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An Introduction to Chemical Science

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151. Cl from Bleaching-Powder.

Experiment 88.--Put a few grams of bleaching- powder into a small
beaker, and set this into a larger one. Cover the latter with
pasteboard or paper, through which passes a thistle-tube reaching
into the small beaker (Fig. 40). Pour through the tube a little
H2SO4 dilated with its volume of H2O.

152. Chlorine Water.--A solution of Cl in water is often useful,
and may be made as follows:-- Experiment 89.--To 3 or 4 crystals
of KClO3 add a few drops of HCl. Heat a minute, and when the gas
begins to disengage, pour in 10 cc. H2O, which dissolves the gas.
2 KClO3 + 4 HCl = 2 KCl + Cl2O4 + 2 H2O + 2 Cl.

153. Bleaching Properties.

Experiment 90.--Put into a receiver of Cl, preferably before
generating it, two pieces of Turkey red cloth, one wet, the other
dry; a small piece of printed paper and a written one; also a red
rose or a green leaf, each wet. Note from which the color is
discharged. If it is not discharged from all, put a little H2O
into the receiver, shake it well, and state what ones are
bleached.

Experiment 91.--(1) Add 5 cc. of Cl water to 5 cc. of indigo
solution. (2) Treat in the same way 5 cc. K2Cr2O7 (potassium
dichromate) solution, and record the results.

Indigo, writing-ink, and Turkey red or madder, are vegetable
pigments; printer's ink contains C, and K2Cr2O7 is a mineral
pigment. State what coloring matters Cl will bleach.

154. Disinfecting Power.

Experiment 92.--Pass a little H2S gas from a generator into a
t.t. containing Cl water. Look for a deposit of S. Notice that
the odor of H2S disappears. H2S + 2 Cl = 2 HCl + S.

155. A Supporter of Combustion.

Experiment 93.--Sprinkle into a receiver of Cl a very little fine
powder or filings of Cu, As, or Sb, and notice the combustion.
Observe that here is a case of combustion in which O does not
take part. Chlorides of the metals are of course formed. Write
the reactions. See whether Cl will support the combustion of
paper or of a stick of wood.

Experiment 94.--Warm 2 or 3 cc. of oil of turpentine (C1OH16) in
an evaporating-dish; dip a piece of tissue paper into it, and
very quickly thrust this into a receiver of Cl. It should take
fire and deposit carbon. C1OH16 + 16 Cl = ? Test the moisture on
the sides of the receiver with litmus. Clean the receiver with a
little petroleum.

Experiment 95.--Prepare a H generator with a lamp-tube bent as in
Figure 41. Light the H, observing the cautions in Experiment 23,
and when well burning, lower the flame into a receiver of Cl.
Observe the change of color which the flame undergoes as it comes
in contact with Cl. Give the reaction for the burning. Test with
litmus any moisture on the sides of the receiver. A mixture of Cl
and H, in direct sunlight combines with explosive violence;
whereas in diffused sunlight it combines slowly, and in darkness
it does not combine. From these experiments state the chief
properties of Cl, and what combustion it will support.

[Figure 41.]

156. Sources and Uses.--The great source of Cl is NaCl, though it
is often made from HCl. Its chief use is in making bleaching-
powder, one pound of which will bleach 300 to 500 pounds of
cloth. Cl is very easily liberated from this powder by a dilute
acid, or, slowly, by taking moisture from the air. Hence its use
as a disinfectant in destroying noxious gases and the germs of
infectious diseases. Cl attacks organic matter and germs as it
does the membrane of the throat or lungs, owing to its affinity
for H.

Cl is the best bleaching agent for cotton goods. It is not
suitable for animal materials, such as silk and wool, as it
attacks their fiber. It does not discharge either mineral or
carbon colors. The chemistry of bleaching is obscure.

As dry material will not bleach, Cl seems to unite with H in H2O
and to set O free. The O then unites with some portion of the
coloring matter, oxidizing it, and breaking up its molecule.
Colors bleached by Cl cannot be restored.

Chapter XXXI.

BROMINE.

Examine bromine, potassium bromide, sodium bromide, magnesium
bromide.

157. Preparation.

Experiment 96.--Pulverize 2 or 3 g. KBr, and mix it with about
the same bulk of MnO2. After putting this into a t.t, add as much
H2SO4, mix them together by shaking, attach a d.t., and conduct
the end of it into a t.t. that is immersed in a bottle of cold
water. Slowly heat the contents of the t.t., and notice the color
of the escaping vapor, and any liquid that condenses in the
receiver. Avoid inhaling the fumes, or getting them into the
eyes.

MnO2 + 2 KBr + 2 H2SO4 = ? Compare this with the equation for
making Cl from NaCl.

158. Tests.

Experiment 97.--Try the bleaching action of Br vapor as in the
case of Cl. Bleach a piece of litmus paper, and try to restore
the color with NH4OH. Explain its bleaching and disinfecting
action. Try the combustibility of As, Sb, and Cu.

159. Description.--Bromine at usual temperatures is a liquid
element; it is the only common one except Hg; it. quickly
evaporates on exposure to air. The chemistry of its manufacture
is like that of Cl; its bleaching and disinfecting powers are
similar to the latter, though they are not quite so strong as
those of Cl. Its affinity for H and for metals is also strongly
marked. A drop of Br on the skin produces a sore slow to heal.
Bromine salts are mainly KBr, NaBr, MgBr2. These in small
quantities accompany NaCl, and are most common in brine springs.
The world's supply of Br comes chiefly from West Virginia and
Ohio, over 300,000 pounds being produced from the salt (NaCl)
wells there in 1884. The water taken from these wells is nearly
evaporated, after which NaCl crystallizes out, leaving a thick
liquid--bittern, or mother liquor--which contains the salts of
Br. The bittern is treated with H2SO4 and Mn02, as above.

For transportation in large quantities, Br has to be made into
the salts NaBr and KBr, on account of the danger attending
leakage or breakage of the receptacles for Br.

160. Uses.--Its chief uses are in photography (page 167),
medicine, as KBr, and analytical chemistry.

Chapter XXXII.

IODINE.

Examine iodine, potassium iodide.

161. Preparation of I.

Experiment 98.--Put into a t.t. 2 or 3 g. of powdered KI mixed
with an equal bulk of MnO2, add H2SO4 enough to cover well, shake
together, complete the apparatus as for making Br, and heat.
Notice the color of the vapor, and any sublimate. The direct
product of the solidification of a vapor is called a sublimate.
The process is sublimation. Observe any crystals formed. Write
the reaction, and compare the process with that for making Br and
Cl. Compare the vapor density of I with that of Br and of Cl.
With that of air. What vapor is heavier than I? What acid and
what base are represented by KI?

162. Tests.

Experiment 99.--(1) Put a crystal of I in the palm of the hand
and watch it for a minute. (2) Put 2 or 3 crystals into a t.t.,
and warm it, meanwhile holding a stirring-rod half-way down the
tube. Notice the vapor, also a sublimate on the sides of the t.t.
and rod. (3) Add to 2 or 3 crystals in a t.t. 5 cc. of alcohol,
C2H5OH; warm it, and see whether a solution is formed. If so, add
5 cc. H2O and look for a ppt. of I. Does this show that I is not
at all soluble in H2O, or not so soluble as in alcohol?

163. Starch Solution and Iodine Test.

Experiment 100.--Pulverize a gram or two of starch, put it into
an evaporating-dish, add 4 or 5 drops of water, and mix; then
heat to the boiling-point 10 cc. H2O in a t.t., and pour it over
the starch, stirring it meanwhile.

(1) Dip into this starch paste a piece of paper, hold it in the
vapor of I, and look for a change of color. (2) Pour a drop of
the starch paste into a clean t.t., and add a drop or two of the
solution of I in alcohol. Add 5 cc. H2O, note the color, then
boil, and finally cool. (3) The presence of starch in a potato or
apple can be shown by putting a drop of I solution in alcohol on
a slice of either, and observing the color. (4) Try to dissolve a
few crystals of I in 5 cc. H2O by boiling. If it does not
disappear, see whether any has dissolved, by touching a drop of
the water to starch paste. This should show that I is slightly
soluble in water.

164. Iodo-Starch Paper.

Experiment 101.--Add to some starch paste that contains no I 5
cc. of a solution of KI, and stir the mixture. Why is it not
colored blue? Dip into this several strips of paper, dry them,
and save for use. This paper is called iodo-starch paper, and is
used as a test for ozone, chlorine, etc. Bring a piece of it in
contact with the vapor of chlorine, bromine, or ozone, and notice
the blue color.

Experiment 102.--Add a few drops of chlorine water to 2cc. of the
starch and KI solution in 10 cc. H2O. This should show the same
effect as the previous experiment.

165. Explanation.--Only free I, not compounds of it, will color
starch blue. It must first be set free from KI. Ozone, chlorine,
etc., have a strong affinity for K, and when brought in contact
with KI they unite with K and set free I, which then acts on the
starch present. Com- plete the equation: KI + Cl = ?

166. Occurrence.--The ultimate source of I is sea water, of which
it constitutes far too small a percentage to be separated
artificially. Sea-weeds, or algae, especially those growing in
the deep sea, absorb its salts--NaI, KI, etc.--from the water. It
thus forms a part of the plant, and from this much of the I of
commerce is obtained. Algae are collected in the spring, on the
coasts of Ireland, Scotland, and Normandy, where rough weather
throws them up. They are dried, and finally burned or distilled;
the ashes are leached to dissolve I salts; the water is nearly
evaporated, and the residue is treated with H2SO4, and MnO2, as
in the case of Br and Cl. I also occurs in Chili, as NaI and
NaIO3, mixed with NaNO3. This is an important source of the I
supply.

167. Uses.--I is much used in medicine, and was formerly employed
in taking daguerreotypes and photographs. Its solution in alcohol
or in ether is known as tincture of iodine.

168. Fluorine.--F, Cl, Br, I, are called halogens or haloids, and
exist in compounds--salts--in sea water. F is the most active of
all elements, combining with every element except O. Until
recently it has never been isolated, for as soon as set free from
one compound it attacks the nearest substance, and seems to be as
much averse to combining with itself, or to existing in the
elementary state, as to uniting with O. It is supposed to be a
gas, and, as is claimed, has lately been isolated by electrolysis
from HF in a Pt U-tube. Fluorite (CaF2) and cryolite (Al2F6 + 6
NaF) are its two principal mineral sources. The enamel of the
teeth contains F in composition.

CHAPTER XXXIII.

THE HALOGENS.

169. Halogens Compared.--The elements F, Cl, Br, I, form a
natural group. Their properties, as well as those of their
compounds, vary in a step-by-step way, as seen below. F is
sometimes an exception. They are best remembered by comparing
them with one another. Notice:

1. Similarity of name-ending. Each name ends in ine.

2. Similarity of origin. Salt water is the ultimate source of
all, except F.

3. Similarity of valence. Each is usually a monad.

4. Similarity of preparation. Cl, Br, I, are obtained from their
salts by means of MnO2 end H2SO4.

5. Variation in occurrence. Cl occurs in sea-salt, Br in sea-
water, I in sea-weed.

6. Variation in color; F being colorless, Cl green, Br red, I
violet.

7. Gradation in sp. gr.; F 19, Cl 35.5, Br 80, I 127.

8. Gradation in state, corresponding to sp. gr.; F being a light
gas, Cl a heavy gas, Br a liquid, I a solid.

9. Corresponding gradation in their usual chemical activity; F
being most active, then Cl, Br, and I.

10. Corresponding gradation in the strength of the H acids; the
strongest being HF, the next, HCl, etc.

11. Corresponding gradation in the explosibility of their N
compounds; the strongest NCl3, the next, NBr3, etc.

12. Corresponding gradation in the number of H and O acids; Cl 4,
Br 3, I 2.

170. Compounds.--The following are some of the oxides, acids, and
salts of the halogens. Name them.


CI2O (+H2O=) 2 HClO. The salts are hypochlorites, as Ca(ClO)2.
Cl2O3 (+H20=) 2 HClO2. The salts are chlorites, as KClO2.
Cl2O4
-- HClO3 The salts are chlorates, as KClO3.
-- HClO4 The salts are perchlorates, as KClO4,
-- HBrO The salts are ? KBrO,
-- -- The salts are wanting.
-- HBrO3. The salts are ? KBrO3,
-- HBrO4. The salts are ? KBrO4,
-- -- The salts are wanting.
-- -- The salts are wanting.
I2O5 (+H2O=) 2 HIO3. The salts are ? KIO3.
-- HIO4. The salts are ? KIO4.


F forms no oxides, and no acids except HF. HF, HCl, HBr, HI, are
striking illustrations of acids with no O. HClO4 is a very strong
oxidizing agent. A drop of it will set paper on fire, or with
powdered charcoal explode violently. This is owing to the ease
with which it gives up 0. Notice why its molecule is broken up
more readily than HC103. The higher the molecular tower, or the
more atoms it contains, the greater its liability to fall. Some
organic compounds contain hundreds of atoms, and hence are easily
broken down, or, as we say, are unstable. Inorganic compounds
are, as a rule, much more stable than organic ones. It is not
always true, however, that the compound with the least number of
atoms is the most stable. SO2 is more stable than SO3, but H2SO3
is less so than H2SO4.
Chapter XXXIV.

VAPOR DENSITY AND MOLECULAR WEIGHT.

Examine a liter measure, in the form of a cube,--cubic decimeter,
--and a cubic centimeter.

171. Gaseous Weights and Volumes.--A liter of H, at 0 degrees
and 760 mm., weighs nearly 0.09 g. This weight is called a crith.
Find the weight of H in the following, in criths and in grams: 15
1., 0.07 1., 50.3 1., 0.035 1., 0.6 1..

It has been estimated that there are (10) 24. molecules of H in a
liter. Does the number vary for different gases? The weight of a
molecule of H in parts of a crith is 1/(10) 24.; in parts of a
gram .09/(10) 24.. If the H molecule is composed of 2 atoms, what
is the weight of its atom in fractions of a crith? What in
fractions of a gram? The weight of the H atom is a microcrith.
What part of a crith is a microcrith?

172. Vapor Density.--Vapor density, or specific gravity referred
to H as the standard, (Physics) is the ratio of the weight of a
given volume of a gas or vapor to the weight of the same volume
of H. A liter of steam weighs nine times as much as a liter of H.
Its vapor density is therefore nine. For convenience, a definite
volume of H is usually taken as the standard, viz., the H atom.
The volume of the H atom and that of the half-molecule of H2O, or
of any gas are identical, each being represented by one square.
If, then, the standard of vapor density is the H atom, half the
molecular weight of a gas must be its vapor density, since it is
evident that we thus compare the weights of equal volumes. The
vapor density of H2O, steam, is found from the symbol as follows:
(2 + 16) / 2 = 9. To obtain the vapor density of any compound
from the formula, we have only to divide its molecular weight by
two. Find the vapor density of HCl, N2O, NO, C12H22O11, Cl, CO2,
HF, SO2. Explain each case.

The half-molecule, instead of the whole, is taken; because our
standard is the hydrogen atom, the smallest portion of matter, by
weight, known to science.

How many criths in a liter of HCl? How many grams? Compute the
number of criths and of grams in one liter of the compounds whose
symbols appear above.

PROBLEMS.

(1) A certain volume of H weighs 0.36 g. at standard temperature
and pressure. How many liters does it contain? If one liter
weighs 0.09 g., to weigh 0.36 g. it will take 0.36 / 0.09 = 4
liters.

(2) How many liters, or criths, of H in 63 g.? 2.7 g.? 1 g.? 5
g.? 250 g.? Explain each.

(3) Suppose the gas to be twice as heavy as H, how many liters in
0.36 g.? A liter of the gas will weigh 0.18 g. (0.09 X 2). In
0.36 g. there will be 0.36 / 0.18 = 2. Answer the question for 63
g., 2.7 g., etc.

(4) How many liters of Cl in each of the above numbers of grams?

(5) How many of HCl? H2O (steam)? CO2? Explain fully every case.

Vapor density is very easily determined from the formula by the
method given above. But in practice the formula is obtained from
the vapor density, and hence the method there given has to be
reversed.

173. Vapor Density of Oxygen.--Suppose we were to obtain the
vapor density of O. We should carefully seal and weigh a given
volume, say a liter, at a noted temperature and barometric
pressure, which are reducedto 0 degrees and 760 mm, and compare
it with the weight of the same volume of H. This has been done
repeatedly, and O has been found to weigh 16 times as much as H,
volume for volume, or, more exactly, 15.96+. Now a liter of each
gas has the same number of molecules, therefore the O molecule
weighs 16 times the H molecule. The half-molecule of each has the
same proportion, and the vapor density of O is 16. Atomic weight
is obtained in a very different way.

PROBLEMS.

(1) A liter of Cl is found to weigh 3.195 g. Compute its vapor
density, and explain fully.

(2) A liter of Hg vapor, under standard conditions, weighs 9 g.
Find its vapor density, and explain.

The vapor density of only a few elements has been satisfactorily
determined. See page 12. Some cannot be vaporized; others can be,
but only under conditions which prevent weighing them. The vapor
density of very many compounds also is unknown.

(3) A liter of CO2 weighs 1.98 g. Find the vapor density, and
from that the molecular weight, remembering that the latter is
twice the former. See whether it corresponds to that obtained
from the formula, CO2. This is,in fact, the way a formula is
ascertained, if the atomic weights of its elements are known.

(4) A liter of a compound gas weighs 2.88 g. Analysis shows that
its weight is half S and half O. As the atomic weight of S is 32,
and that of O is 16, what is the symbol for the gas?

Solution. Its molecular weight is 64, i.e. (2.88=0.09) X 2, of
which 32 is S and 32 O. The atomic weight of S is 32, hence there
is one atom of S, while of O there are two atoms. The formula is
SO2.

(5) A liter of a compound gas, which is found to contain 1 C and
3 O by weight, weighs 1.26 g. What is its formula? Atomic weights
are taken from page 12. Prove your answer.

(6) A liter of a compound of N and O weighs 1.98 g. The N is
7/11; and the O 4/11. What is the gas?

(7) A compound of N and H gas weighs 0.765 g. to the liter. The N
is 14/17 of the whole, the H 3/17. What gas is it? CHAPTER XXXV.

ATOMIC WEIGHT.

174. Definition.--We have seen that the molecular weight of a
compound, as well as of most elements, is obtained from the vapor
density by doubling the latter. It remains to explain how atomic
weights are obtained. The term is rather misleading. The atomic
weight of an element is its least combining weight, the smallest
portion that enters into chemical union, which is, of course, the
weight of an atom.

175. Atomic Weight of Oxygen.--Suppose we wish to find the atomic
weight of oxygen. We must find the smallest proportion by weight
in which it occurs in any compound. This can only be done by
analyzing all the compounds of O that can be vaporized. As
illustrative of these compounds take the six following:--


Wt. of other
Names. V. d. Mol. Wt. Wt. of O. Elem. Symbol.
Carbon monoxide... 14 28 16 12 ?
Carbon dioxide.... 22 44 32 12 ?
Hydrogen monoxide... 9 18 16 2 ?
Nitrogen monoxide... 22 44 16 28 ?
Nitrogen trioxide... 38 76 48 28 ?
Nitrogen pentoxide... 54 108 80 28 ?


176. Molecular Symbols.--From the vapor density of the gases--
column 2--we obtain their molecular weight-- column 3. To find
the proportion of O, it must be separated by chemical means from
its compounds and separately weighed. These relative weights are
given in column 4. Now the smallest weight of O which unites in
any case is its atomic weight. If any compound of O should in
future be found in which its combining weight is 8 or 4, that
would be called its atomic weight. By dividing the numbers in
column 4, wt. of O, by 16, the atomic weight of O, we obtain the
number of O atoms in the molecule. Subtracting the weights of O
from the molecular weights, we have the parts of the other
elements, column 5, and dividing these by the atomic weight of
the respective elements, we have the number of atoms of those
elements, these last, combined with the number of O atoms, give
the symbol. In this way complete the last column.

Show how to get the atomic weight of Cl from these compounds,
arranging them in tabular form, and completing as above: HCl,
KCl, NaCl, ZnCl2, MgCl2; the atomic weight of N in these: N2O,
NO, NH3.

177. Molecular and Atomic Volumes.--We thus see that vapor
density and atomic weight are obtained in two quite different
ways. In the case of elements the two are usually identical, i.e.
with the few whose vapor density is known; but this is not always
true, and it leads to interesting conclusions regarding atomic
volume. In O both vapor density and atomic weight are 16. This
gives 2 atoms of O to the molecule, i.e. the molecular weight /
the atomic weight. The size of an O atom is therefore half the
gaseous molecule, and is represented by one square. S has a vapor
density and an atomic weight of 32 each. Compute the number of
atoms in the molecule. Compute for I, in which the two are
identical, 127. P has an atomic weight of 31, while its vapor
density is 62. Its molecule must consist of 4 atoms, each half
the size of the H atom, The vapor density of As is 150, the
atomic weight 75. Compute the number of atoms in its molecule,
and represent their relative size. Hg has an atomic weight of
200, a vapor density of 100. Compute as before, and compare the
results with those on page 12. Ozone has an atomic weight of 16,
a vapor density 24. Compute.

Chapter XXXVI.

DIFFUSION AND CONDENSATION OF GASES.

178. Diffusion of Gases.--Oxygen is 16 times as heavy as H. If
the two gases were mixed, without combining, in a confined space,
it might be supposed that O would settle to the bottom and H rise
to the top. This would, in fact, take place at first, but only
for an instant, for all gases tend to diffuse or become
intimately mixed. The lighter the gas the more quickly it
diffuses.

179. Law of Diffusion of Gases.--The diffusibility of gases
varies inversely as the square roots of their vapor densities.
Compare the diffusibility of H with that of O. dif. H:dif. O::
sqrt(16): sqrt(1), or dif: H: dif. O:: 4: 1.

That is to say, if H and O be set free from separate receivers in
a room, the H will become intermingled with the atmosphere four
times as quickly as the O. Compare the diffusibility of O and N;
of Cl and H. Take the atomic weights of these, since they are the
same as the vapor densities. In case of a compound gas, half the
molecular weight must be taken for the vapor density; e.g. dif.
N20: dif. O.:: sqrt(16): sqrt(22).

180. Cause.--Diffusion is due to molecular motion; the lighter
the gas the more rapid the vibration of its molecules. Compare
the diffusibility of CO2 and that of Cl; of HCl and SO2; of HF
and I.

181. Liquefaction and Solidification of Gases.--Water boils at
100 degrees, under standard pressure, though evaporating at all
temperatures; it vaporizes at a lower point if the pressure be
less, as on a mountain, and at a higher temperature if the
pressure be greater, as at points below the sea level. Alcohol
boils at 78 degrees, standard pressure, and every liquid has a
point of temperature and pressure above which it must pass into
the gaseous state. Likewise every gas has a critical temperature
above which it cannot be liquefied at any pressure.

This condition was not recognized formerly, and before 1877, O,
H, N, C4, CO, NO, etc., had not been liquefied, though put under
a pressure of more than 2,000 atmospheres. They were called
permanent gases. In 1877 Cailletet and Pictet liquefied and
solidified these and others. The lowest temperature, about -225
degrees, was produced by suddenly releasing the pressure from
solid N to 4mm, which caused it rapidly to evaporate.
Evaporation, especially under diminished pressure, always lowers
the temperature by withdrawing heat.

These low degrees are indicated by a H thermometer, or if too low
for that, by a "thermo-electric couple" of copper and German
silver.

The pupil can easily liquefy SO, by passing it through a U-tube
which is surrounded by a mixture of ice and salt in a large
receiver. At the meeting of the American Association for the
Advancement of Science in 1887, a solid brick of CO2 was seen and
handled by the members, Liquid H is steel blue.

A few results obtained under a pressure of one atmosphere are:--
Boiling Points: C2H4--102 degrees; CH4--184 degrees; O--181
degrees; N --194 degrees; CO--190 degrees; NO--154 degrees; Air--
191 degrees.

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