14-1 Liquid Nitrogen and Kelvin Temperatures
Purpose: To explore the effects of a super cool temperatures on substances, to see how the
melting points and boiling points of substances differ, and learn about the Kelvin scale.
Procedure:
Steps
1. Place a mercury
thermometer into the
LN2 and observe the
mercury reading.
2. The instructor will
place a sample of
liquid mercury and a
sample of alcohol into
the LN2.
3. Clear your lab table
and the instrutor will
pour some LN2 on
your table. Don't let it
touch you.
4. Stick out your
hand, palm side
down, and the
instructor will pour
on tiny amount of
LN2 on the back of
your hand. (Note:
Remove all jewelry
beforehand.)
5. Observe the
behavior when 10 ml
of LN2 are poured by
the instructor into a
graduated
cylinder.
6. Pour the LN2 into
the plastic bottle and
immediately place a
balloon over the
opening. Don't allow
the balloon to come
off.
7. Take off the
balloon without
releasing the contents
and then have a
member of the
group breathe in the
contents.
Predictions
Observations
Explanations
8. Blow up the
balloon and tie it off.
Press down the
balloon in the large
container of LN2 for
a minute and then
hold up the balloon to
the light and observe
its contents.
9. Fill a beaker 1/2
full of water. A
sample of LN2 will be
poured into the
beaker.
10. Fill a second
beaker 1/2 full of
water, stir in a few
drops of dish
detergent. LN2 will
be added.
11. Secure a plastic
bottle in a clamp on a
stand. Aim it away
from all glass and
people. Cork it firmly
when LN2 is added
inside.
12. A racquetball that
has been sitting in
LN2 will be hit with a
bat.
14-2. Determining Phases relative to Melting & Boiling Points
Generally a substance will remain a solid below its freezing point, become a gas above the boiling
point, and assume a liquid between the two phase changes.
Substance
water
mercury
oxygen
nitrogen
iron
Freezing Point ('C)
0
-39
-218
-209
1535
Boiling Point ('C)
100
357
-183
-196
2750
Use the table of freezing and boiling points above to indicate the state of matter with (s), (l) or (g)
for each substance at a given temperature.
1. Dry ice remains at -78'C.
H2O_____, Hg_____, O2_____, N2_____, Fe_____
2. Body temperature is about 37'C.
H2O_____, Hg_____, O2_____, N2_____, Fe_____
3. A match flame is about 1200'C.
H2O_____, Hg_____, O2_____, N2_____, Fe_____
4. Hydrogen freezes at about -250'C.
H2O_____, Hg_____, O2_____, N2_____, Fe_____
14-3 Equilibrium
The diagram above shows the behavior of molecules in the liquid and vapor phase inside a jar.
Notice that at any given moment both evaporation and condensation are occurring.
D1. Why would it be foolhardy to seal air tight the container of liquid nitrogen? Why is not
dangerous to hold propane gas in a sealed container?
D2. Compare the rate of evaporation to condensation for the following circumstances.
a. The temperature rises.
b. The temperature falls.
c. The temperature remains
constant.
D3. Define equilibrium
14-4 Phases versus Temperature: Heating & Cooling Curves
1. Why does the temperature remain flat during phase changes?
2. What is the source of energy to fuel all storms?
14-5 Extra Credit Assignment: Write a fictional story about your favorite water molecule starting
out somewhere, somehow as an ice molecule; rising all the way to the gas phase; and then
eventually returning to the solid phase. Be creative.
14-6 Phases versus Pressure and Temperature
1. Define NORMAL atmospheric pressure?
2. Define NORMAL boiling point and mark the graph below with an A on the corresponding
spot.
3. Define NORMAL melting point and mark the graph below with an B on the corresponding
spot.
4. The TRIPLE POINT is where all three phases - gas, liquid and solid are in equilibrium. What
are the pressures and temperatures that correspond to the triple points of water and similarly
carbon dioxide?
5. Define SUBLIMATION. At what temperature and pressure does dry ice NORMALLY undergo
sublimation?
6. What is the minimum pressure required to liquify dry ice?
7. Imagine you go ice skating and the conditions include NORMAL air pressure at -5oC. Draw a
line from this spot corresponding with increasing pressure. What phase will you enter that
allows you to slide smoothly across the ice?
8. You have a tomato from your garden. It is at NORMAL pressure and 25 oC. Find this point on
the graph for water. Then draw a line corresponding with lowering the temperature and then
lowering the pressure until you reach a point corresponding with freeze-drying the tomato for
preservation.
9. A refrigerant easily changes back and forth between the liquid and gaseous phases. When it
evaporates it withdraws heat like sweating cools the body. When the refrigerant condenses it
releases heat like the humidity that fuels a thunder storm. Draw 2 lines corresponding to the
increasing and decreasing pressures produced by the pump in your refrigerator in order to
evaporate and recondense the refrigerant to cool your food.
14-7 Vapor Pressure
D1. Why does the vapor pressure increase with temperature? (Consider the motions of
molecules.)
THE BOILING POINT IS REACHED WHEN THE VAPOR PRESSURE EQUALS THE EXTERIOR
PRESSURE.
D3. Examine the figure below closely. What pressure corresponds to the normal boiling point of
water?
D4. What are the normal boiling points of ethyl alcohol and diethyl ether according to the graph?
(What would you predict about the boiling point of ethylene glycol?)
Use the vapor pressure graph from the last page to find the corresponding temperature or
pressures for the following questions.
1. If in Denver water boils at 94 'C, estimate the atmospheric pressure there.
2. Estimate what pressure we would have to lower to in order to boil water at 70 ‘C.
3. If at the top of Mt. Everest the air pressure is only 500 Torr, then at what temperature would
alcohol boil there?
More Vapor Pressure Questions
4. Water and a roast are sealed inside a pressure cooker. How does the pressure cooker enable
us to cook the roast faster than just boiling it in water?
5. You are driving across the desert on the way to Las Vegas and your car overheats. You open
the hood to check the radiator and the cap says DANGER. What will happen if you open the
cap? What is the value of allowing the cooling system to build up pressure?
6. Why do you have to often cook foods longer in Denver?