Download 17 Change in Phase

17 Change in Phase
Matter around us exists in four
common phases (or states).
• Ice, for example, is the solid phase of H2O. Add
energy, and you add motion to the rigid
molecular structure, which breaks down to form
H2O in the liquid phase, water. Add more energy,
and the liquid changes to the gaseous phase. Add
still more energy, and the molecules break into
ions and electrons, giving the plasma phase. The
phase of matter depends on its temperature and
the pressure that is exerted on it. Changes of
phase almost always require a transfer of energy
Evaporation
• change of phase from liquid to gas that occurs
at the surface of a liquid.
• Cooling
Condensation
• The opposite of evaporation
• the changing of a gas to a liquid.
• When gas molecules near the surface of a liquid
are attracted to the liquid, they strike the surface
with increased kinetic energy and become part of
the liquid.
• In collisions with low-energy molecules in the
liquid, excess kinetic energy is shared with the
liquid, increasing the liquid temperature.
• Condensation is a warming process.
Condensation in the Atmosphere
• There is always some water vapor in the air.
• A measure of the amount of this water vapor is
called humidity (mass of water per volume of air)
• Weather reports often use the term relative
humidity—the ratio of the amount of water vapor
currently in the air at a given temperature to the
largest amount of water vapor the air can contain
at that temperature. 1
• Air that contains as much vapor as it can is
saturated.
Why is it common for clouds to form
where there are updrafts of warm moist
air?
• Warm air rises. As it rises, it expands.
• As it expands, it chills and water-vapor
molecules are slowed.
• Lower-speed molecular collisions result
in water molecules sticking together.
• If there are larger and slower-moving
particles or ions present, water vapor
condenses upon these particles, and,
with sufficient buildup, we have a cloud
Boiling
• 100°C for water at atmospheric pressure—
• molecules are energetic enough to exert a vapor
pressure as great as the pressure of the
surrounding water (which is due mainly to
atmospheric pressure).
• The motion of water-vapor molecules in the
bubble of steam (much enlarged) creates a gas
pressure (called the vapor pressure) that
counteracts the atmospheric and water pressure
against the bubble.
The tight lid of a pressure cooker holds
pressurized water vapor above the
water’s surface, and this inhibits boiling.
In this way, the boiling temperature of
the water is increased to above 100°C.
• lowered pressure (as at high altitudes) decreases
the boiling point of the liquid. So we see that
boiling depends not only on temperature but on
pressure as well.
Geysers
• A geyser is a periodically erupting pressure cooker.
It consists of a long, narrow, vertical hole into
which underground streams seep
• The column of water is heated by volcanic heat
• exceeding 100°C.
•
As the water gushes out, the pressure on the
remaining water is reduced. It then boils rapidly
and erupts with great force
Boiling Is a Cooling Process
• Both Evaporation and boiling are cooling
processes
• When 100°C water at atmospheric pressure is
boiling, its temperature remains constant.
• That means it cools as fast as it warms.
• If cooling didn’t take place, continued input of
energy to a pot of boiling water would result in a
continued increase in temperature. The reason a
pressure cooker reaches higher temperatures is
because it prevents normal boiling, which, in
effect, prevents cooling.
Boiling and Freezing at the Same Time
•
We usually boil water by the application of heat. But we can boil water by the
reduction of pressure.
•
We can dramatically show the cooling effect of evaporation and boiling when
room-temperature water is placed in a vacuum jar
•
If the pressure in the jar is slowly reduced by a vacuum pump, the water will start
to boil. The boiling process removes heat from the water left in the dish, which
cools to a lower temperature. As the pressure is further reduced, more and more
of the slower-moving molecules boil away. Continued boiling results in a lowering
of temperature until the freezing point of approximately 0°C is reached.
•
Continued cooling by boiling causes ice to form over the surface of the bubbling
water.
Melting and Freezing
•
Melting- If enough heat is absorbed, the attractive forces between the molecules will no longer be
able to hold them together.
•
Freezing is the converse of this process. As energy is withdrawn from a liquid, molecular motion
diminishes until finally the molecules, on the average, are moving slowly enough so that the
attractive forces between them are able to cause cohesion. The molecules then vibrate about fixed
positions and form a solid.
•
At atmospheric pressure, water freezes at 0°C—unless such substances as sugar or salt are
dissolved in it.
•
Dissolved substances cause a lower freezing point
•
Solute ions grab electrons from the hydrogen atoms in H2O and impede crystal formation. The
result of this interference by “foreign” ions is that slower motion is required for the formation of
the six-sided ice-crystal structures.
•
The ice first formed is almost always pure H2O.
Regelation
The wire gradually
passes through the ice
without cutting it in
half.
• This phenomenon of melting under pressure and
freezing again when the pressure is reduced
• It is one of the properties of water that distinguishes it
from other materials.
• The making of snowballs is a good example of
regelation. When we compress the snow with our
hands, we cause a slight melting of the ice crystals;
when pressure is removed, refreezing occurs and binds
the snow together. Making snowballs is difficult in very
cold weather because the pressure we can apply is not
enough to melt the snow
Energy and Changes of Phase
Heat pumps
•
A refrigerator is a “heat pump.” It transfers heat out of a cold environment and into a warm
environment. When the process is reversed, the heat pump is an air conditioner. In both cases,
external energy operates the device.
•
This is accomplished by a liquid of low boiling point, the refrigerant, which is pumped into the
cooling unit, where it turns into a gas.
•
can extract heat from water that is pumped in from nearby underground pipes.
•
Water underground is relatively warm. In the Midwest and the Central Plains, subsoil
temperature below a meter deep is about 13°C (55°F) year-round—warmer than the air in
wintertime.
•
Heat is extracted from the water (just as from food in a refrigerator) by the vaporization of a
common refrigerant. The vaporized refrigerant is then pumped to condensation coils, where it
condenses and gives off heat to warm the home. The cooled water is returned to the ground
outside, where it warms back up to ground temperature and repeats the cycle.
A graph showing the energy involved in the heating and
the changes of phase of 1 g of H2O.
energy involved in the heating and the
changes of phase of 1 g of H2O.
•
a 1-gram piece of ice at a temperature of –50°C in a closed container that is placed on a
stove to heat. A thermometer in the container reveals a slow increase in temperature
up to 0°C. Then an amazing thing happens. The temperature remains at 0°C even
though heat input continues. Rather than getting warmer, the ice begins to melt. In
order for the whole gram of ice to melt, 80 calories (335 joules) of energy is absorbed
by the ice, not even raising its temperature a fraction of a degree. Only when all the ice
melts will each additional calorie (4.18 joules) absorbed by the water increase its
temperature by 1°C until the boiling temperature, 100°C, is reached. Again, as energy is
added, the temperature remains constant while more and more of the gram of water is
boiled away and becomes steam. The water must absorb 540 calories (2255 joules) of
heat energy to vaporize the whole gram. Finally, when all the water has become steam
at 100°C, the temperature begins to rise once more. It will continue to rise as long as
energy is added.
• Heat of vaporization is either the energy
required to separate molecules from the liquid
phase or the energy released when gas
condenses to the liquid phase.
Heat of fusion is either the energy needed to
separate molecules from the solid phase or
the energy released when bonds form in a
liquid that change it to the solid phase.
Water’s heat of vaporization is huge. The energy needed to
vaporize a quantity of boiling water is nearly seven times the
energy needed to melt the same amount of ice.
• The amount of energy required to change a unit mass
of any substance from solid to liquid (and vice versa) is
called the latent heat of fusion for the substance. (The
word latent reminds us that this is thermal energy
hidden from the thermometer
• The amount of energy required to change any
substance from liquid to gas (and vice versa) is called
the latent heat of vaporization for the substance. For
water, we have seen this is a whopping 540 calories per
gram (2255 joules per gram). 6 It so happens that these
relatively high values are due to the strong forces
between water molecules—hydrogen bonds.
On a cold day, hot water freezes faster than warm water because
of the energy that leaves the hot water during rapid evaporation
• The rate of cooling by rapid evaporation is
very high because each evaporating gram of
water draws at least 540 calories from the
water left behind. This is an enormous
amount of energy compared with the 1 calorie
per Celsius degree that is drawn from each
gram of water that cools by thermal
conduction.
Paul Ryan tests the hotness of molten
lead by dragging his wetted finger
through it.
• How can they do this?
• The low conductivity of wooden
coals, however is the principal
reason the feet of barefoot
firewalkers are not burned.