Chemistry
Lecture 7d
Temperature and Phase Transitions
Temperature
• Science Definition: average kinetic energy
• Measures how hot or cold something is.
• Units: Celsius (˚C), Kelvin (K)
Absolute Zero
• Temperature in which all particles stop moving.
• -273.15 ˚C
• Scientists have yet to attain this temperature.
Kelvin Scale
• Used in science to measure temperature.
• Based on the concept of absolute zero.
• There are no negative numbers.
• Conversion:
TK = (TC +273) K
Convert me to Kelvin!
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2.
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5.
35 C
100 C
-15 C
127 C
-23 C
Err… Change me back!
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273 K
146 K
47 K
89 K
0K
As pressure increases
or the temperature
decreases, the
molecules are pushed
together and the
substance moves
towards becoming a
solid. As the pressure
decreases or the
temperature increases,
the molecules move
farther apart and the
substance moves
towards becoming a
gas.
The Phase Diagram
Shows the standard curve for the
phases of any substance.
The triple point is the
temperature and pressure at
which all three phases of a
substance can exist at the same
time. This point differs for each
substance.
At the left are two phase diagrams for
water. On the lower graph we have
specified the line for 1 atmosphere, and
we can see that the point where this
crosses the melting point line is 0ºC
(273K), and the point where the 1
atmosphere line crosses the boiling point
is 100ºC (373K). Note at the triple point of
water, as long as the atmospheric
pressure stays low, no matter what the
temperature is, water sublimates from a
solid to a gas. Remember this for future
discussions on the possibility of liquid
water on the surface of Mars, where the
atmospheric pressure is 0.6% that of
Earth's, or roughly 4.56 mm.
The melting curve of ice/water is very special. It has a negative slope due to the fact that when ice
melts, the molar volume decreases. Ice actually melts at a lower temperature at higher pressures.
(Think of how an ice skater glides along on her skates; the ice momentarily melts due to her weight,
and then refreezes when the pressure is released.)
The phase diagram
for water.
You must be able to look at
a phase diagram and
determine a substance’s
phase based on given
temperature and pressure.
Lets look at some other
substances.
A phase diagram for carbon dioxide illustrates the more common forward slope
of the melting point line. Notice that the triple point of carbon dioxide is well
above 1 atmosphere. Notice also that at 1 atmosphere carbon dioxide can only
be the solid or the gas. Liquid carbon dioxide does not exist at 1 atmosphere.
Dry ice (solid carbon dioxide) has a temperature of -78.5 degrees F (-61
degrees C, 212 K) at room pressure, which is why you can get a serious burn
(actually frostbite) from holding it in your hands. Although carbon dioxide liquid
doesn't exist at normal room pressures, it does exist at slightly elevated
pressure.
At left is a qualitative phase
diagram for nitrogen. Its triple point
occurs at an atmospheric pressure
of 0.123 and a temperature of
63.15 K. At lower pressures,
nitrogen will sublimate. Remember
this point when we discuss Triton, a
moon of Neptune, towards the end
of the quarter. The normal melting
and boiling point for nitrogen (that
is, at 1 atmos.) is 63.3 and 77.4 K
(-320 degrees F!!) respectively.
Liquid nitrogen is cold.
The Exception
As water cools, the molecules expand rather than
move closer together. This is due to the shape of the
molecule. So more space between the molecules
allows ice to be larger than liquid water…this is
molecules found in liquid form.
why ice floats. •Water
Notice the compact nature of the
molecules.
•Water molecules found in solid form.
Notice the molecules have spread 
further apart than in liquid form.
Temperature Depression and Elevation
Have you ever put salt into a pot of water to cook pasta?
What for?
Actually, when you add salt to water, you raise the
boiling temperature. Water alone boils at 100ºC. Water
mixed with salt boils at over 101 ºC, allowing the pasta
to cook faster. This is known as temperature elevation:
adding a substance to a liquid to raise the boiling
temperature.
Adding a substance to a liquid can
also lower the freezing temperature,
known as temperature depression.
Freezing Point Depression
Boiling Point Elevation