SOLUTIONS
~Definitions.~ When a substance disappears in a liquid in such a way as to
thoroughly mix with it and to be lost to sight as an individual body,
the resulting liquid is called a _solution_. The liquid in which the
substance dissolves is called the _solvent_, while the dissolved
substance is called the _solute_.
~Classes of solutions.~ Matter in any one of its physical states may
dissolve in a liquid, so that we may have solutions of gases, of
liquids, and of solids. Solutions of liquids in liquids are not often
mentioned in the following pages, but the other two classes will become
very familiar in the course of our study, and deserve special attention.
SOLUTION OF GASES IN LIQUIDS
[Illustration: Fig. 30]
It has already been stated that oxygen, hydrogen, and nitrogen are
slightly soluble in water. Accurate study has led to the conclusion that
all gases are soluble to some extent not only in water but in many other
liquids. The amount of a gas which will dissolve in a liquid depends
upon a number of conditions, and these can best be understood by
supposing a vessel B (Fig. 30), to be filled with the gas and inverted
over the liquid. Under these circumstances the gas cannot escape or
become mixed with another gas.
~Circumstances affecting the solubility of gases.~ A number of
circumstances affect the solubility of a gas in a liquid.
1. _Nature of the gas._ Other conditions being equal, each gas has its
own peculiar solubility, just as it has its own special taste or odor.
The solubility of gases varies between wide limits, as will be seen from
the following table, but as a rule a given volume of a liquid will not
dissolve more than two or three times its own volume of a gas.
_Solubility of Gases in Water_
1 l. of water at 760 mm. pressure and at 0° will dissolve:
Ammonia 1148.00 l.
Hydrochloric acid 503.00
Sulphur dioxide 79.79
Carbon dioxide 1.80
Oxygen 41.14 cc.
Hydrogen 21.15
Nitrogen 20.03
In the case of very soluble gases, such as the first three in the table,
it is probable that chemical combination between the liquid and the gas
takes place.
2. _Nature of the liquid._ The character of the liquid has much
influence upon the solubility of a gas. Water, alcohol, and ether have
each its own peculiar solvent power. From the solubility of a gas in
water, no prediction can be made as to its solubility in other liquids.
3. _Influence of pressure._ It has been found that the weight of gas
which dissolves in a given case is proportional to the pressure exerted
upon the gas. If the pressure is doubled, the weight of gas going into
solution is doubled; if the pressure is diminished to one half of its
original value, half of the dissolved gas will escape. Under high
pressure, large quantities of gas can be dissolved in a liquid, and when
the pressure is removed the gas escapes, causing the liquid to foam or
_effervesce_.
4. _Influence of temperature._ In general, the lower the temperature of
the liquid, the larger the quantity of gas which it can dissolve. 1000
volumes of water at 0° will dissolve 41.14 volumes of oxygen; at 50°,
18.37 volumes; at 100° none at all. While most gases can be expelled
from a liquid by boiling the solution, some cannot. For example, it is
not possible to expel hydrochloric acid gas completely from its solution
by boiling.
SOLUTION OF SOLIDS IN LIQUIDS
This is the most familiar class of solutions, since in the laboratory
substances are much more frequently used in the form of solutions than
in the solid state.
~Circumstances affecting the solubility of a solid.~ The solubility of a
solid in a liquid depends upon several factors.
1. _Nature of the solid._ Other conditions being the same, solids vary
greatly in their solubility in liquids. This is illustrated in the
following table:
_Table of Solubility of Solids at 18°_
100 cc. of water will dissolve:
Calcium chloride 71.0 g.
Sodium chloride 35.9
Potassium nitrate 29.1
Copper sulphate 21.4
Calcium sulphate 0.207
No solids are absolutely insoluble, but the amount dissolved may be so
small as to be of no significance for most purposes. Thus barium
sulphate, one of the most insoluble of common substances, dissolves in
water to the extent of 1 part in 400,000.
2. _Nature of the solvent._ Liquids vary much in their power to dissolve
solids. Some are said to be good solvents, since they dissolve a great
variety of substances and considerable quantities of them. Others have
small solvent power, dissolving few substances, and those to a slight
extent only. Broadly speaking, water is the most general solvent, and
alcohol is perhaps second in solvent power.
3. _Temperature._ The weight of a solid which a given liquid can
dissolve varies with the temperature. Usually it increases rapidly as
the temperature rises, so that the boiling liquid dissolves several
times the weight which the cold liquid will dissolve. In some instances,
as in the case of common salt dissolved in water, the temperature has
little influence upon the solubility, and a few solids are more soluble
in cold water than in hot. The following examples will serve as
illustrations:
_Table of Solubility at 0° and at 100°_
100 cc. of water will dissolve:
At 0° At 100°
Calcium chloride 49.6 g. 155.0 g.
Sodium chloride 35.7 39.8
Potassium nitrate 13.3 247.0
Copper sulphate 15.5 73.5
Calcium sulphate 0.205 0.217
Calcium hydroxide 0.173 0.079
~Saturated solutions.~ A liquid will not dissolve an unlimited quantity of
a solid. On adding the solid to the liquid in small portions at a time,
it will be found that a point is reached at which the liquid will not
dissolve more of the solid at that temperature. The solid and the
solution remain in contact with each other unchanged. This condition may
be described by saying that they are in equilibrium with each other. A
solution is said to be _saturated_ when it remains unchanged in
concentration in contact with some of the solid. The weight of the solid
which will completely saturate a definite volume of a liquid at a given
temperature is called the _solubility_ of the substance at that
temperature.
~Supersaturated solutions.~ When a solution, saturated at a given
temperature, is allowed to cool it sometimes happens that no solid
crystallizes out. This is very likely to occur when the vessel used is
perfectly smooth and the solution is not disturbed in any way. Such a
solution is said to be _supersaturated_. That this condition is unstable
can be shown by adding a crystal of the solid to the solution. All of
the solid in excess of the quantity required to saturate the solution at
this temperature will at once crystallize out, leaving the solution
saturated. Supersaturation may also be overcome in many cases by
vigorously shaking or stirring the solution.
~General physical properties of solutions.~ A few general statements may
be made in reference to the physical properties of solutions.
1. _Distribution of the solid in the liquid._ A solid, when dissolved,
tends to distribute itself uniformly through the liquid, so that every
part of the solution has the same concentration. The process goes on
very slowly unless hastened by stirring or shaking the solution. Thus,
if a few crystals of a highly colored substance such as copper sulphate
are placed in the bottom of a tall vessel full of water, it will take
weeks for the solution to become uniformly colored.
2. _Boiling points of solutions._ The boiling point of a liquid is
raised by the presence of a substance dissolved in it. In general the
extent to which the boiling point of a solvent is raised by a given
substance is proportional to the concentration of the solution, that
is, to the weight of the substance dissolved in a definite weight of the
solvent.
3. _Freezing points of solutions._ A solution freezes at a lower
temperature than the pure solvent. The lowering of the freezing point
obeys the same law which holds for the raising of the boiling point: the
extent of lowering is proportional to the weight of dissolved substance,
that is, to the concentration of the solution.
~Electrolysis of solutions.~ Pure water does not appreciably conduct the
electric current. If, however, certain substances such as common salt
are dissolved in the water, the resulting solutions are found to be
conductors of electricity. Such solutions are called _electrolytes_.
When the current passes through an electrolyte some chemical change
always takes place. This change is called _electrolysis_.
[Illustration: Fig. 31]
The general method used in the electrolysis of a solution is illustrated
in Fig. 31. The vessel D contains the electrolyte. Two plates or rods,
A and B, made of suitable material, are connected with the wires
from a battery (or dynamo) and dipped into the electrolyte, as shown in
the figure. These plates or rods are called _electrodes_. The electrode
connected with the zinc plate of the battery is the negative electrode
or _cathode_, while that connected with the carbon plate is the positive
electrode or _anode_.
~Theory of electrolytic dissociation.~ The facts which have just been
described in connection with solutions, together with many others, have
led chemists to adopt a theory of solutions called _the theory of
electrolytic dissociation_. The main assumptions in this theory are the
following.
1. _Formation of ions._ Many compounds when dissolved in water undergo
an important change. A portion of their molecules fall apart, or
_dissociate_, into two or more parts, called _ions_. Thus sodium nitrate
(NaNO_{3}) dissociates into the ions Na and NO_{3}; sodium chloride,
into the ions Na and Cl. These ions are free to move about in the
solution independently of each other like independent molecules, and for
this reason were given the name ion, which signifies a wanderer.
2. _The electrical charge of ions._ Each ion carries a heavy electrical
charge, and in this respect differs from an atom or molecule. It is
evident that the sodium in the form of an ion must differ in some
important way from ordinary sodium, for sodium ions, formed from sodium
nitrate, give no visible evidence of their presence in water, whereas
metallic sodium at once decomposes the water. The electrical charge,
therefore, greatly modifies the usual chemical properties of the
element.
3. _The positive charges equal the negative charges._ The ions formed by
the dissociation of any molecule are of two kinds. One kind is charged
with positive electricity and the other with negative electricity;
moreover the sum of all the positive charges is always equal to the sum
of all the negative charges. The solution as a whole is therefore
electrically neutral. If we represent dissociation by the usual chemical
equations, with the electrical charges indicated by + and - signs
following the symbols, the dissociation of sodium chloride molecules is
represented thus:
NaCl --> Na^{+}, Cl^{-}.
The positive charge on each sodium ion exactly equals the negative
charge on each chlorine ion. Sodium sulphate dissociates, as shown in
the equation
Na_{2}SO_{4} --> 2Na^{+}, SO_{4}^{--}.
Here the positive charge on the two sodium ions equals the double
negative charge on the SO_{4} ion.
4. _Not all compounds dissociate._ Only those compounds dissociate whose
solutions form electrolytes. Thus salt dissociates when dissolved in
water, the resulting solution being an electrolyte. Sugar, on the other
hand, does not dissociate and its solution is not a conductor of the
electric current.
5. _Extent of dissociation differs in different liquids._ While
compounds most readily undergo dissociation in water, yet dissociation
often occurs to a limited extent when solution takes place in liquids
other than water. In the discussion of solutions it will be understood
that the solvent is water unless otherwise noted.
~The theory of electrolytic dissociation and the properties of solutions.~
In order to be of value, this theory must give a reasonable explanation
of the properties of solutions. Let us now see if the theory is in
harmony with certain of these properties.
~The theory of electrolytic dissociation and the boiling and freezing
points of solutions.~ We have seen that the boiling point of a solution
of a substance is raised in proportion to the concentration of the
dissolved substance. This is but another way of saying that the change
in the boiling point of the solution is proportional to the number of
molecules of the dissolved substance present in the solution.
It has been found, however, that in the case of electrolytes the boiling
point is raised more than it should be to conform to this law. If the
solute dissociates into ions, the reason for this becomes clear. Each
ion has the same effect on the boiling point as a molecule, and since
their number is greater than the number of molecules from which they
were formed, the effect on the boiling point is abnormally great.
In a similar way, the theory furnishes an explanation of the abnormal
lowering of the freezing point of electrolytes.
~The theory of electrolytic dissociation and electrolysis.~ The changes
taking place during electrolysis harmonize very completely with the
theory of dissociation. This will become clear from a study of the
following examples.
[Illustration: Fig. 32]
1. _Electrolysis of sodium chloride._ Fig. 32 represents a vessel in
which the electrolyte is a solution of sodium chloride (NaCl). According
to the dissociation theory the molecules of sodium chloride dissociate
into the ions Na^{+} and Cl^{-}. The Na^{+} ions are attracted to the
cathode owing to its large negative charge. On coming into contact with
the cathode, the Na^{+} ions give up their positive charge and are then
ordinary sodium atoms. They immediately decompose the water according to
the equation
Na + H_{2}O = NaOH + H,
and hydrogen is evolved about the cathode.
The chlorine ions on being discharged at the anode in similar manner may
either be given off as chlorine gas, or may attack the water, as
represented in the equation
2Cl + H_{2}O = 2HCl + O.
2. _Electrolysis of water._ The reason for the addition of sulphuric
acid to water in the preparation of oxygen and hydrogen by electrolysis
will now be clear. Water itself is not an electrolyte to an appreciable
extent; that is, it does not form enough ions to carry a current.
Sulphuric acid dissolved in water is an electrolyte, and dissociates
into the ions 2 H^{+} and SO_{4}^{--}. In the process of electrolysis of
the solution, the hydrogen ions travel to the cathode, and on being
discharged escape as hydrogen gas. The SO_{4} ions, when discharged at
the anode, act upon water, setting free oxygen and once more forming
sulphuric acid:
SO_{4} + H_{2}O = H_{2}SO_{4} + O.
The sulphuric acid can again dissociate and the process repeat itself as
long as any water is left. Hence the hydrogen and oxygen set free in the
electrolysis of water really come directly from the acid but indirectly
from the water.
3. _Electrolysis of sodium sulphate._ In a similar way, sodium sulphate
(Na_{2}SO_{4}), when in solution, gives the ions 2 Na^{+} and
SO_{4}^{--}. On being discharged, the sodium atoms decompose water about
the cathode, as in the case of sodium chloride, while the SO_{4} ions
when discharged at the anode decompose the water, as represented in the
equation
SO_{4} + H_{2}O = H_{2}SO_{4} + O
[Illustration: Fig. 33]
That new substances are formed at the cathode and anode may be shown in
the following way. A U-tube, such as is represented in Fig. 33, is
partially filled with a solution of sodium sulphate, and the liquid in
one arm is colored with red litmus, that in the other with blue litmus.
An electrode placed in the red solution is made to serve as cathode,
while one in the blue solution is made the anode. On allowing the
current to pass, the blue solution turns red, while the red solution
turns blue. These are exactly the changes which would take place if
sodium hydroxide and sulphuric acid were to be set free at the
electrodes, as required by the theory.
~The properties of electrolytes depend upon the ions present.~ When a
substance capable of dissociating into ions is dissolved in water, the
properties of the solution will depend upon two factors: (1) the ions
formed from the substance; (2) the undissociated molecules. Since the
ions are usually more active chemically than the molecules, most of the
chemical properties of an electrolyte are due to the ions rather than to
the molecules.
The solutions of any two substances which give the same ion will have
certain properties in common. Thus all solutions containing the copper
ion (Cu^{++}) are blue, unless the color is modified by the presence of
ions or molecules having some other color.
EXERCISES
1. Distinguish clearly between the following terms: electrolysis,
electrolyte, electrolytic dissociation, ions, solute, solvent, solution,
saturated solution, and supersaturated solution.
2. Why does the water from some natural springs effervesce?
3. (a) Why does not the water of the ocean freeze? (b) Why will ice
and salt produce a lower temperature than ice alone?
4. Why does shaking or stirring make a solid dissolve more rapidly in a
liquid?
5. By experiment it was found that a certain volume of water was
saturated at 100° with 114 g. of potassium nitrate. On cooling to 0° a
portion of the substance crystallized. (a) How many grams of the
substance remained in solution? (b) What was the strength of the
solution at 18°? (c) How much water had been used in the experiment?
6. (a) 10 g. of common salt were dissolved in water and the solution
evaporated to dryness; what weight of solid was left? (b) 10 g. of
zinc were dissolved in hydrochloric acid and the solution evaporated to
dryness; what weight of solid was left?
7. Account for the fact that sugar sometimes deposits from molasses,
even when no evaporation has taken place.
8. (a) From the standpoint of the theory of electrolytic dissociation,
write the simple equation for a dilute solution of copper sulphate
(CuSO_{4}); this solution is blue. (b) In the same manner, write one
for sodium sulphate; this solution is colorless. (c) How would you
account for the color of the copper sulphate solution?
9. (a) As in the preceding exercise, write a simple equation for a
dilute solution of copper chloride (CuCl_{2}); this solution is blue.
(b) In the same manner, write one for sodium chloride; this solution
is colorless. To what is the blue color due?
10. What component is present in concentrated sulphuric acid that is
almost wanting in very dilute sulphuric acid?
11. Why will vegetables cook faster when boiled in strong salt water
than when boiled in pure water?
12. How do you explain the foaming of soda water?