide is applied to the last name of
binaries. How many parts by weight of Na and of Cl in NaCl? What
is the molecular weight, i.e. the weight of its molecule? Name
KCl. How many atoms in its molecule? Parts by weight of each
element? Molecular weight? Does the symbol stand for more than
one molecule? How many molecules in 4 NaCl? How many atoms of Na
and of Cl? Name these: HCl, NaBr, NaI, KBr, AgCl, AgI, HBr, HI,
HF, HgO, ZnO, ZnS, MgO, CaO. Compute the proportion by weight of
each element in the last three.

A coefficient before the symbol of a compound includes all the
elements of the symbol, and shows the number of molecules. How
many in these: 6 KBr? 3 Sn0? 12 NaCl? How many atoms of each
element in the above?

An exponent, always written below, applies only to the element
after which it is written, and shows the number of atoms. Explain
these: AuCl3, ZnCl2, Hg2Cl2.

Write symbols for four molecules of sodium bromide, one of silver
iodide (always omit coefficient one), eight of potassium bromide,
ten of hydrogen chloride; also for one molecule of each of these:
hydrogen fluoride, potassium iodide, silver chloride.

In all the above cases the elements have united atom for atom.
Some elements will not so unite. In CaCl2 how many atoms of each
element? Parts by weight of each? Give molecular weight. Is the
size of the molecule thereby changed? Name these, give the number
of atoms of each element in the molecule, and the proportion by
weight, also their molecular weights: AuCl3, ZnCl2, MnCl2, Na2O,
K2S, H3P, H4C.

Principal Elements.
Name.	   Sym. At. Wt. Valence.  Vap.D.  At.Vol.   Mol.Vol. State.
Aluminium  Al	  27.   II, IV     ...       ...        ...   Solid

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