COMPOUNDS OF HYDROGEN AND OXYGEN; WATER AND HYDROGEN DIOXIDE
WATER
~Historical.~ Water was long regarded as an element. In 1781 Cavendish
showed that it is formed by the union of hydrogen and oxygen. Being a
believer in the phlogiston theory, however, he failed to interpret his
results correctly. A few years later Lavoisier repeated Cavendish's
experiments and showed that water must be regarded as a compound of
hydrogen and oxygen.
~General methods employed for the determination of the composition of a
compound.~ The composition of a compound may be determined by either of
two general processes these are known as _analysis_ and _synthesis_.
1. _Analysis_ is the process of decomposing a compound into its
constituents and determining what these constituents are. The analysis
is _qualitative_ when it results in merely determining what elements
compose the compound; it is _quantitative_ when the exact percentage of
each constituent is determined. Qualitative analysis must therefore
precede quantitative analysis, for it must be known what elements, are
in a compound before a method can be devised for determining exactly how
much of each is present.
2. _Synthesis_ is the process of forming a compound from its constituent
parts. It is therefore the reverse of analysis. Like analysis, it may be
either qualitative or quantitative.
~Application of these methods to the determination of the composition of
water.~ The determination of the composition of water is a matter of
great interest not only because of the importance of the compound but
also because the methods employed illustrate the general methods of
analysis and synthesis.
~Methods based on analysis.~ The methods based on analysis may be either
qualitative or quantitative in character.
[Illustration: Fig. 18]
1. _Qualitative analysis._ As was stated in the study of oxygen, water
may be separated into its component parts by means of the electric
current. The form of apparatus ordinarily used for effecting this
analysis is shown in Fig. 18. A platinum wire, to the end of which is
attached a small piece of platinum foil (about 15 mm. by 25 mm.), is
fused through each of the tubes B and D, as shown in the figure. The
stopcocks at the ends of these tubes are opened and water, to which has
been added about one tenth of its volume of sulphuric acid, is poured
into the tube A until the side tubes B and D are completely
filled. The stopcocks are then closed. The platinum wires extending into
the tubes B and D are now connected with the wires leading from two
or three dichromate cells joined in series. The pieces of platinum foil
within the tubes thus become the electrodes, and the current flows from
one to the other through the acidulated water. As soon as the current
passes, bubbles of gas rise from each of the electrodes and collect in
the upper part of the tubes. The gas rising from the negative electrode
is found to be hydrogen, while that from the positive electrode is
oxygen. It will be seen that the volume of the hydrogen is approximately
double that of the oxygen. Oxygen is more soluble in water than
hydrogen, and a very little of it is also lost by being converted into
ozone and other substances. It has been found that when the necessary
corrections are made for the error due to these facts, the volume of the
hydrogen is exactly double that of the oxygen.
Fig. 19 illustrates a simpler form of apparatus, which may be used in
place of that shown in Fig. 18. A glass or porcelain dish is partially
filled with water to which has been added the proper amount of acid. Two
tubes filled with the same liquid are inverted over the electrodes. The
gases resulting from the decomposition of the water collect in the
tubes.
[Illustration: Fig. 19]
2. _Quantitative analysis._ The analysis just described is purely
qualitative and simply shows that water contains hydrogen and oxygen. It
does not prove the absence of other elements; indeed it does not prove
that the hydrogen and oxygen are present in the proportion in which they
are liberated by the electric current. The method may be made
quantitative, however, by weighing the water decomposed and also the
hydrogen and oxygen obtained in its decomposition. If the combined
weights of the hydrogen and oxygen exactly equal the weight of the water
decomposed, then it would be proved that the water consists of hydrogen
and oxygen in the proportion in which they are liberated by the electric
current. This experiment is difficult to carry out, however, so that the
more accurate methods based on synthesis are used.
~Methods based on synthesis.~ Two steps are necessary to ascertain the
exact composition of water by synthesis: (1) to show by qualitative
synthesis that water is formed by the union of oxygen with hydrogen; (2)
to determine by quantitative synthesis in what proportion the two
elements unite to form water. The fact that water is formed by the
combination of oxygen with hydrogen was proved in the preceding chapter.
The quantitative synthesis may be made as follows:
[Illustration: Fig. 20]
The combination of the two gases is brought about in a tube called a
eudiometer. This is a graduated tube about 60 cm. long and 2 cm. wide,
closed at one end (Fig. 20). Near the closed end two platinum wires are
fused through the glass, the ends of the wires within the tube being
separated by a space of 2 mm or 3 mm. The tube is entirely filled with
mercury and inverted in a vessel of the same liquid. Pure hydrogen is
passed into the tube until it is about one fourth filled. The volume of
the gas is then read off on the scale and reduced to standard
conditions. Approximately an equal volume of pure oxygen is then
introduced and the volume again read off and reduced to standard
conditions. This gives the total volume of the two gases. From this the
volume of the oxygen introduced may be determined by subtracting from
it the volume of the hydrogen. The combination of the two gases is now
brought about by connecting the two platinum wires with an induction
coil and passing a spark from one wire to the other. Immediately a
slight explosion occurs. The mercury in the tube is at first depressed
because of the expansion of the gases due to the heat generated, but at
once rebounds, taking the place of the gases which have combined to form
water. The volume of the water in the liquid state is so small that it
may be disregarded in the calculations. In order that the temperature of
the residual gas and the mercury may become uniform, the apparatus is
allowed to stand for a few minutes. The volume of the gas is then read
off and reduced to standard conditions, so that it may be compared with
the volumes of the hydrogen and oxygen originally taken. The residual
gas is then tested in order to ascertain whether it is hydrogen or
oxygen, experiments having proved that it is never a mixture of the two.
From the information thus obtained the composition of the water may be
calculated. Thus, suppose the readings were as follows:
Volume of hydrogen taken 20.3 cc.
Volume of hydrogen and oxygen 38.7
Volume of oxygen 18.4
Volume of gas left after combination has taken place (oxygen) 8.3
The 20.3 cc. of hydrogen have combined with 18.4 cc. minus 8.3 cc. (or
10.1 cc.) of oxygen; or approximately 2 volumes of hydrogen have
combined with 1 of oxygen. Since oxygen is 15.88 times as heavy as
hydrogen, the proportion by weight in which the two gases combine is 1
part of hydrogen to 7.94 of oxygen.
~Precaution.~ If the two gases are introduced into the eudiometer in the
exact proportions in which they combine, after the combination has taken
place the liquid will rise and completely fill the tube. Under these
conditions, however, the tube is very likely to be broken by the sudden
upward rush of the liquid. Hence in performing the experiment care is
taken to introduce an excess of one of the gases.
~A more convenient form of eudiometer.~ A form of eudiometer (Fig. 21)
different from that shown on page 43 is sometimes used to avoid the
calculations necessary in reducing the volumes of the gases to the same
conditions of temperature and pressure in order to make comparisons.
With this apparatus it is possible to take the readings of the volumes
under the same conditions of temperature and pressure, and thus compare
them directly. The apparatus (Fig. 21) is filled with mercury and the
gases introduced into the tube A. The experiment is carried out as in
the preceding one, except that before taking the reading of the gas
volumes, mercury is either added to the tube B or withdrawn from it by
means of the stopcock C, until it stands at exactly the same height in
both tubes. The gas inclosed in tube A is then under atmospheric
pressure; and since but a few minutes are required for performing the
experiment, the conditions of temperature and pressure may be regarded
as constant. Hence the volumes of the hydrogen and oxygen and of the
residual gas may be read off from the tube and directly compared.
[Illustration: Fig. 21]
~Method used by Berzelius and Dumas.~ The method used by these
investigators enables us to determine directly the proportion by weight
in which the hydrogen and oxygen combine. Fig. 22 illustrates the
apparatus used in making this determination. B is a glass tube
containing copper oxide. C and D are glass tubes filled with calcium
chloride, a substance which has great affinity for water. The tubes B
and C, including their contents, are carefully weighed, and the
apparatus connected as shown in the figure. A slow current of pure
hydrogen is then passed through A, and that part of the tube B which
contains copper oxide is carefully heated. The hydrogen combines with
the oxygen present in the copper oxide to form water, which is absorbed
by the calcium chloride in tube C. The calcium chloride in tube D
prevents any moisture entering tube C from the air. The operation is
continued until an appreciable amount of water has been formed. The
tubes B and C are then weighed once more. The loss of weight in the
tube B will exactly equal the weight of oxygen taken up from the
copper oxide in the formation of the water. The gain in weight in the
tube C will exactly equal the weight of the water formed. The
difference in these weights will of course equal the weight of the
hydrogen present in the water formed.
[Illustration: Fig. 22]
~Dumas' results.~ The above method for the determination of the
composition of water was first used by Berzelius in 1820. The work was
repeated in 1843 by Dumas, the average of whose results is as follows:
Weight of water formed 236.36 g.
Oxygen given up by the copper oxide 210.04
------
Weight of hydrogen present in water 26.32
According to this experiment the ratio of hydrogen to oxygen in water is
therefore 26.32 to 210.04, or as l to 7.98
~Morley's results.~ The American chemist Morley has recently determined
the composition of water, extreme precautions being taken to use pure
materials and to eliminate all sources of error. The hydrogen and oxygen
which combined, as well as the water formed, were all accurately
weighed. According to Morley's results, 1 part of hydrogen by weight
combines with 7.94 parts of oxygen to form water.
~Comparison of results obtained.~ From the above discussions it is easy to
see that it is by experiment alone that the composition of a compound
can be determined. Different methods may lead to slightly different
results. The more accurate the method chosen and the greater the skill
with which the experiment is carried out, the more accurate will be the
results. It is generally conceded by chemists that the results obtained
by Morley in reference to the composition of water are the most accurate
ones. In accordance with these results, then, _water must be regarded as
a compound containing hydrogen and oxygen in the proportion of 1 part by
weight of hydrogen to 7.94 parts by weight of oxygen_.
~Relation between the volume of aqueous vapor and the volumes of the
hydrogen and oxygen which combine to form it.~ When the quantitative
synthesis of water is carried out in the eudiometer as described above,
the water vapor formed by the union of the hydrogen and oxygen at once
condenses. The volume of the resulting liquid is so small that it may be
disregarded in making the calculations. If, however, the experiment is
carried out at a temperature of 100° or above, the water-vapor formed is
not condensed and it thus becomes possible to compare the volume of the
vapor with the volumes of hydrogen and oxygen which combined to form it.
This can be accomplished by surrounding the arm A of the eudiometer
(Fig. 23) with the tube B through which is passed the vapor obtained
by boiling some liquid which has a boiling point above 100°. In this way
it has been proved that 2 volumes of hydrogen and 1 volume of oxygen
combine to form exactly 2 volumes of water vapor, the volumes all being
measured under the same conditions of temperature and pressure. It will
be noted that the relation between these volumes may be expressed by
whole numbers. The significance of this very important fact will be
discussed in a subsequent chapter.
[Illustration: Fig. 23]
~Occurrence of water.~ Water not only covers about three fourths of the
surface of the earth, and is present in the atmosphere in the form of
moisture, but it is also a common constituent of the soil and rocks and
of almost every form of animal and vegetable organism. The human body is
nearly 70% water. This is derived not only from the water which we drink
but also from the food which we eat, most of which contains a large
percentage of water. Thus potatoes contain about 78% of water, milk 85%,
beef over 50%, apples 84%, tomatoes 94%.
~Impurities in water.~ Chemically pure water contains only hydrogen and
oxygen. Such a water never occurs in nature, however, for being a good
solvent, it takes up certain substances from the rocks and soil with
which it comes in contact. When such waters are evaporated these
substances are deposited in the form of a residue. Even rain water,
which is the purest form occurring in nature, contains dust particles
and gases dissolved from the atmosphere. The foreign matter in water is
of two kinds, namely, _mineral_, such as common salt and limestone, and
_organic_, that is the products of animal and vegetable life.
~Mineral matter in water.~ The amount and nature of the mineral
matter present in different waters vary greatly, depending on
the character of the rocks and soil with which the waters come
in contact. The more common of the substances present are
common salt and compounds of calcium, magnesium, and iron. One
liter of the average river water contains about 175 mg. of
mineral matter. Water from deep wells naturally contains more
mineral matter than river water, generally two or three times
as much, while sea water contains as much as 35,000 mg. to the
liter.
~Effect of impurities on health.~ The mineral matter in water does not,
save in very exceptional cases, render the water injurious to the human
system. In fact the presence of a certain amount of such matter is
advantageous, supplying the mineral constituents necessary for the
formation of the solid tissues of the body. The presence of organic
matter, on the other hand, must always be regarded with suspicion. This
organic matter may consist not only of the products of animal and
vegetable life but also of certain microscopic forms of living organisms
which are likely to accompany such products. Contagious diseases are
known to be due to the presence in the body of minute living organisms
or germs. Each disease is caused by its own particular kind of germ.
Through sewage these germs may find their way from persons afflicted
with disease into the water supply, and it is principally through the
drinking water that certain of these diseases, especially typhoid fever,
are spread. It becomes of great importance, therefore, to be able to
detect such matter when present in drinking water as well as to devise
methods whereby it can be removed or at least rendered harmless.
~Analysis of water.~ The mineral analysis of a water is, as the
name suggests, simply the determination of the mineral matter
present. Sanitary analysis, on the other hand, is the
determination of the organic matter present. The physical
properties of a water give no conclusive evidence as to its
purity, since a water may be unfit for drinking purposes and
yet be perfectly clear and odorless. Neither can any reliance
be placed on the simple methods often given for testing the
purity of water. Only the trained chemist can carry out such
methods of analysis as can be relied upon.
[Illustration: Fig. 24]
~Purification of water.~ Three general methods are used for the
purification of water, namely, _distillation_, _filtration_, and
_boiling_.
1. _Distillation._ The most effective way of purifying natural waters is
by the process of distillation. This consists in boiling the water and
condensing the steam. Fig. 24 illustrates the process of distillation,
as commonly conducted in the laboratory. Ordinary water is poured into
the flask A and boiled. The steam is conducted through the condenser
B, which consists essentially of a narrow glass tube sealed within a
larger one, the space between the two being filled with cold water,
which is admitted at C and escapes at D. The inner tube is thus kept
cool and the steam in passing through it is condensed. The water formed
by the condensation of the steam collects in the receiver E and is
known as _distilled_ water. Such water is practically pure, since the
impurities are nonvolatile and remain in the flask A.
~Commercial distillation.~ In preparing distilled water on a
large scale, the steam is generated in a boiler or other metal
container and condensed by passing it through a pipe made of
metal, generally tin. This pipe is wound into a spiral and is
surrounded by a current of cold water. Distilled water is used
by the chemist in almost all of his work. It is also used in
the manufacture of artificial ice and for drinking water.
~Fractional distillation.~ In preparing distilled water, it is
evident that if the natural water contains some substance which
is volatile its vapor will pass over and be condensed with the
steam, so that the distillate will not be pure water. Even such
mixtures, however, may generally be separated by repeated
distillation. Thus, if a mixture of water (boiling point 100°)
and alcohol (boiling point 78°) is distilled, the alcohol,
having the lower boiling point, tends to distill first,
followed by the water. The separation of the two is not
perfect, however, but may be made nearly so by repeated
distillations. The process of separating a mixture of volatile
substances by distillation is known as _fractional
distillation_.
2. _Filtration._ The process of distillation practically removes all
nonvolatile foreign matter, mineral as well as organic. In purifying
water for drinking purposes, however, it is only necessary to eliminate
the latter or to render it harmless. This is ordinarily done either by
filtration or boiling. In filtration the water is passed through some
medium which will retain the organic matter. Ordinary charcoal is a
porous substance and will condense within its pores the organic matter
in water if brought in contact with it. It is therefore well adapted to
the construction of filters. Such filters to be effective must be kept
clean, since it is evident that the charcoal is useless after its pores
are filled. A more effective type of filter is the Chamberlain-Pasteur
filter. In this the water is forced through a porous cylindrical cup,
the pores being so minute as to strain out the organic matter.
~City filtration beds.~ For purifying the water supply of cities,
large filtration beds are prepared from sand and gravel, and
the water is allowed to filter through these. Some of the
impurities are strained out by the filter, while others are
decomposed by the action of certain kinds of bacteria present
in the sand. Fig. 25 shows a cross section of a portion of the
filter used in purifying the water supply of Philadelphia. The
water filters through the sand and gravel and passes into the
porous pipe A, from which it is pumped into the city mains.
The filters are covered to prevent the water from freezing in
cold weather.
[Illustration: Fig. 25]
3. _Boiling._ A simpler and equally efficient method for purifying water
for drinking purposes consists in boiling the water. It is the germs in
water that render it dangerous to health. These germs are living forms
of matter. If the water is boiled, the germs are killed and the water
rendered safe. While these germs are destroyed by heat, cold has little
effect upon them. Thus Dewar, in working with liquid hydrogen, exposed
some of these minute forms of life to the temperature of boiling
hydrogen (-252°) without killing them.
~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° and boils at 100°
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° 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°,
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° and 100° 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 × 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° in place of 4°;
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° 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?