THE CHLORINE FAMILY
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| | | |
| ATOMIC | MELTING | BOILING | COLOR AND STATE
| WEIGHT | POINT | POINT |
______________|________|_________|_________|______________________
| | | |
Fluorine (F) | 19.00 | -223° | -187° | Pale yellowish gas.
Chlorine (Cl) | 35.45 | -102° | -33.6° | Greenish-yellow gas.
Bromine (Br) | 79.96 | -7° | 59° | Red liquid.
Iodine (I) | 126.97 | 107° | 175° | Purplish-black solid.
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~The family.~ The four elements named in the above table form a strongly
marked family of elements and illustrate very clearly the way in which
the members of a family in a periodic group resemble each other, as well
as the character of the differences which we may expect to find between
the individual members.
1. _Occurrence._ These elements do not occur in nature in the free
state. The compounds of the last three elements of the family are found
extensively in sea water, and on this account the name _halogens_,
signifying "producers of sea salt," is sometimes applied to the family.
2. _Properties._ As will be seen by reference to the table, the melting
points and boiling points of the elements of the family increase with
their atomic weights. A somewhat similar gradation is noted in their
color and state. One atom of each of the elements combines with one atom
of hydrogen to form acids, which are gases very soluble in water. The
affinity of the elements for hydrogen is in the inverse order of their
atomic weights, fluorine having the strongest affinity and iodine the
weakest. Only chlorine and iodine form oxides, and those of the former
element are very unstable. The elements of the group are univalent in
their compounds with hydrogen and the metals.
FLUORINE
~Occurrence.~ The element fluorine occurs in nature most abundantly as the
mineral fluorspar (CaF_{2}), as cryolite (Na_{3}AlF_{6}), and in the
complex mineral apatite (3 Ca_{3}(PO_{4})_{2}·CaF_{2}).
~Preparation.~ All attempts to isolate the element resulted in failure
until recent years. Methods similar to those which succeed in the
preparation of the other elements of the family cannot be used; for as
soon as the fluorine is liberated it combines with the materials of
which the apparatus is made or with the hydrogen of the water which is
always present. The preparation of fluorine was finally accomplished by
the French chemist Moissan by the electrolysis of hydrofluoric acid.
Perfectly dry hydrofluoric acid (HF) was condensed to a liquid and
placed in a U-shaped tube made of platinum (or copper), which was
furnished with electrodes and delivery tubes, as shown in Fig. 52. This
liquid is not an electrolyte, but becomes such when potassium fluoride
is dissolved in it. When this solution was electrolyzed hydrogen was set
free at the cathode and fluorine at the anode.
[Illustration: Fig. 52]
~Properties.~ Fluorine is a gas of slightly yellowish color, and can be
condensed to a liquid boiling at -187° under atmospheric pressure. It
solidifies at -223°. It is extremely active chemically, being the most
active of all the elements at ordinary temperatures.
It combines with all the common elements save oxygen, very often with
incandescence and the liberation of much heat. It has a strong affinity
for hydrogen and is able to withdraw it from its compounds with other
elements. Because of its great activity it is extremely poisonous.
Fluorine does not form any oxides, neither does it form any oxygen
acids, in which respects it differs from the other members of the
family.
~Hydrofluoric acid~ (HF). Hydrofluoric acid is readily obtained from
fluorspar by the action of concentrated sulphuric acid. The equation is
CaF_{2} + H_{2}SO_{4} = CaSO_{4} + 2HF.
In its physical properties it resembles the binary acids of the other
elements of this family, being, however, more easily condensed to a
liquid. The anhydrous acid boils at 19° and can therefore be prepared at
ordinary pressures. It is soluble in all proportions in water, and a
concentrated solution--about 50%--is prepared for the market. Its fumes
are exceedingly irritating to the respiratory organs, and several
chemists have lost their lives by accidentally breathing them.
[Illustration: HENRI MOISSAN (French) (1853-1907)
Famous for his work with the electric furnace at high temperatures;
prepared artificial diamonds, together with many new binary compounds
such as carbides, silicides, borides, and nitrides; isolated fluorine
and studied its properties and its compounds very thoroughly]
~Chemical properties.~ Hydrofluoric acid, like other strong acids, readily
acts on bases and metallic oxides and forms the corresponding fluorides.
It also dissolves certain metals such as silver and copper. It acts very
vigorously upon organic matter, a single drop of the concentrated acid
making a sore on the skin which is very painful and slow in healing. Its
most characteristic property is its action upon silicon dioxide
(SiO_{2}), with which it forms water and the gas silicon tetrafluoride
(SiF_{4}), as shown in the equation
SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.
Glass consists of certain compounds of silicon, which are likewise acted
on by the acid so that it cannot be kept in glass bottles. It is
preserved in flasks made of wax or gutta-percha.
~Etching.~ Advantage is taken of this reaction in etching designs
upon glass. The glass vessel is painted over with a protective
paint upon which the acid will not act, the parts which it is
desired to make opaque being left unprotected. A mixture of
fluorspar and sulphuric acid is then painted over the vessel
and after a few minutes the vessel is washed clean. Wherever
the hydrofluoric acid comes in contact with the glass it acts
upon it, destroying its luster and making it opaque, so that
the exposed design will be etched upon the clear glass. Frosted
glass globes are often made in this way.
The etching may also be effected by covering the glass with a
thin layer of paraffin, cutting the design through the wax and
then exposing the glass to the fumes of the acid.
~Salts of hydrofluoric acid,--fluorides.~ A number of the fluorides are
known, but only one of them, calcium fluoride (CaF_{2}), is of
importance. This is the well-known mineral fluorspar.
CHLORINE
~Historical.~ While studying the action of hydrochloric acid upon the
mineral pyrolusite, in 1774, Scheele obtained a yellowish, gaseous
substance to which he gave a name in keeping with the phlogiston theory
then current. Later it was supposed to be a compound containing oxygen.
In 1810, however, the English chemist Sir Humphry Davy proved it to be
an element and named it chlorine.
~Occurrence.~ Chlorine does not occur free in nature, but its compounds
are widely distributed. For the most part it occurs in combination with
the metals in the form of chlorides, those of sodium, potassium, and
magnesium being most abundant. Nearly all salt water contains these
substances, particularly sodium chloride, and very large salt beds
consisting of chlorides are found in many parts of the world.
~Preparation.~ Two general methods of preparing chlorine may be mentioned,
namely, the laboratory method and the electrolytic method.
1. _Laboratory method._ In the laboratory chlorine is made by warming
the mineral pyrolusite (manganese dioxide, MnO_{2}) with concentrated
hydrochloric acid. The first reaction, which seems to be similar to the
action of acids upon oxides in general, is expressed in the equation
MnO_{2} + 4HCl = MnCl_{4} + 2H_{2}O.
The manganese compound so formed is very unstable, however, and breaks
clown according to the equation
MnCl_{4} = MnCl_{2} + 2Cl.
Instead of using hydrochloric acid in the preparation of chlorine it
will serve just as well to use a mixture of sodium chloride and
sulphuric acid, since these two react to form hydrochloric acid. The
following equations will then express the changes:
(1) 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
(2) MnO_{2} + 4 HCl = MnCl_{2} + 2Cl + 2H_{2}O.
(3) MnCl_{2} + H_{2}SO_{4} = MnSO_{4} + 2HCl.
Combining these equations, the following equation expressing the
complete reaction is obtained:
2NaCl + MnO_{2} + 2H_{2}SO_{4} = MnSO_{4} + Na_{2}SO_{4} + 2H_{2}O + 2Cl.
Since the hydrochloric acid liberated in the third equation is free to
act upon manganese dioxide, it will be seen that all of the chlorine
originally present in the sodium chloride is set free.
The manganese dioxide and the hydrochloric acid are brought
together in a flask, as represented in Fig. 53, and a gentle
heat is applied. The rate of evolution of the gas is regulated
by the amount of heat applied, and the gas is collected by
displacement of air. As the equations show, only half of the
chlorine present in the hydrochloric acid is liberated.
[Illustration: Fig. 53]
2. _Electrolytic method._ Under the discussion of electrolysis (p. 102)
it was shown that when a solution of sodium chloride is electrolyzed
chlorine is evolved at the anode, while the sodium set free at the
cathode reacts with the water to form hydrogen, which is evolved, and
sodium hydroxide, which remains in solution. A great deal of the
chlorine required in the chemical industries is now made in this way in
connection with the manufacture of sodium hydroxide.
~Physical properties.~ Chlorine is a greenish-yellow gas, which has a
peculiar suffocating odor and produces a very violent effect upon the
throat and lungs. Even when inhaled in small quantities it often
produces all the symptoms of a hard cold, and in larger quantities may
have serious and even fatal action. It is quite heavy (density = 2.45)
and can therefore be collected by displacement of air. One volume of
water under ordinary conditions dissolves about three volumes of
chlorine. The gas is readily liquefied, a pressure of six atmospheres
serving to liquefy it at 0°. It forms a yellowish liquid which
solidifies at -102°.
~Chemical properties.~ At ordinary temperatures chlorine is far more
active chemically than any of the elements we have so far considered,
with the exception of fluorine; indeed, it is one of the most active of
all elements.
1. _Action on metals._ A great many metals combine directly with
chlorine, especially when hot. A strip of copper foil heated in a burner
flame and then dropped into chlorine burns with incandescence. Sodium
burns brilliantly when heated strongly in slightly moist chlorine. Gold
and silver are quickly tarnished by the gas.
2. _Action on non-metals._ Chlorine has likewise a strong affinity for
many of the non-metals. Thus phosphorus burns in a current of the gas,
while antimony and arsenic in the form of a fine powder at once burst
into flame when dropped into jars of the gas. The products formed in all
cases where chlorine combines with another element are called
_chlorides_.
3. _Action on hydrogen._ Chlorine has a strong affinity for hydrogen,
uniting with it to form hydrochloric acid. A jet of hydrogen burning in
the air continues to burn when introduced into a jar of chlorine, giving
a somewhat luminous flame. A mixture of the two gases explodes violently
when a spark is passed through it or when it is exposed to bright
sunlight. In the latter case it is the light and not the heat which
starts the action.
4. _Action on substances containing hydrogen._ Not only will chlorine
combine directly with free hydrogen but it will often abstract the
element from its compounds. Thus, when chlorine is passed into a
solution containing hydrosulphuric acid, sulphur is precipitated and
Hydrochloric acid formed. The reaction is shown by the following
equation:
H_{2}S + 2Cl = 2HCl + S.
With ammonia the action is similar:
NH_{3} + 3Cl = 3HCl + N.
The same tendency is very strikingly seen in the action of chlorine upon
turpentine. The latter substance is largely made up of compounds having
the composition represented by the formula C_{10}H_{16}. When a strip of
paper moistened with warm turpentine is placed in a jar of chlorine
dense fumes of hydrochloric acid appear and a black deposit of carbon is
formed. Even water, which is a very stable compound, can be decomposed
by chlorine, the oxygen being liberated. This may be shown in the
following way:
[Illustration: Fig. 54]
If a long tube of rather large diameter is filled with a strong
solution of chlorine in water and inverted in a vessel of the
same solution, as shown in Fig. 54, and the apparatus is placed
in bright sunlight, very soon bubbles of a gas will be observed
to rise through the solution and collect in the tube. An
examination of this gas will show that it is oxygen. It is
liberated from water in accordance with the following equation:
H_{2}O + 2Cl = 2HCl + O.
5. _Action on color substances,--bleaching action._ If strips of
brightly colored cloth or some highly colored flowers are placed in
quite dry chlorine, no marked change in color is noticed as a rule. If,
however, the cloth and flowers are first moistened, the color rapidly
disappears, that is, the objects are bleached. Evidently the moisture as
well as the chlorine is concerned in the action, and a study of the case
shows that the chlorine has combined with the hydrogen of the water. The
oxygen set free oxidizes the color substance, converting it into a
colorless compound. It is evident from this explanation that chlorine
will only bleach those substances which are changed into colorless
compounds by oxidation.
6. _Action as a disinfectant._ Chlorine has also marked germicidal
properties, and the free element, as well as compounds from which it is
easily liberated, are used as disinfectants.
~Nascent state.~ It will be noticed that oxygen when set free from water
by chlorine is able to do what ordinary oxygen cannot do, for both the
cloth and the flowers are unchanged in the air which contains oxygen. It
is generally true that the activity of an element is greatest at the
instant of liberation from its compounds. To express this fact elements
at the instant of liberation are said to be in the _nascent state_. It
is nascent oxygen which does the bleaching.
~Hydrochloric acid~ (_muriatic acid_) (HCl). The preparation of
hydrochloric acid may be discussed under two general heads:
1. _Laboratory preparation._ The product formed by the burning of
hydrogen in chlorine is the gas hydrochloric acid. This substance is
much more easily obtained, however, by treating common salt (sodium
chloride) with sulphuric acid. The following equation shows the
reaction:
2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
The dry salt is placed in a flask furnished with a funnel tube and an
exit tube, the sulphuric acid is added, and the flask gently warmed. The
hydrochloric acid gas is rapidly given off and can be collected by
displacement of air. The same apparatus can be used as was employed in
the preparation of chlorine (Fig. 53).
When a _solution_ of salt is treated with sulphuric acid there
is no very marked action. The hydrochloric acid formed is very
soluble in water, and so does not escape from the solution;
hence a state of equilibrium is soon reached between the four
substances represented in the equation. When _concentrated_
sulphuric acid, in which hydrochloric acid is not soluble, is
poured upon dry salt the reaction is complete.
2. _Commercial preparation._ Commercially, hydrochloric acid is prepared
in connection with the manufacture of sodium sulphate, the reaction
being the same as that just given. The reaction is carried out in a
furnace, and the hydrochloric acid as it escapes in the form of gas is
passed into water in which it dissolves, the solution forming the
hydrochloric acid of commerce. When the materials are pure a colorless
solution is obtained. The most concentrated solution has a density of
1.2 and contains 40% HCl. The commercial acid, often called _muriatic
acid_, is usually colored yellow by impurities.
~Composition of hydrochloric acid.~ When a solution of hydrochloric acid
is electrolyzed in an apparatus similar to the one in which water was
electrolyzed (Fig. 18), chlorine collects at the anode and hydrogen at
the cathode. At first the chlorine dissolves in the water, but soon the
water in the one tube becomes saturated with it, and if the stopcocks
are left open until this is the case, and are then closed, it will be
seen that the two gases are set free in equal volumes.
When measured volumes of the two gases are caused to unite it is found
that one volume of hydrogen combines with one of chlorine. Other
experiments show that the volume of hydrochloric acid formed is just
equal to the sum of the volumes of hydrogen and chlorine. Therefore one
volume of hydrogen combines with one volume of chlorine to form two
volumes of hydrochloric acid gas. Since chlorine is 35.18 times as heavy
as hydrogen, it follows that one part of hydrogen by weight combines
with 35.18 parts of chlorine to form 36.18 parts of hydrochloric acid.
~Physical properties.~ Hydrochloric acid is a colorless gas which has an
irritating effect when inhaled, and possesses a sour, biting taste, but
no marked odor. It is heavier than air (density = 1.26) and is very
soluble in water. Under standard conditions 1 volume of water dissolves
about 500 volumes of the gas. On warming such a solution the gas
escapes, until at the boiling point the solution contains about 20% by
weight of HCl. Further boiling will not drive out any more acid, but the
solution will distill with unchanged concentration. A more dilute
solution than this will lose water on boiling until it has reached the
same concentration, 20%, and will then distill unchanged. Under high
pressure the gas can be liquefied, 28 atmospheres being required at 0°.
Under these conditions it forms a colorless liquid which is not very
active chemically. It boils at -80° and solidifies at -113°. The
solution of the gas in water is used almost entirely in the place of the
gas itself, since it is not only far more convenient but also more
active.
~Chemical properties.~ The most important chemical properties of
hydrochloric acid are the following:
1. _Action as an acid._ In aqueous solution hydrochloric acid has very
strong acid properties; indeed, it is one of the strongest acids. It
acts upon oxides and hydroxides, converting them into salts:
NaOH + HCl = NaCl + H_{2}O,
CuO + 2HCl = CuCl_{2} + H_{2}O.
It acts upon many metals, forming chlorides and liberating hydrogen:
Zn + 2HCl = ZnCl_{2} + 2H,
Al + 3HCl = AlCl_{3} + 3H.
Unlike nitric and sulphuric acids it has no oxidizing action, so that
when it acts on metals hydrogen is always given off.
2. _Relation to combustion._ Hydrochloric acid gas is not readily
decomposed, and is therefore neither combustible nor a supporter of
combustion.
3. _Action on oxidizing agents._ Although hydrochloric acid is
incombustible, it can be oxidized under some circumstances, in which
case the hydrogen combines with oxygen, while the chlorine is set free.
Thus, when a solution of hydrochloric acid acts upon manganese dioxide
part of the chlorine is set free:
MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl.
~Aqua regia.~ It has been seen that when nitric acid acts as an oxidizing
agent it usually decomposes, as represented in the equation
2HNO_{3} = H_{2}O + 2NO + 3O.
The oxygen so set free may act on hydrochloric acid:
6HCl + 3O = 3H_{2}O + 6Cl.
The complete equation therefore is
2HNO_{3} + 6HCl = 4H_{2}O + 2NO + 6Cl.
When concentrated nitric and hydrochloric acids are mixed this reaction
goes on slowly, chlorine and some other substances not represented in
the equation being formed. The mixture is known as _aqua regia_ and is
commonly prepared by adding one volume of nitric acid to three volumes
of hydrochloric acid. It acts more powerfully upon metals and other
substances than either of the acids separately, and owes its strength
not to acid properties but to the action of the nascent chlorine which
it liberates. Consequently, when it acts upon metals such as gold it
converts them into chlorides, and the reaction can be represented by
such equations as
Au + 3Cl = AuCl_{3}.
~Salts of hydrochloric acid,--chlorides.~ The chlorides of all the metals
are known and many of them are very important compounds. Some of them
are found in nature, and all can be prepared by the general method of
preparing salts. Silver chloride, lead chloride, and mercurous chloride
are insoluble in water and acids, and can be prepared by adding
hydrochloric acid to solutions of compounds of the respective elements.
While the chlorides have formulas similar to the fluorides, their
properties are often quite different. This is seen in the solubility of
the salts. Those metals whose chlorides are insoluble form soluble
fluorides, while many of the metals which form soluble chlorides form
insoluble fluorides.
~Compounds of chlorine with oxygen and hydrogen.~ Chlorine combines with
oxygen and hydrogen to form four different acids. They are all quite
unstable, and most of them cannot be prepared in pure form; their salts
can easily be made, however, and some of them will be met with in the
study of the metals. The formulas and names of these acids are as
follows:
HClO hypochlorous acid.
HClO_{2} chlorous acid.
HClO_{3} chloric acid.
HClO_{4} perchloric acid.
~Oxides of chlorine.~ Two oxides are known, having the formulas Cl_{2}O
and ClO_{2}. They decompose very easily and are good oxidizing agents.
BROMINE
~Historical.~ Bromine was discovered in 1826 by the French chemist
Ballard, who isolated it from sea salt. He named it bromine (stench)
because of its unbearable fumes.
~Occurrence.~ Bromine occurs almost entirely in the form of bromides,
especially as sodium bromide and magnesium bromide, which are found in
many salt springs and salt deposits. The Stassfurt deposits in Germany
and the salt waters of Ohio and Michigan are especially rich in
bromides.
~Preparation of bromine.~ The laboratory method of preparing bromine is
essentially different from the commercial method.
[Illustration Fig. 55]
1. _Laboratory method._ As in the case of chlorine, bromine can be
prepared by the action of hydrobromic acid (HBr) on manganese dioxide.
Since hydrobromic acid is not an article of commerce, a mixture of
sulphuric acid and a bromide is commonly substituted for it. The
materials are placed in a retort arranged as shown in Fig. 55. The end
of the retort just touches the surface of the water in the test tube. On
heating, the bromine distills over and is collected in the cold
receiver. The equation is
2NaBr + 2H_{2}SO_{4} + MnO_{2} = Na_{2}SO_{4} + MnSO_{4}
+ 2H_{2}O + 2Br.
2. _Commercial method._ Bromine is prepared commercially from the waters
of salt wells which are especially rich in bromides. On passing a
current of electricity through such waters the bromine is first
liberated. Any chlorine liberated, however, will assist in the reaction,
since free chlorine decomposes bromides, as shown in the equation
NaBr + Cl = NaCl + Br.
When the water containing the bromine is heated, the liberated bromine
distills over into the receiver.
~Physical properties.~ Bromine is a dark red liquid about three times as
heavy as water. Its vapor has a very offensive odor and is most
irritating to the eyes and throat. The liquid boils at 59° and
solidifies at -7°; but even at ordinary temperatures it evaporates
rapidly, forming a reddish-brown gas very similar to nitrogen peroxide
in appearance. Bromine is somewhat soluble in water, 100 volumes of
water under ordinary conditions dissolving 1 volume of the liquid. It is
readily soluble in carbon disulphide, forming a yellow solution.
~Chemical properties and uses.~ In chemical action bromine is very similar
to chlorine. It combines directly with many of the same elements with
which chlorine unites, but with less energy. It combines with hydrogen
and takes away the latter element from some of its compounds, but not
so readily as does chlorine. Its bleaching properties are also less
marked.
Bromine finds many uses in the manufacture of organic drugs and
dyestuffs and in the preparation of bromides.
~Hydrobromic acid (HBr).~ When sulphuric acid acts upon a bromide
hydrobromic acid is set free:
2NaBr + H_{2}SO_{4} = Na_{2}SO_{4} + 2HBr.
At the same time some bromine is set free, as may be seen from the red
fumes which appear, and from the odor. The explanation of this is found
in the fact that hydrobromic acid is much less stable than hydrochloric
acid, and is therefore more easily oxidized. Concentrated sulphuric acid
is a good oxidizing agent, and oxidizes a part of the hydrobromic acid,
liberating bromine:
H_{2}SO_{4} + 2HBr = 2H_{2}O + SO_{2} + 2Br.
~Preparation of pure hydrobromic acid.~ A convenient way to make
pure hydrobromic acid is by the action of bromine upon moist
red phosphorus. This can be done with the apparatus shown in
Fig. 56. Bromine is put into the dropping funnel A, and red
phosphorus, together with enough water to cover it, is placed
in the flask B. By means of the stopcock the bromine is
allowed to flow drop by drop into the flask, the reaction
taking place without the application of heat. The equations are
(1) P + 3Br = PBr_{3},
(2) PBr_{3} + 3H_{2}O = P(OH)_{3} + 3HBr.
[Illustration Fig. 56]
The U-tube C contains glass beads which have been moistened
with water and rubbed in red phosphorus. Any bromine escaping
action in the flask acts upon the phosphorus in the U-tube. The
hydrobromic acid is collected in the same way as hydrochloric
acid.
~Properties.~ Hydrobromic acid very strikingly resembles hydrochloric acid
in physical and chemical properties. It is a colorless, strongly fuming
gas, heavier than hydrochloric acid and, like it, is very soluble in
water. Under standard conditions 1 volume of water dissolves 610 volumes
of the gas. Chemically, the chief point in which it differs from
hydrochloric acid is in the fact that it is much more easily oxidized,
so that bromine is more readily set free from it than chlorine is from
hydrochloric acid.
~Salts of hydrobromic acid,--bromides.~ The bromides are very similar to
the chlorides in their properties. Chlorine acts upon both bromides and
free hydrobromic acid, liberating bromine from them:
KBr + Cl = KCl + Br,
HBr + Cl = HCl + Br.
Silver bromide is extensively used in photography, and the bromides of
sodium and potassium are used as drugs.
~Oxygen compounds.~ No oxides of bromine are surely known, and
bromine does not form so many oxygen acids as chlorine does.
Salts of hypobromous acid (HBrO) and bromic acid (HBrO_{3}) are
known.
IODINE
~Historical.~ Iodine was discovered in 1812 by Courtois in the ashes of
certain sea plants. Its presence was revealed by its beautiful violet
vapor, and this suggested the name iodine (from the Greek for violet
appearance).
~Occurrence.~ In the combined state iodine occurs in very small quantities
in sea water, from which it is absorbed by certain sea plants, so that
it is found in their ashes. It occurs along with bromine in salt springs
and beds, and is also found in Chili saltpeter.
~Preparation.~ Iodine may be prepared in a number of ways, the principal
methods being the following:
1. _Laboratory method._ Iodine can readily be prepared in the laboratory
from an iodide by the method used in preparing bromine, except that
sodium iodide is substituted for sodium bromide. It can also be made by
passing chlorine into a solution of an iodide.
[Illustration: Fig. 57]
2. _Commercial method._ Commercially iodine was formerly prepared from
seaweed (kelp), but is now obtained almost entirely from the deposits of
Chili saltpeter. The crude saltpeter is dissolved in water and the
solution evaporated until the saltpeter crystallizes. The remaining
liquors, known as the "mother liquors," contain sodium iodate
(NaIO_{3}), in which form the iodine is present in the saltpeter. The
chemical reaction by which the iodine is liberated from this compound is
a complicated one, depending on the fact that sulphurous acid acts upon
iodic acid, setting iodine free. This reaction is shown as follows:
2HIO_{3} + 5H_{2}SO_{3} = 5H_{2}SO_{4} + H_{2}O + 2I.
~Purification of iodine.~ Iodine can be purified very
conveniently in the following way. The crude iodine is placed
in an evaporating dish E (Fig. 57), and the dish is set upon
the sand bath S. The iodine is covered with the inverted
funnel F, and the sand bath is gently heated with a Bunsen
burner. As the dish becomes warm the iodine rapidly evaporates
and condenses again on the cold surface of the funnel in
shining crystals.
This process, in which a solid is converted into a vapor and is
again condensed into a solid without passing through the liquid
state, is called _sublimation_.
~Physical properties.~ Iodine is a purplish-black, shining, heavy solid
which crystallizes in brilliant plates. Even at ordinary temperatures it
gives off a beautiful violet vapor, which increases in amount as heat is
applied. It melts at 107° and boils at 175°. It is slightly soluble in
water, but readily dissolves in alcohol, forming a brown solution
(tincture of iodine), and in carbon disulphide, forming a violet
solution. The element has a strong, unpleasant odor, though by no means
as irritating as that of chlorine and bromine.
~Chemical properties.~ Chemically iodine is quite similar to chlorine and
bromine, but is still less active than bromine. It combines directly
with many elements at ordinary temperatures. At elevated temperatures it
combines with hydrogen, but the reaction is reversible and the compound
formed is quite easily decomposed. Both chlorine and bromine displace it
from its salts:
KI + Br = KBr + I,
KI + Cl = KCl + I.
When even minute traces of iodine are added to thin starch paste a very
intense blue color develops, and this reaction forms a delicate test for
iodine. Iodine is extensively used in medicine, especially in the form
of a tincture. It is also largely used in the preparation of dyes and
organic drugs, iodoform, a substance used as an antiseptic, has the
formula CHI_{3}.
~Hydriodic acid (HI).~ This acid cannot be prepared in pure condition by
the action of sulphuric acid upon an iodide, since the hydriodic acid
set free is oxidized by the sulphuric acid just as in the case of
hydrobromic acid, but to a much greater extent. It can be prepared in
exactly the same way as hydrobromic acid, iodine being substituted for
bromine. It can also be prepared by passing hydrosulphuric acid into
water in which iodine is suspended. The equation is
H_{2}S + 2I = 2HI + S.
The hydriodic acid formed in this way dissolves in the water.
~Properties and uses.~ Hydriodic acid resembles the corresponding acids of
chlorine and bromine in physical properties, being a strongly fuming,
colorless gas, readily soluble in water. Under standard conditions 1
volume of water dissolves about 460 volumes of the gas. It is, however,
more unstable than either hydrochloric or hydrobromic acids, and on
exposure to the air it gradually decomposes in accordance with the
equation
2HI + O = H_{2}O + 2I.
Owing to the slight affinity between iodine and hydrogen the acid easily
gives up its hydrogen and is therefore a strong reducing agent. This is
seen in its action on sulphuric acid.
The salts of hydriodic acid, the iodides, are, in general, similar to
the chlorides and bromides. Potassium iodide (KI) is the most familiar
of the iodides and is largely used in medicine.
~Oxygen compounds.~ Iodine has a much greater affinity for oxygen
than has either chlorine or bromine. When heated with nitric
acid it forms a stable oxide (I_{2}O_{5}). Salts of iodic acid
(HIO_{3}) and periodic acid (HIO_{4}) are easily prepared, and
the free acids are much more stable than the corresponding
acids of the other members of this family.
GAY-LUSSAC'S LAW OF VOLUMES
In the discussion of the composition of hydrochloric acid it was stated
that one volume of hydrogen combines with one volume of chlorine to form
two volumes of hydrochloric acid. With bromine and iodine similar
combining ratios hold good. These facts recall the simple volume
relations already noted in the study of the composition of steam and
ammonia. These relations may be represented graphically in the following
way:
+---+ +----+ +------+ +------+
| H | + | Cl | = | H Cl | + | H Cl |
+---+ +----+ +------+ +------+
+---+ +---+ +---+ +--------+ +--------+
| H | | H | + | O | = | H_{2}O | + | H_{2}O |
+---+ +---+ +---+ +--------+ +--------+
+---+ +---+ +---+ +---+ +--------+ +--------+
| H | | H | | H | + | N | = | NH_{3} | + | NH_{3} |
+---+ +---+ +---+ +---+ +--------+ +--------+
In the early part of the past century Gay-Lussac, a distinguished French
chemist, studied the volume relations of many combining gases, and
concluded that similar relations always hold. His observations are
summed up in the following law: _When two gases combine chemically there
is always a simple ratio between their volumes, and between the volume
of either one of them and that of the product, provided it is a gas._ By
a simple ratio is meant of course the ratio of small whole numbers, as
1 : 2, 2 : 3.
EXERCISES
1. How do we account for the fact that liquid hydrofluoric acid is not
an electrolyte?
2. Why does sulphuric acid liberate hydrofluoric acid from its salts?
3. In the preparation of chlorine, what advantages are there in treating
manganese dioxide with a mixture of sodium chloride and sulphuric acid
rather than with hydrochloric acid?
4. Why must chlorine water be kept in the dark?
5. What is the derivation of the word nascent?
6. What substances studied are used as bleaching agents? To what is the
bleaching action due in each case?
7. What substances studied are used as disinfecting agents?
8. What is meant by the statement that hydrochloric acid is one of the
strongest acids?
9. What is the meaning of the phrase _aqua regia_?
10. Cl_{2}O is the anhydride of what acid?
11. A solution of hydriodic acid on standing turns brown. How is this
accounted for?
12. How can bromine vapor and nitrogen peroxide be distinguished from
each other?
13. Write the equations for the reaction taking place when hydriodic
acid is prepared from iodine, phosphorus, and water.
14. From their behavior toward sulphuric acid, to what class of agents
do hydrobromic and hydriodic acids belong?
15. Give the derivation of the names of the elements of the chlorine
family.
16. Write the names and formulas for the binary acids of the group in
the order of the stability of the acids.
17. What is formed when a metal dissolves in each of the following?
nitric acid; dilute sulphuric acid; concentrated sulphuric acid;
hydrochloric acid; aqua regia.
18. How could you distinguish between a chloride, a bromide, and an
iodide?
19. What weight of sodium chloride is necessary to prepare sufficient
hydrochloric acid to saturate 1 l. of water under standard conditions?
20. On decomposition 100 l. of hydrochloric acid would yield how many
liters of hydrogen and chlorine respectively, the gases being measured
under the same conditions? Are your results in accord with the
experimental facts?