2-2 Vaporization and expansion
Heated water evaporates and becomes water vapor
in a phenomenon known as evaporation, but this process
change from the liquid phase to the gaseous phase
is accompanied by expanding volume, a phenomenon
known as evaporative expansion. According to the
Ideal Gas Equation on the relationship between the
temperature and pressure of a gas, when this expansion
occurs in a closed system the internal pressure
of the system increases in proportion to the temperature.
For example, at 0 °C
and 1 atm, 1 cm3
of water becomes 1.24 L when vaporized, in other
words, it expands to 1,240 times its liquid volume.
This phenomenon is shown in the following manner.
The Ideal Gas Equation
PV = nRT
We can substitute the following
values:
P = 1 atm
R = 0.082 atm/ °C
• mol
T = 273°C,
and
n = 1/18 mol/gm
in the following formula:
V = nRT/P
to confirm that the result is
1.24 L.
For example, when water (liquid) 18 cm3
evaporates, it becomes:
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vaporizes
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Water (liquid)
18 cm3 (gm) |
→
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water vapor
(gas) 22.4 L |
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1,240 times
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2-3 Freezing and expansion
Volume expansion due to freezing occurs on
a very small scale compared to vaporization
expansion, and it also relies much less on
temperature. When 1 cm3
of water freezes at 0 °C
and 1 atm, the volume becomes 1.09 cm3
, a 1.09 times increase. This value can be
found in the following manner. The density
of water at 0 °C
and 1 atm is 1 gm/cm3
, so the volume of 1 cm3
of water has a weight WL of 1 gm. When this
freezes, at 0 °C
and 1 atm, the density ds of ice = 0.917 gm/cm3
, so the volume can be found as follows.
In other words, when 1 cm3
of water (liquid) freezes:
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freezing
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| Water (liquid)
1 cm3 |
→
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ice (solid)
1.09 cm3
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1.09 times
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The reason for this freezing expansion
is as follows.
Within liquid water are groups
of water molecules that are linked together
by hydrogen bonds (bonds formed when the positively
charged H atoms in the molecules form mutual
electrostatic attractions with negatively
charged atoms in other molecules, in this
case oxygen atoms). When the temperature drops,
the thermal motion of water decreases and
the groups become larger, and at around 0 °C
most of the water molecules are linked by
hydrogen bonds. Then, as the temperature drops
below 0 °C ,
the water molecules lose the ability to move
freely, becoming crystallized. Therefore,
the water molecules in ice form an orderly
crystal lattice. This crystalline structure
contains a large number of gaps, and so the
volume occupied is greater than water. Most
substances become denser as the temperature
drops and less dense as the temperature rises.
(Temperature is inversely proportional to
density, forming the basis for the use of
substances such as alcohol or mercury in thermometers.)
This property of water differs from other
substances, and is an exception to the general
rule that the solid phase is less dense. When
ice melts and water molecules become able
to move, molecules are able to enter the gaps
causing water to become more dense, and so
ice is able to float. (Example, icebergs)
Expressing the existence of water as humidity
refers to the amount of water vapor in the
air. The unit of humidity generally used as
an environmental factor in environmental testing
is called relative humidity, which is based
on the capacity standard. Relative humidity
is defined by the following formula.
Where,
RH = relative humidity,
e = the moisture content
actually present of the air's uniform capacity
at the uniform temperature, and
E = the maximum moisture
content possible at the temperature condition
in e, at the saturation moisture content in
the air.
This moisture content is also
shown in proportion to a certain water vapor
pressure. |
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| 3. Chemical properties of temperature
and moisture |
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Thermal
energy, the index of temperature, is
not matter so it is not directly involved
in chemical reactions. Thermal energy
can penetrate or leave matter. It can
come into being and be extinguished
inside matter, and can intensify or
calm the motion of molecules forming
matter. It can effectively speed or
slow chemical reactions. At the same
time, chemical reactions occurring inside
matter can create new thermal energy
or exhaust thermal energy as a product
of the reaction. However, when the reacting
substances of a chemical reaction are
consumed, the reaction will stop regardless
of temperature.
Thermal energy plays a supporting
role in relation to moisture, but the
main role in this domain is the moisture
itself, and its absolute mass. Most
chemical reactions occur in solvents,
so we can assume that water molecules
play some sort of role. |
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3-2 Properties of
water
The more pure the water,
the lower its ability to conduct electricity,
but regardless of how much it is purified,
water will always have some level of
conductivity. This is because an extremely
small amount of H2O
ionizes into H+
and OH-
and maintains an ionization equilibrium
as shown in the following formula.
This ionization equilibrium
is not only true for pure water, but
also holds true for water in aqueous
solutions of acids, bases, salts, and
so on. This relationship can be shown
using the equilibrium constant. In pure
water and weak aqueous solutions the
relationship can definitely be seen
as constant, and can be described with
the following formula.
This Kw is called the ionic
product of water, and is a constant
that changes according to the temperature.
This constant value is always maintained
in any sort of aqueous solution, whether
pure water, acid, base, salt or other.
In pure water it is [H+
] = [OH-
]. By measuring the electrical conductivity
of pure water, this value can be established
as 1.0 x 10-7
mol/ L at 25°C.
Putting this value in the above formula
gives us
However, as we have already
noted, the ionic product changes according
to temperature, and the higher the temperature
the greater the value of the ionic product.
Furthermore, as a property
of water, one molecule has the same
characteristics as one drop or one cup
of water (a giant molecule). Also, ions
such as the hydrogen ion H+
and the hydroxide ion OH-
have an extremely high mobility ratio.
In other words, H+
and [H3O]+
(oxo-nium) ions move toward the cathode,
and the OH-
ions move toward the anode.
Water is generally very
good at dissolving ionic compounds,
but at room temperature reacts with
metals such as sodium, potassium, and
calcium. Water vapor reacts with iron
to create hydrogen and form oxides and
hydroxides, and hydrolyzes with substances
such as nonmetallic halides and esters.
(Hydrolysis is dissolution inside the
molecules that compose the substance,
i.e., changing the bonds inside the
molecules.) A comparatively large number
of substances react in this way. |
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),
and the two hydrogen atoms enclose the
oxygen atom at an angle of approximately
105°. 