A B C D E F G H I J K L M N O P R S T U V W X Z

An Introduction to Chemical Science

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86. Properties and Tests.

Experiment 53.--(1) Note the color of the prepared liquid. (2)
Put a drop on the finger; then wash it off at once. (3) Dip a
quill or piece of white silk into it; then wash off the acid.
What color is imparted to animal substances? (4) Add a little to
a few bits of Cu turnings, or to a Cu coin. Write the equation.
(5) To 2 cc.indigo solution, add 2 cc. HNO3. State the leading
properties of HNO3, from these tests.

87. Chemically Pure HNO3 is a Colorless Liquid.-- The yellow
color of that prepared in Experiment 52 is due to liquid NO2
dissolved in it. It is then called fuming HNO3, and is very
strong. NO2 is formed at a high temperature.

Commercial or ordinary HNO3, is made from NaNO3, this being
cheaper than KNO3; it is about half water.

88. Uses. HNO3 is the basis of many nitrates, as AgNO3, used for
photography, Ba(NO3)2 and Sr(NO3)2 for fire-works, and others for
dyeing and printing calico; it is employed in making aqua regia,
sulphuric acid, nitro-glycerine, gun-cotton, aniline colors,
zylonite, etc.

Enough experiments have been performed to answer the question
whether some acids can be prepared from their salts. H2SO4 is not
so made, because no acid is strong enough to act on its salts. In
making HCl, HNO3, etc., sulphuric acid was used, being the
strongest.

AQUA REGIA.

89. Preparation and Action. Experiment 54.--Into a t.t. put 2 cc.
HNO3, and 14 qcm. of either Au leaf or Pt. Warm in a flame. If
the metal is pure, no action takes place. Into another tube put 6
cc. HCl and add a similar leaf. Heat this also. There should be
no action. Pour the contents of one t.t. into the other. Note the
effect. Which is stronger, one of the acids, or the combination
of the two? Note the odor. It is that of Cl. 3HCl + HNO3 = NOCl +
2H2O + Cl2. This reaction is approximate only. The strength is
owing to nascent chlorine, which unites with Au. Au + 3Cl =
AuCl3. If Pt be used, PtCl4 is produced. No other acid except
nitro-hydrochloric will dissolve Au or Pt; hence the ancients
called it aqua regia, or king of liquids. It must be made as
wanted, since it cannot be kept and retain its strength.

CHAPTER XIX.

SULPHURIC ACID.

90. Preparation.

Experiment 55.--Having fitted a cork with four or five
perforations to a large t.t., pass a d.t. from three of these to
three smaller t.t., leaving the others open to the air, as in
Figure 28. Into one t.t. put 5 cc. H2O, into another 5 g. Cu
turnings and 10 cc. H2SO4, into the third 5 g. Cu turnings and 10
cc. dilute HNO3, half water. Hang on a ring stand, and slowly
heat the tubes containing H2O and H2SO4. Notice the fumes that
pass into the large t.t.

Trace out and apply to Figure 28 these reactions:--

(1) Cu + 2 H2SO4 = CuSO4 + 2 H2O + SO2.

(2) 3 Cu + 8 HNO3 = 3 Cu(NO3)2+ 4 H2O + 2 NO.

(3) NO + O = NO2.

(4) SO2 + H2O + NO2 =H2SO4 + NO.

(4) comes from combining the gaseous products in (1), (2), (3).
In (3), NO takes an atom of O from the air, becoming NO2, and at
once gives it up, to the H2SO3 (H2O + SO2), making H2SO4, and
again goes through the same operation of taking up O and passing
it along. NO is thus called a carrier of O. It is a reducing
agent, while NO2 is an oxidizing agent. This is a continuous
process, and very important, since it changes useless H2SO3 into
valuable H2SO4. If exposed to the air, H2SO3 would very slowly
take up O and become H2SO4.

Instead of the last experiment, this may be employed if
preferred: Burn a little S in a receiver. Put into an
evaporating-dish, 5 cc. HNO3, and dip a paper or piece of cloth
into it. Hang the paper in the receiver of SO2, letting no HNO3
drop from it. Continue this operation till a small quantity of
liquid is found in the bottle. The fumes show that HNO3 has lost
O. 2 HNO3 + SO2 = H2SO4 + 2 NO2.

91. Tests for H2SO4.

Experiment 56.--(1) Test the liquid with litmus. (2) Transfer it
to a t.t., and add an equal volume of BaCl2 solution. H2SO4 +
BaCl2 = ? Is BaSO4 soluble? (3) Put one drop H2SO4 from the
reagent bottle in 10 cc. H2O in a clean t.t., and add 1 cc. BaCl2
solution. Look for any cloudiness. This is the characteristic
test for H2SO4 and soluble sulphates, and so delicate that one
drop in a liter of H2O can be detected. (4) Instead of H2SO4, try
a little Na2SO4 solution. (5) Put two or three drops of strong
H2SO4 on writing-paper, and evaporate, high over a flame, so as
not to burn the paper. Examine it when dry. (6) Put a stick into
a t.t. containing 2 cc. H2SO4, and note the effect. (7) Review
Experiment 5. (8) Into an e.d. pour 5 cc. H2O, and then 15 cc.
H2SO4. Stir it meantime with a small t.t. containing 2 or 3 cc.
NH4OH, and notice what takes place in the latter; also note the
heat of the e.d.

The effects of (5), (6), (7), and (8) are due to the intense
affinity which H2SO4 has for H2O. So thirsty is it that it even
abstracts H and O from oxalic acid in the right proportion to
form H2O, combines them, and then absorbs the water.

92. Affinity for Water.--This acid is a desiccator or dryer, and
is used to take moisture from the air and prevent metallic
substances from rusting. In this way it dilutes itself, and may
increase its weight threefold. In diluting, the acid must always
be poured into the water slowly and with stirring, not water into
the acid, since, as H2O is lighter than H2SO4, heat enough may be
set free at the surface of contact to cause an explosion.
Contraction also takes place, as may be shown by accurately
measuring each liquid in a graduate, before mixing, and again
when cold. The mixture occupies less volume than the sum of the
two volumes. For the best results the volume of the acid should
be about three times that of the water.

93. Sulphuric Acid made on a Large Scale involves the same
principles as shown in Experiment 55, excepting that S02 is
obtained by burning S or roasting FeS2 (pyrite),

[Fig. 29.]

and HNO3 is made on the spot from NaNO3 and H2SO4. SO2 enters a
large leaden chamber, often 100 to 300 feet long, and jets of
steam and small portions of HNO3 are also forced in. The "chamber
acid" thus formed is very dilute, and must be evaporated first in
leaden pans, and finally in glass or platinum retorts, since
strong H2SO4, especially if hot, dissolves lead. See Experiment
124. Study Figure 29, and write the reactions. 2 HNO3 breaks up
into 2 NO2, H2O, and O. 94. Importance.--Sulphuric acid has been
called, next to human food, the most indispensable article known.
There is hardly a product of modern civilization in the
manufacture of which it is not directly or indirectly used.
Nearly a million tons are made yearly in Great Britain alone. It
is the basis of all acids, as Na2CO3 is of alkalies. It is the
life of chemical industry, and the quantity of it consumed is an
index of a people's civilization. Only a few of its uses can be
stated here. The two leading ones are the reduction of Ca3(PO4)2
for artificial manures and the sodium carbonate manufacture.
Foods depend on the productiveness of soils and on fertilizers,
and thus indirectly our daily bread is supplied by means of this
acid; and from sodium carbonate glass, soap, saleratus, baking-
powders, and most alkalies are made directly or indirectly. H2SO4
is employed in bleaching, dyeing, printing, telegraphy,
electroplating, galvanizing iron and wire, cleaning metals,
refining Au and Ag, making alum, blacking, vitriols, glucose,
mineral waters, ether, indigo, madder, nitroglycerine, gun-
cotton, parchment, celluloid, etc., etc.

FUMING SULPHURIC ACID.

95. Nordhausen or Fuming Sulphuric Acid, H2S207 used in
dissolving indigo and preparing coal-tar pigments, is made by
distilling FeSO4. 4FeSO4 + H2O = H2S207 + 2Fe203 + 2S02. This was
the original sulphuric acid. It is also formed when S03 is
dissolved in H2SO4. When exposed to the air, S03 escapes with
fuming.

CHAPTER XX.

AMMONIUM HYDRATE.

96. Preparation of Bases.--We have seen that many acids are made
by acting on a salt of the acid required, with a stronger acid.
This is the direct way. The following experiments will show that
bases may be prepared in a similar way by acting on salts of the
base required with other bases, which we may regard as stronger
than the ones to be obtained.

97. Preparation of NH4OH and NH3.

Experiment 57.--Powder 10 g. ammonium chloride, NH4Cl, in a mortar
and mix with 10 g. calcium hydrate, Ca(OH)2; recently slaked lime
is the best. Cover with water in a flask, and connect with Woulff
bottles, as for making HCl (Fig. 22); heat the flask for fifteen
minutes or more. The experiment may be tried on a smaller scale
with a t.t. if desired.

The reaction is: 2NH4Cl + Ca(OH)2 = CaCl2 + 2NH4OH. NH4OH is
broken up into NH3, ammonia gas, and water. NH4OH = NH3 + H2O.
These pass over into the first bottle, where the water takes up
the NH3, for which it has great affinity. One volume of water at
0° will absorb more than 1000 volumes of NH3. Thus NH4OH may be
called a solution of NH3, in H2O. Write the reaction.

Experiment 58.--Powder and mix 2 or 3 g. each of ammonium
nitrate, NH4NO3, and Ca(OH)2; put them into a t.t., and heat
slowly. Note the odor. 2NH4NO3 + Ca(OH)2 = ?

98. Tests.

Experiment 59.--(1) Generate a little of the gas in a t.t., and
note the odor. (2) Test the gas with wet red litmus paper. (3)
Put a little HCl into an e.d., and pass over it the fumes of NH3
from a d.t. Note the result, and write the equation. (4) Fill a
small t.t. with the gas by upward displacement; then, while still
inverted, put the mouth of the t.t. into water. Explain the rise
of the water. (5) How might NH4Cl be obtained from the NH4OH in
the Woulff bottles? (6) Test the liquid in each bottle with red
litmus paper. (7) Add some from the first bottle to 5 or 10 cc.
of a solution of FeSO4 or FeCl2, and look for a ppt. State the
reaction.

99. Formation.--Ammonia, hartshorn, exists in animal and
vegetable compounds, in salts, and, in small quantities, in the
atmosphere. Rain washes it from the atmosphere into the soil;
plants take it from the soil; animals extract it from plants.
Coal, bones, horns, etc., are the chief sources of it, and from
them it is obtained by distillation. It results also from
decomposing animal matter. NH3 can be produced by the direct
union of N and H, only by an electric discharge or by ozone. It
may be collected over Hg like other gases that are very soluble
in water.

100. Uses. --Ammonium hydrate, NH4OH, and ammonia, NH3, are used
in chemical operations, in making artificial ice, and to some
extent in medicine; from them also may be obtained ammonium
salts. State what you would put with NH4OH to obtain (NH4)2SO4.
To obtain NH4NO3. The use of NH4OH in the laboratory may be
illustrated by the following experiment:--

Experiment 60.--Into a t.t. put 10 cc. of a solution of ferrous
sulphate, FeSO4. Into another put 10 cc. of sodium sulphate
solution, Na2SO4. Add a little NH4OH to each. Notice a ppt. in
the one case but none in the other. If solutions of these two
compounds were mixed, the metals Fe and Na could be separated by
the addition of NH4OH, similar to the separation of Ag and Cu by
HCl. Try the experiment.

CHAPTER XXI.

SODIUM HYDRATE.

101. Preparation.

Experiment 61.--Dissolve 3 g. sodium carbonate, Na2CO3, in 10 or
15 cc. H2O in an e.d., and bring it to the boiling-point. Then
add to this a mixture of 1 or 2 g. calcium hydrate, Ca(OH)2, in 5
or 10cc. H2O. It will not dissolve. Boil the whole for five
minutes. Then pour off the liquid which holds NaOH in solution.
Evaporate if desired. This is the usual mode of preparing NaOH.

The reaction is Na2CO3 + Ca(OH)2 = 2NaOH + CaCO3. The residue is
Ca(OH)2 and CaCO3; the solution contains NaOH, which can be
solidified by evaporating the water. Sodium hydrate is an
ingredient in the manufacture of hard soap, and for this use
thousands of tons are made annually, mostly in Europe. It is an
important laboratory reagent, its use being similar to that of
ammonium hydrate. Exposed to the air, it takes up water and CO2,
forming a mixture of NaOH and Na2CO3. It is one of the strongest
alkalies, and corrodes the skin.

Experiment 62.--Put 20 cc. of H2O in a receiver. With the forceps
take a piece of Na, not larger than half a pea, from the naphtha
in which it is kept, drop it into the H2O, and at once cover the
receiver loosely with paper or cardboard. Watch the action, as
the Na decomposes H2O. HOH + Na = NaOH + H. If the water be hot
the action is so rapid that enough heat is produced to set the H
on fire. That the gas is H can be shown by putting the Na under
the mouth of a small inverted t.t., filled with cold water, in a
water-pan. Na rises to the top, and the t.t. fills with H, which
can be tested. NaOH dissolves in the water.102. Properties.

Experiment 63.--(1) Test with red litmus paper the solutions
obtained in the last two experiments. (2) To 5cc.of alum
solution, K2A12(SO4)4, add 2cc.of the liquid, and notice the
color and form of the ppt.

POTASSIUM HYDRATE.

103. KOH is made in the Same Way as NaOH.

Describe the process in full (Experiment 61), and give the
equation.

Experiment 64.--Drop a small piece of K into a receiver of H2O,
as in Experiment 62. The K must be very small, and the experiment
should not be watched at too close a range. The receiver should
not be covered with glass, but with paper. The H burns, uniting
with O of the air. The purple color is imparted by the burning,
or oxidation of small particles of K. Write the equation for the
combustion of each.

H2O might be considered the symbol of an acid, since it is the
union of H and a negative element; or, if written HOH, it might
be called a base, since it has a positive element and the (OH)
radical. It is neutral to litmus, and on this account might be
called a salt. It is better, however, to call it simply an oxide.

Potassium hydrate, caustic potash, is employed for the
manufacture of soft soap. As a chemical reagent its action is
almost precisely like that of caustic soda, though it is usually
considered a stronger base, as K is a more electro-positive
element than Na.

CALCIUM HYDRATE.

104. Calcium Hydrate, the Most Common of the Bases, is nearly as
important to them as H2SO4 is to acids. Since it is used to make
the other bases, it might be called the strongest base; as H2SO4
is often called the strongest acid. The strength of an acid or
base, however,depends on the substance to which it is applied, as
well as on itself, and for most purposes this one is classified
as a weaker base than the three previously described.

Sulphuric acid, the most useful of the acids, is not made
directly from its salts, but has to be synthesized. Calcium
hydrate is also made by an indirect process, as follows:

CaCO3, i.e. limestone, marble, etc., is burnt in kilns with C, a
process which separates the gas, CO2, according to the reaction:
CaCO3 = CaO + CO2. CaO is unslaked lime, or quick-lime. On
treating this with water, slaked lime, Ca(OH)2 is formed, with
generation of great heat. CaO + H2O = Ca(OH)2. Its affinity for
H2O is so great that it takes the latter from the air, if
exposed.

Experiment 65.--Saturate some unslaked lime with water, in an
e.d., and look for the results stated above, leaving it as long
as may be necessary.

105. Resume.--From the experiments in the last few chapters on
the three divisions of chemical compounds, acids, bases and
salts, we have seen (1) that acids and bases are the chemical
opposites of each other; (2) that salts are formed by the union
of acids and bases; (3) that some acids can be obtained from
their salts by the action of a stronger acid; (4) that some bases
can be got from salts by the similar action of other bases; (5)
that the strongest acids and bases, as well as others, may be
obtained in an indirect way by synthesis.

CHAPTER XXII.

OXIDES OF NITROGEN.

106. There are five oxides of N, only two of which are important.

NITROGEN MONOXIDE (N2O).

107. Preparation.

Experiment 66.--Put into a flask, holding 200cc, lOg of ammonium
nitrate, NH4NO3; heat it over wire gauze or asbestus in an iron
plate, having a d.t. connected with a large t.t., which is held
in a receiver of water, and from this t.t., another d.t. passing
into a pneumatic trough, so as to collect the gas over water
(Fig. 30). Have all the bearings tight. The reaction is NH4NO3 =
2H2O + N2O. The t.t. is for collecting the H2O.

[Fig. 30.]

Note the color of the liquid in the t.t.; taste a drop, and test
it with litmus. If the flask is heated too fast, some NO is
formed, and this taking O from the air makes NO2, which liquefies
and gives an acid reaction and a red color. Some NH4NO3 is also
liable to be carried over.

108. Properties.

Experiment 67.--Test the gas in the receiver with a burning stick
and a glowing one, and compare the combustion with that in O.
N20may also be tested with S and P, if desired. N is set free in
each case. Write the reactions.

Nitrogen monoxide or protoxide, the nitrous oxide of dentists,
when inhaled, produces insensibility to pain,-- anaesthesia,--
and, if continued, death from suffocation. Birds die in half a
minute from breathing it. Mixed with one-fourth O, and inhaled
for a minute or two, it produces intoxication and laughter, and
hence is called laughing gas. As made in Experiment 66, it
contains Cl and NO, as impurities, and should not be breathed.

NITROGEN DIOXIDE (NO, OR N2O2).

109. Preparation.

Experiment 68.--Into a t.t. or receiver put 5g Cu turnings, add 5
cc. H2O and 5 cc. HNO3. Collect the gas like H, over water. 3Cu +
8HNO3 = ? What two products will be left in the generator? Notice
the color of the liquid. This color is characteristic of Cu
salts. Notice also the red fumes of NO2.

110. Properties.

Experiment 69.--Test the gas with a burning stick, admitting as
little air as possible. Test it with burning S. NO is not a
supporter of C and S combustion. Put a small bit of P in a
deflagrating-spoon, and when it is vigorously burning, lower it
into the gas. It should continue to burn. State the reaction.
What combustion will NO support? Note that NO is half N, while
N2O is two-thirds N, and account for the difference in supporting
combustion.

NITROGEN TETROXIDE (NO2 or N2O4).

111. Preparation.

Experiment 70.--Lift from the water-pan a receiver of NO, and
note the colored fumes. They are NO2, or N2O4, nitrogen
tetroxide. NO + O = NO2. Is NO combustible? What is the source of
O in the experiment?OXIDES OF NITROGEN.

NITROGEN TRIOXIDE (N2O3).

112. Preparation.

Experiment 71.--Put into a t.t. 1 g. of starch and 1 cc. of HNO3.
Heat the mixture for a minute. The red fumes are N2O3 and NO2.

Nitrogen pentoxide, N2O5, is an unimportant solid. United with
water it forms HNO3. N2O5 + H2O = 2HNO3.

CHAPTER XXIII.

LAWS OF DEFINITE AND OF MULTIPLE PROPORTION.

113. Weight and Volume.--We have seen that water contains two
parts of H by volume to one part of O; or, by weight, two parts
of H to sixteen of O. These proportions are invariable, or no
symbol for water would be possible. Every compound in the same
way has an unvarying proportion of elements.

114. Law of Definite Proportion.--In a given compound the
proportion of any element by weight, or, if a gas, by volume is
always constant. Apply the law, by weight and by volume, to
these: HCl, NH3, H2S, N2O.

There is another law of equal importance in chemistry, which the
compounds of N and O well illustrate.


Weight. Volume.
N. O. N. O.
Nitrogen protoxide N2O 28 16 2 1
Nitrogen dioxide N2O2 28 32 2 2
Nitrogen trioxide. N2O3 28 48 2 3
Nitrogen tetroxide N2O4 28 64 2 4
Nitrogen pentoxide N2O5 28 80 2 5


Note that the proportion of O by weight is in each case a
multiple of the first, 16. Also that the proportion by volume of
O is a multiple of that in the first compound. In this example
the N remains the same. If that had varied in the different
compounds, it would also havevaried by a multiple of the smallest
proportion. This is true in all compounds.

115. Law of Multiple Proportion.--Whenever one element combines
with another in more than one proportion, it always combines in
some multiple, one or more, of its least combining weight, or, if
a gas, of its least combining volume.

The least combining weight of an element is its atomic weight;
and it is this fact of a least combining weight that leads us to
believe the atom to be indivisible.

Apply the law in the case of P2O, P2O3, P2O5; in HClO, HClO2,
HClO3, HClO4, arranging the symbols, weights, and volumes in a
table, as above.

The volumetric proportions of each element in the oxides of
nitrogen are exhibited below.


_ + _ + _ = __
N + N + O = N2O

_ + _ + _ + _ = __
N + N + O + O = N2O2

_ + _ + _ + _ + _ = __
N + N + O + O + O = N2O3

_ + _ + _ + _ + _ + _ = __
N + N + O + O + O + O = N2O4

_ + _ + _ + _ + _ + _ + _ = __
N + N + O + O + O + O + O = N2O5

CHAPTER XXIV.

CARBON PROTOXIDE.

116. Preparation.

Experiment 72.--Put into a flask, of 200 cc., 5 g. of oxalic acid
crystals, H2C2O4, and 25 cc. H2SO4. Have the d.t. pass into a
solution of NaOH in a Woulff bottle (Fig. 31), and collect
the gas over water. Heat the flask slowly, and avoid inhaling the
gas.

117. Tests.

Experiment 73.--Remove a receiver of the gas, and try to light
the latter with a splinter. Is it combustible, or a supporter of
(C) combustion? What is the color of the flame? When the
combustion ceases, shake up a little lime water with the gas left
in the receiver. What gas has been formed by the combustion, as
shown by the test? See page 80. Give the reaction for the
combustion.

We have seen that H2SO4 has great affinity for H2O. Oxalic acid
consists of H, C, O in the right proportion to form H2O, CO2, and
CO. H2SO4 withdraws H and O in the right proportion to form
water, unites them, and then absorbs the water, leaving the C and
O to combine and form CO2 and CO. NaOH solution removes CO2 from
the mixture, forming Na2CO3, and leaves CO. Write both reactions.

118. Carbon Protoxide, called also carbon monoxide, carbonic
oxide, etc., is a gas, having no color or taste, butpossessing a
faint odor. It is very poisonous. Being the lesser oxide of C, it
is formed when C is burned in a limited supply of O, whereas CO2
is always produced when O is abundant. The formation of each is
well shown by tracing the combustion in a coal fire. Air enters
at the bottom, and CO2 is first formed. C + 2O = CO2. As this gas
passes up, the white-hot coal removes one atom of O, leaving CO.
CO2 + C - 2CO. At the top, if the draft be open, a blue flame
shows the combustion of CO. CO + O = CO2. The same reduction of
CO2 takes place in the iron furnace, and whenever there is not
enough oxygen to form CO2, the product is CO.

Great care should be taken that this gas does not escape into the
room, as one per cent has proved fatal. Not all of it is burned
at the top of the coal; and when the stove door is open, the
upper drafts should be open also. It is the most poisonous of the
gases from coal; hence the danger from sleeping in a room having
a coal fire.

119. Water Gas.--CO is one of the constituents of "water gas,"
which, by reason of its cheapness, is supplanting gas made from
coal, as an illuminator, in some cities. It is made by passing
superheated steam over red-hot charcoal or coke. C unites with
the O of H2O, forming CO, and sets H free, thus producing two
inflammable gases. C + H2O --? As neither of these gives much
light, naphtha is distilled and mixed with them in small
quantities to furnish illuminating power See page 183.

CHAPTER XXV.

CARBON DIOXIDE.

120. Preparation.

Experiment 74.--Put into a t.t., or a bottle with a d.t. and a
thistle-tube, 10 or 20 g. CaCO3, marble in lumps; add as many
cubic centimeters of H2O, and half as much HCl, and collect the
gas by downward displacement (Fig. 39). Add more acid as needed.
CaCO3 + 2 HCl = CaCl2 + H2CO3. H2CO3 = H2O + CO2. H2CO3 is a very
weak compound, and at once breaks up. By some, its existence as a
compound is doubted.

121. Tests.

Experiment 75.--(1) Put a burning and a glowing stick into the
t.t. or bottle. (2) Hold the end of the d.t. directly against the
flame of a small burning stick. Does the gas support combustion?
(3) Pour a receiver of the gas over a candle flame. What does
this show of the weight of the gas? (4) Pass a little CO2 into
some H2O (Fig. 32), and test it with litmus. Give the reaction
for the solution of CO2 in H2O.

Experiment 76.--Put into a t.t. 51 cc. of clear Ca(OH)2 solution,
i.e. lime water; insert in this the end of a d.t. from a CO2
generator (Fig. 32). Notice any ppt. formed. It is CaCO3. Let the
action continue until the ppt. disappears and the liquid is
clear. Then remove the d.t., boil the clear liquid for a minute,
and notice whether the ppt. reappears.

122. Explanation.

Ca(OH)2 + CO2 = CaCO3 + H2O. The curious phenomena of this
experiment are explained by the solubility of CaCO3 in water
containing CO2, and its insolu-bility in water, having no CO2.
When all the Ca(OH)3 is combined, or changed to CaCO3, the excess
of CO2 unites with H2O, forming the weak acid H2CO3, which
dissolves the precipitate, CaCO3, and gives a clear liquid. On
heating this, H2CO3 gives up its CO2, and CaCO3 is
reprecipitated, not being soluble in pure water.

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