An Elementary Study of Chemistry

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~Self-purification of water.~ It has long been known that water
contaminated with organic matter tends to purify itself when exposed to
the air. This is due to the fact that the water takes up a small amount
of oxygen from the air, which gradually oxidizes the organic matter
present in the water. While water is undoubtedly purified in this way,
the method cannot be relied upon to purify a contaminated water so as to
render it safe for drinking purposes.

~Physical properties.~ Pure water is an odorless and tasteless liquid,
colorless in thin layers, but having a bluish tinge when observed
through a considerable thickness. It solidifies at 0 deg. and boils at 100 deg.
under the normal pressure of one atmosphere. If the pressure is
increased, the boiling point is raised. When water is cooled it steadily
contracts until the temperature of 4 deg. is reached: it then expands. Water
is remarkable for its ability to dissolve other substances, and is the
best solvent known. Solutions of solids in water are more frequently
employed in chemical work than are the solid substances, for chemical
action between substances goes on more readily when they are in solution
than it does when they are in the solid state.

~Chemical properties.~ Water is a very stable substance, or, in other
words, it does not undergo decomposition readily. To decompose it into
its elements by heat alone requires a very high temperature; at 2500 deg.,
for example, only about 5% of the entire amount is decomposed. Though
very stable towards heat, water can be decomposed in other ways, as by
the action of the electrical current or by certain metals.

~Heat of formation and heat of decomposition are equal.~ The fact
that a very high temperature is necessary to decompose water
into hydrogen and oxygen is in accord with the fact that a
great deal of heat is evolved by the union of hydrogen and
oxygen; for it has been proved that the heat necessary to
decompose a compound into its elements (heat of decomposition)
is equal to the heat evolved in the formation of a compound
from its elements (heat of formation).

~Water of crystallization.~ When a solid is dissolved in water and the
resulting solution is allowed to evaporate, the solid separates out,
often in the form of crystals. It has been found that the crystals of
many compounds, although perfectly dry, give up a definite amount of
water when heated, the substance at the same time losing its crystalline
form. Such water is called _water of crystallization_. This varies in
amount with different compounds, but is perfectly definite for the same
compound. Thus, if a perfectly dry crystal of copper sulphate is
strongly heated in a tube, water is evolved and condenses on the sides
of the tube, the crystal crumbling to a light powder. The weight of the
water evolved is always equal to exactly 36.07% of the weight of copper
sulphate crystals heated. The water must therefore be in chemical
combination with the substance composing the crystal; for if simply
mixed with it or adhering to it, not only would the substance appear
moist but the amount present would undoubtedly vary. The combination,
however, must be a very weak one, since the water is often expelled by
even a gentle heat. Indeed, in some cases the water is given up on
simple exposure to air. Such compounds are said to be _efflorescent_.
Thus a crystal of sodium sulphate (Glauber's salt) on exposure to air
crumbles to a fine powder, owing to the escape of its water of
crystallization. Other substances have just the opposite property: they
absorb moisture when exposed to the air. For example, if a bit of dry
calcium chloride is placed in moist air, in the course of a few hours it
will have absorbed sufficient moisture to dissolve it. Such substances
are said to be _deliquescent_. A deliquescent body serves as a good
drying or _desiccating_ agent. We have already employed calcium chloride
as an agent for absorbing the moisture from hydrogen. Many substances,
as for example quartz, form crystals which contain no water of
crystallization.

~Mechanically inclosed water.~ Water of crystallization must be
carefully distinguished from water which is mechanically
inclosed in a crystal and which can be removed by powdering the
crystal and drying. Thus, when crystals of common salt are
heated, the water inclosed in the crystal is changed into steam
and bursts the crystal with a crackling sound. Such crystals
are said to _decrepitate_. That this water is not combined is
proved by the fact that the amount present varies and that it
has all the properties of water.

~Uses of water.~ The importance of water in its relation to life and
commerce is too well known to require comment. Its importance to the
chemist has also been pointed out. It remains to call attention to the
fact that it is used as a standard in many physical measurements. Thus
0 deg. and 100 deg. on the centigrade scale are respectively the freezing and
the boiling points of water under normal pressure. The weight of 1 cc.
of water at its point of greatest density is the unit of weight in the
metric system, namely, the gram. It is also taken as the unit for the
determination of the density of liquids and solids as well as for the
measurement of amounts of heat.

HYDROGEN DIOXIDE

~Composition.~ As has been shown, 1 part by weight of hydrogen combines
with 7.94 parts by weight of oxygen to form water. It is possible,
however, to obtain a second compound of hydrogen and oxygen differing
from water in composition in that 1 part by weight of hydrogen is
combined with 2 x 7.94, or 15.88 parts, of oxygen. This compound is
called _hydrogen dioxide_ or _hydrogen peroxide_, the prefixes _di-_ and
_per-_ signifying that it contains more oxygen than hydrogen oxide,
which is the chemical name for water.

~Preparation.~ Hydrogen dioxide cannot be prepared cheaply by the direct
union of hydrogen and oxygen, and indirect methods must therefore be
used. It is commonly prepared by the action of a solution of sulphuric
acid on barium dioxide. The change which takes place may be indicated as
follows:

sulphuric acid + barium dioxide = barium sulphate + hydrogen dioxide
-------------- -------------- --------------- ----------------
hydrogen barium barium hydrogen
sulphur oxygen sulphur oxygen
oxygen oxygen

In other words, the barium and hydrogen in the two compounds exchange
places. By this method a dilute solution of the dioxide in water is
obtained. It is possible to separate the dioxide from the water by
fractional distillation. This is attended with great difficulties,
however, since the pure dioxide is explosive. The distillation is
carried on under diminished pressure so as to lower the boiling points
as much as possible; otherwise the high temperature would decompose the
dioxide.

~Properties.~ Pure hydrogen dioxide is a colorless sirupy liquid having a
density of 1.49. Its most characteristic property is the ease with which
it decomposes into water and oxygen. One part by weight of hydrogen is
capable of holding firmly only 7.94 parts of oxygen. The additional 7.94
parts of oxygen present in hydrogen dioxide are therefore easily
evolved, the compound breaking down into water and oxygen. This
decomposition is attended by the generation of considerable heat. In
dilute solution hydrogen dioxide is fairly stable, although such a
solution should be kept in a dark, cool place, since both heat and light
aid in the decomposition of the dioxide.

~Uses.~ Solutions of hydrogen dioxide are used largely as oxidizing
agents. The solution sold by druggists contains 3% of the dioxide and is
used in medicine as an antiseptic. Its use as an antiseptic depends upon
its oxidizing properties.

EXERCISES

1. Why does the chemist use distilled water in making solutions, rather
than filtered water?

2. How could you determine the total amount of solid matter dissolved in
a sample of water?

3. How could you determine whether a given sample of water is distilled
water?

4. How could the presence of air dissolved in water be detected?

5. How could the amount of water in a food such as bread or potato be
determined?

6. Would ice frozen from impure water necessarily be free from disease
germs?

7. Suppose that the maximum density of water were at 0 deg. in place of 4 deg.;
what effect would this have on the formation of ice on bodies of water?

8. Is it possible for a substance to contain both mechanically inclosed
water and water of crystallization?

9. If steam is heated to 2000 deg. and again cooled, has any chemical change
taken place in the steam?

10. Why is cold water passed into C instead of D (Fig. 24)?

11. Mention at least two advantages that a metal condenser has over a
glass condenser.

12. Draw a diagram of the apparatus used in your laboratory for
supplying distilled water.

13. 20 cc. of hydrogen and 7 cc. of oxygen are placed in a eudiometer
and the mixture exploded. (a) How many cubic centimeters of aqueous
vapor are formed? (b) What gas and how much of it remains in excess?

14. (a) What weight of water can be formed by the combustion of 100 L
of hydrogen, measured under standard conditions? (b)What volume of
oxygen would be required in (a)? (c)What weight of potassium
chlorate is necessary to prepare this amount of oxygen?

15. What weight of oxygen is present in 1 kg. of the ordinary hydrogen
dioxide solution? In the decomposition of this weight of the dioxide
into water and oxygen, what volume of oxygen (measured under standard
conditions) is evolved?

CHAPTER V

THE ATOMIC THEORY

~Three fundamental laws of matter.~ Before we can gain any very definite
idea in regard to the structure of matter, and the way in which
different kinds of substances act chemically upon each other, it is
necessary to have clearly in view three fundamental laws of matter.
These laws have been established by experiment, and any conception which
may be formed concerning matter must therefore be in harmony with them.
The laws are as follows:

~Law of conservation of matter.~ This law has already been touched upon in
the introductory chapter, and needs no further discussion. It will be
recalled that it may be stated thus: _Matter can neither be created nor
destroyed, though it can be changed from one form into another._

~Law of definite composition.~ In the earlier days of chemistry there was
much discussion as to whether the composition of a given compound is
always precisely the same or whether it is subject to some variation.
Two Frenchmen, Berthollet and Proust, were the leaders in this
discussion, and a great deal of most useful experimenting was done to
decide the question. Their experiments, as well as all succeeding ones,
have shown that the composition of a pure chemical compound is always
exactly the same. Water obtained by melting pure ice, condensing steam,
burning hydrogen in oxygen, has always 11.18% hydrogen and 88.82% oxygen
in it. Red oxide of mercury, from whatever source it is obtained,
contains 92.6% mercury and 7.4% oxygen. This truth is known as _the law
of definite composition_, and may be stated thus: _The composition of a
chemical compound never varies._

~Law of multiple proportion.~ It has already been noted, however, that
hydrogen and oxygen combine in two different ratios to form water and
hydrogen dioxide respectively. It will be observed that this fact does
not contradict the law of definite composition, for entirely different
substances are formed. These compounds differ from each other in
composition, but the composition of each one is always constant. This
ability of two elements to unite in more than one ratio is very
frequently observed. Carbon and oxygen combine in two different ratios;
nitrogen and oxygen combine to form as many as five distinct compounds,
each with its own precise composition.

In the first decade of the last century John Dalton, an English
school-teacher and philosopher, endeavored to find some rule which holds
between the ratios in which two given substances combine. His studies
brought to light a very simple relation, which the following examples
will make clear. In water the hydrogen and oxygen are combined in the
ratio of 1 part by weight of hydrogen to 7.94 parts by weight of oxygen.
In hydrogen dioxide the 1 part by weight of hydrogen is combined with
15.88 parts by weight of oxygen. The ratio between the amounts of oxygen
which combine with the same amount of hydrogen to form water and
hydrogen dioxide respectively is therefore 7.94: 15.88, or 1: 2.

[Illustration: JOHN DALTON (English) (1766-1844)

Developed the atomic theory; made many studies on the properties and the
composition of gases. His book entitled "A New System of Chemical
Philosophy" had a large influence on the development of chemistry]

Similarly, the element iron combines with oxygen to form two oxides, one
of which is black and the other red. By analysis it has been shown that
the former contains 1 part by weight of iron combined with 0.286 parts
by weight of oxygen, while the latter contains 1 part by weight of iron
combined with 0.429 parts by weight of oxygen. Here again we find that
the amounts of oxygen which combine with the same fixed amount of iron
to form the two compounds are in the ratio of small whole numbers, viz.,
2:3.

Many other examples of this simple relation might be given, since it has
been found to hold true in all cases where more than one compound is,
formed from the same elements. Dalton's law of multiple proportion
states these facts as follows: _When any two elements,_ A _and_ B,
_combine to form more than one compound, the amounts of_ B _which unite
with any fixed amount of_ A _bear the ratio of small whole numbers to
each other_.

~Hypothesis necessary to explain the laws of matter.~ These three
generalizations are called _laws_, because they express in concise
language truths which are found by careful experiment to hold good in
all cases. They do not offer any explanation of the facts, but merely
state them. The human mind, however, does not rest content with the mere
bare facts, but seeks ever to learn the explanation of the facts. A
suggestion which is offered to explain such a set of facts is called an
_hypothesis_. The suggestion which Dalton offered to explain the three
laws of matter, called the _atomic hypothesis_, was prompted by his view
of the constitution of matter, and it involves three distinct
assumptions in regard to the nature of matter and chemical action.
Dalton could not prove these assumptions to be true, but he saw that if
they were true the laws of matter become very easy to understand.

~Dalton's atomic hypothesis.~ The three assumptions which Dalton made in
regard to the nature of matter, and which together constitute the atomic
hypothesis, are these:

1. All elements are made up of minute, independent particles which
Dalton designated as _atoms_.

2. All atoms of the same element have equal masses; those of different
elements have different masses; in any change to which an atom is
subjected its mass does not change.

3. When two or more elements unite to form a compound, the action
consists in the union of a definite small number of atoms of each
element to form a small particle of the compound. The smallest particles
of a given compound are therefore exactly alike in the number and kinds
of atoms which they contain, and larger masses of the substances are
simply aggregations of these least particles.

~Molecules and atoms.~ Dalton applied the name atom not only to the minute
particles of the elements but also to the least particles of compounds.
Later Avogadro, an Italian scientist, pointed out the fact that the two
are different, since the smallest particle of an element is a unit,
while that of a compound must have at least two units in it. He
suggested the name _molecule_ for the least particle of a compound which
can exist, retaining the name _atom_ for the smallest particle of an
element. In accordance with this distinction, we may define the atom and
the molecule as follows: _An atom is the smallest particle of an element
which can exist. A molecule is the smallest particle of a compound which
can exist._ It will be shown in a subsequent chapter that sometimes two
or more atoms of the same element unite with each other to form
molecules of the element. While the term atom, therefore, is applicable
only to elements, the term molecule is applicable both to elements and
compounds.

~The atomic hypothesis and the laws of matter.~ Supposing the atomic
hypothesis to be true, let us now see if it is in harmony with the laws
of matter.

1. _The atomic hypothesis and the law of conservation of matter._ It is
evident that if the atoms never change their masses in any change which
they undergo, the total quantity of matter can never change and the law
of conservation of matter must follow.

2. _The atomic hypothesis and the law of definite composition._
According to the third supposition, when iron combines with sulphur the
union is between definite numbers of the two kinds of atoms. In the
simplest case one atom of the one element combines with one atom of the
other. If the sulphur and the iron atoms never change their respective
masses when they unite to form a molecule of iron sulphide, all iron
sulphide molecules will have equal amounts of iron in them and also of
sulphur. Consequently any mass made up of iron sulphide molecules will
have the same fraction of iron by weight as do the individual iron
sulphide molecules. Iron sulphide, from whatever source, will have the
same composition, which is in accordance with the law of definite
composition.

3. _The atomic hypothesis and the law of multiple proportion._ But this
simplest case may not always be the only one. Under other conditions one
atom of iron might combine with two of sulphur to form a molecule of a
second compound. In such a case the one atom of iron would be in
combination with twice the mass of sulphur that is in the first
compound, since the sulphur atoms all have equal masses. What is true
for one molecule will be true for any number of them; consequently when
such quantities of these two compounds are selected as are found to
contain the same amount of iron, the one will contain twice as much
sulphur as the other.

The combination between the atoms may of course take place in other
simple ratios. For example, two atoms of one element might combine with
three or with five of the other. In all such cases it is clear that the
law of multiple proportion must hold true. For on selecting such numbers
of the two kinds of molecules as have the same number of the one kind of
atoms, the numbers of the other kind of atoms will stand in some simple
ratio to each other, and their weights will therefore stand in the same
simple ratio.

~Testing the hypothesis.~ Efforts have been made to find compounds which
do not conform to these laws, but all such attempts have resulted in
failure. If such compounds should be found, the laws would be no longer
true, and the hypothesis of Dalton would cease to possess value. When an
hypothesis has been tested in every way in which experiment can test it,
and is still found to be in harmony with the facts in the case, it is
termed a _theory_. We now speak of the atomic theory rather than of the
atomic hypothesis.

~Value of a theory.~ The value of a theory is twofold. It aids in the
clear understanding of the laws of nature because it gives an
intelligent idea as to why these laws should be in operation.

A theory also leads to discoveries. It usually happens that in testing a
theory much valuable work is done, and many new facts are discovered.
Almost any theory in explaining given laws will involve a number of
consequences apart from the laws it seeks to explain. Experiment will
soon show whether these facts are as the theory predicts they will be.
Thus Dalton's atomic theory predicted many properties of gases which
experiment has since verified.

~Atomic weights.~ It would be of great advantage in the study of chemistry
if we could determine the weights of the different kinds of atoms. It is
evident that this cannot be done directly. They are so small that they
cannot be seen even with a most powerful microscope. It is calculated
that it would take 200,000,000 hydrogen atoms placed side by side to
make a row one centimeter long. No balance can weigh such minute
objects. It is possible, however, to determine their relative
weights,--that is, how much heavier one is than another. _These relative
weights of the atoms are spoken of as the atomic weights of the
elements._

If elements were able to combine in only one way,--one atom of one with
one atom of another,--the problem of determining the atomic weights
would be very simple. We should merely have to take some one convenient
element as a standard, and find by experiment how much of each other
element would combine with a fixed weight of it. The ratios thus found
would be the same ratios as those between the atoms of the elements, and
thus we should have their relative atomic weights. The law of multiple
proportion calls attention to the fact that the atoms combine in other
ratios than 1: 1, and there is no direct way of telling which one, if
any, of the several compounds in a given case is the one consisting of a
single atom of each element.

If some way were to be found of telling how much heavier the entire
molecule of a compound is than the atom chosen as a standard,--that is,
of determining the molecular weights of compounds,--the problem could be
solved, though its solution would not be an entirely simple matter.
There are ways of determining the molecular weights of compounds, and
there are other experiments which throw light directly upon the relative
weights of the atoms. These methods cannot be described until the facts
upon which they rest have been studied. It will be sufficient for the
present to assume that these methods are trustworthy.

~Standard for atomic weights.~ Since the atomic weights are merely
relative to some one element chosen as a standard, it is evident that
any one of the elements may serve as this standard and that any
convenient value may be assigned to its atom. At one time oxygen was
taken as this standard, with the value 100, and the atomic weights of
the other elements were expressed in terms of this standard. It would
seem more rational to take the element of smallest atomic weight as the
standard and give it unit value; accordingly hydrogen was taken as the
standard with an atomic weight of 1. Very recently, however, this unit
has been replaced by oxygen, with an atomic weight of 16.

~Why oxygen is chosen as the standard for atomic weights.~ In the
determination of the atomic weight of an element it is necessary to find
the weight of the element which combines with a definite weight of
another element, preferably the element chosen as the standard. Since
oxygen combines with the elements far more readily than does hydrogen to
form definite compounds, it is far better adapted for the standard
element, and has accordingly replaced hydrogen as the standard. Any
definite value might be given to the weight of the oxygen atom. In
assigning a value to it, however, it is convenient to choose a whole
number, and as small a number as possible without making the atomic
weight of any other element less than unity. For these reasons the
number 16 has been chosen as the atomic weight of oxygen. This makes
the atomic weight of hydrogen equal to 1.008, so that there is but
little difference between taking oxygen as 16 and hydrogen as 1 for the
unit.

The atomic weights of the elements are given in the Appendix.

EXERCISES

1. Two compounds were found to have the following compositions: (a)
oxygen = 69.53%, nitrogen = 30.47%; (b) oxygen = 53.27%, nitrogen =
46.73%. Show that the law of multiple proportion holds in this case.

2. Two compounds were found to have the following compositions: (a)
oxygen = 43.64%, phosphorus = 56.36%; (b) oxygen = 56.35%, phosphorus
= 43.65%. Show that the law of multiple proportion holds in this case.

3. Why did Dalton assume that all the atoms of a given element have the
same weight?

CHAPTER VI

CHEMICAL EQUATIONS AND CALCULATIONS

~Formulas.~ Since the molecule of any chemical compound consists of a
definite number of atoms, and this number never changes without
destroying the identity of the compound, it is very convenient to
represent the composition of a compound by indicating the composition of
its molecules. This can be done very easily by using the symbols of the
atoms to indicate the number and the kind of the atoms which constitute
the molecule. HgO will in this way represent mercuric oxide, a molecule
of which has been found to contain 1 atom each of mercury and oxygen.
H_{2}O will represent water, the molecules of which consist of 1 atom of
oxygen and 2 of hydrogen, the subscript figure indicating the number of
the atoms of the element whose symbol precedes it. H_{2}SO_{4} will
stand for sulphuric acid, the molecules of which contain 2 atoms of
hydrogen, 1 of sulphur, and 4 of oxygen. The combination of symbols
which represents the molecule of a substance is called its _formula_.

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