This Lesson Plan was developed by:
Dr. Reuben Collins and Dr. Tom Furtak
Physics Department, Colorado School of Mines
Golden, CO 80401
How Cold is Cold?
Objectives

To understand temperature as a condition of matter that we associate with hot and cold

To appreciate the range of temperature beyond every day experience

To understand that the properties of matter depend on temperature

To know that an important property of matter that depends on temperature is its phase
(solid, liquid, gas)

To know that different materials change from one phase to another at different
temperatures

To know that air contains nitrogen as a gas at room temperature

To understand that a material like nitrogen can be converted to a liquid in a special
refrigerator that cools and pressurizes it

To observe that the gas phase of a material occupies a large volume, compared to its
liquid or solid phase

To observe that some solid materials become brittle as they get very cold while others
become tough
Main Lesson
Background
Liquid nitrogen is a cryogenic liquid (of or related to low temperatures) and is stored at 77
Kelvin (-320.5º F at sea level). Since liquid nitrogen is so cold it has many uses as a refrigerant
(including keeping some electronic equipment cool and preserving simple living organisms for
long periods of time) and is also used to form inert environments for the preservation of other
materials. Because it’s so cold, the interaction of liquid nitrogen with other materials can lead to
changes in the material properties that are scientifically interesting, some of which will be
described in the following demonstrations.
Precautions
Liquid nitrogen will freeze and destroy cells (in skin and eyes) by contact. Students should stay
a safe distance (6 feet) away from the demonstrations. The presenter(s) should wear eye (safety
goggles), skin (long sleeves), hand (very thick gloves), and foot (no sandals!) protection. The
table under the demonstrations will become very cold, so should be robust or separately
protected by several layers of newspapers.
Materials
 1-pint transparent Pyrex measuring cup
 water
 ordinary red-alcohol glass thermometer
 adding machine paper cut into one-meter strips
 pencil (one for each student)
 meter stick or yard stick (one for each student)
 Liquid nitrogen in 2-liter, transportable dewar
 5-gallon (clean) steel pail
 1-pint Styrofoam container (Pyrex measuring cup can substitute)
 long steel tongs
 long-handled spoon
 hammer
 short length of board (2x4)
 8d flat-head nail
 bag of party balloons
 banana
 racquet ball
 penny
 flower
 tea kettle
 Pyrex test tube
 ice cream ingredients
o ½ gallon of whole milk
o 4 pints of cream (get the good stuff)
o 2 pound bag of white granulated sugar
o liquid vanilla flavoring
o small Styrofoam cups and plastic spoons (one set for each student)
Sequence
Introduction:
Explain that the day’s lesson deals with the influence of temperature on the properties of matter.
Fill the transparent Pyrex measuring cup half-way with water. Show this to the students and
remind them that the temperature of the room is 70 F (or so) by showing them the thermometer.
Moderate a discussion about what’s in the cup and how it looks. It’s actually half-full of air.
The properties of air and of water depend on temperature. Water is a liquid at room temperature.
Air is a mixture of gasses at room temperature. Have the students name the types of gasses that
make up air. They all know about oxygen, but will probably identify greenhouse gasses as the
rest of what’s in air. Make sure they hear that nitrogen gas is the major component of air.
(nitrogen: 78%, oxygen: 21%, small amounts of argon, carbon dioxide, and water).
Broaden the discussion to include the concept of the phases of matter. Talk about the phases of
water. Bring up the concept of temperature and how we measure it. Water changes phase from a
solid (ice) to a liquid at -32 F, and from a liquid to a gas (steam) at 212 F (202 F in Denver).
These phase changes can be used to calibrate a thermometer. Direct the discussion toward the
concept of a thermometer as a number line, with hot at the top and cold at the bottom, just like
the markings on the alcohol/glass thermometer.
Thermometer Activity:
Spread the students out. Each one needs a pencil and a measuring stick (meter or yard). Help
them to build a number line using the adding machine paper. The accompanying table shows
where the marks should be. Make sure that you use the Fahrenheit scale. Start with boiling
water at one end. They should draw a line perpendicular to the paper strip, and write “water
boils” along the line, as well as 212 F. Have them mark and label the following points,
measured down the paper strip, using the inverse column in the table: freezing point of water,
zero degrees, coldest air temperature caused by weather on Earth (Antarctica), boiling point of
nitrogen, freezing point of nitrogen, absolute zero. Make sure that the students have the
measuring stick properly oriented, with the numbers increasing down from the water boiling
point toward colder temperatures.
There are lots of discussions that can be based on the number line thermometers. Could a regular
alcohol/glass thermometer be used to measure all of the points on their thermometers? What
would it feel like to be in a room that was at the same temperature as liquid nitrogen? What type
of gas would there be in the atmosphere of such a room? Ask the students what they think liquid
nitrogen looks like. How about solid nitrogen? What would liquid nitrogen look like if it were
boiling? What would the temperature of liquid nitrogen be if it were boiling? Point out that the
freezing and boiling points of nitrogen are very close in temperature, compared to these points
for water.
Liquid Nitrogen Demonstrations:
Move the class to a semi-circular arrangement centered on the demonstration table. Suit up in
your protective gear. Do as many of these demonstrations as you have time for, asking questions
and discussing things with the students as you go.
Appearance of Liquid Nitrogen: Get rid of the water in the Pyrex measuring cup and dry it out.
Pour liquid nitrogen into the cup to the same level as the water. Have the students identify what
they are seeing (clear liquid coming out, lots of bubbling, bubbling sound, cloud of white smokelike stuff). Explain that the cup is very hot, compared to the liquid nitrogen. It boils when it hits
the cup, just like water would boil if you poured it into a kettle that was removed from a 300 F
oven. After the boiling subsides, hold the cup up for the class to see. What’s in the cup now,
compared to the previous discussion when it was half-full of water? Explain that the smoky stuff
is actually fog—condensed water vapor (like in a cloud) that forms because the nitrogen is so
cold. If the room is humid the outside of the cup will also condense water vapor, which is first a
liquid, then a solid (frost).
What Happens to Cells in Liquid Nitrogen?: Impress upon the class that liquid nitrogen could be
dangerous, if it got on your skin. To show that, dunk a flower into the liquid nitrogen and bang it
on the table after it’s totally frozen. How would you like your finger to do that?...so, stay back.
What happened to the flower? If you warmed the flower up after it had been frozen like that
would it survive? Dunk another flower into the liquid nitrogen and try it. You can put it aside
while you continue with the other demos.
Phase Change from Liquid to Gas: Talk about what happens to the material in a boiling liquid,
like water. Where does the water go? Demonstrate this idea with nitrogen by pouring a small
amount of liquid nitrogen into a tea kettle. As the nitrogen evaporates the nitrogen gas escapes
through the hole in the lid and the kettle whistles, just like it would if it were filled with water on
a hot stove. Demonstrate the volume change as a liquid evaporates by pouring a little liquid
nitrogen into a Pyrex test tube, and quickly covering the opening with a party balloon. Talk
about what’s in the balloon. Some students will still say it’s air. Point out that there is no way
for air to get into the balloon as it gets bigger, so it must be full of nitrogen gas.
Shrinking Balloons: Pass out party balloons and have the students blow them up and tie them
off (even 5th graders can do this). Pour some liquid nitrogen into the 5-gallon pail. Have the
students drop the balloons into the pail one by one. Talk about what’s happening. As the gas in
the balloon gets cold, first the oxygen, then the nitrogen condenses into a liquid. Liquids take up
much less space than gasses, so the balloons shrink. You should be able to get all of the balloons
into the pail. Take them out one by one and put them on the table in front of the pail. They’ll
each reinflate as the nitrogen and oxygen evaporates. This is accompanied by lots of twisting
and turning and cracking sounds…very impressive.
Modified Elastic Properties: The mechanical properties of most things change dramatically
when cooled to liquid nitrogen temperature. Rubber gets very brittle. Demonstrate this with a
racquet ball. Show how it bounces. Dunk it into liquid nitrogen until the boiling stops. Then
drop it on the table. It won’t bounce at all, and will probably shatter into lots of pieces. Even
metals do this. Put a penny on the board and bang it with the hammer. The penny may distort,
as the metal bends. Dunk the penny into the liquid nitrogen and repeat the test. Now, the penny
will break up into little slivers. Put a banana into the 1-pint container and cover it with liquid
nitrogen. If you take it out before the banana is totally frozen, it will be sufficiently tough so that
you can use it to pound a nail into the board.
Ice Cream Activity:
You can make ice cream the old-fashioned way, by using a mixture of salt and ice to freeze the
liquid ice cream at a temperature slightly below the freezing point of water. Or, you can do it the
scientific way, using liquid nitrogen to do the cooling.
Have two people (the stirrer and the liquid nitrogen dispenser) don protective gloves and safety
goggles. Pour the milk, cream, sugar, vanilla flavoring, into the metal bucket Have the stirrer
begin stirring the potion with the big spoon The liquid nitrogen dispenser will slowly pour a
small amount of liquid nitrogen into the bowl. Stir, pour in more, stir, pour in more, and keep
repeating until the mixture solidifies (about 3 to 5 min).
Serve immediately! This stuff is really good.
Additional Demonstrations
Thermal Contraction: One of the properties of matter that depends on temperature is the distance
between the atoms that make up the material. This distance decreases as the temperature is
reduced (thermal contraction). The amount of thermal contraction (and thermal expansion, as
the temperature is increased) depends on the type of material. Two metals that demonstrate
different amounts of thermal contraction can be bonded together in a “bi-metallic strip”. If you
dunk a bi-metallic strip into liquid nitrogen the strip will curl up, as one side contracts more than
the other. This phenomenon is behind the temperature-sensitive spring inside of some types of
thermometers and temperature controllers.
Chemical Activity: Chemical reactions are sensitive to temperature. This can be demonstrated
with an ordinary AA battery, hooked up to a flashlight bulb. Use a battery holder and a bulb
socket that can be purchased at Radio Shack. The wires have to be long enough to enable the
battery to be dunked into the liquid nitrogen without also dunking the bulb. The voltage
generated at the battery terminals is caused by an electrochemical reaction inside of the battery.
As the battery temperature is reduced, the reaction slows down and the light dims.
Follow-Up Activites
Similar demonstrations and experiments can be performed as class activities using solid carbon
dioxide (dry ice). Dry ice converts directly to a gas at a temperature of -109 F. Most
supermarkets receive shipments of frozen food packed in dry ice. You can purchase (or receive
as an educational donation) several large bricks on most days. Transport the dry ice in an
ordinary cooler. It can be broken into smaller pieces with a screwdriver and hammer. Be sure to
wear eye protection and use heavy gloves when handling dry ice.
Observe the phase change
Dry ice sublimes directly from a solid to a gas. Demonstrate this to the class by placing an ice
cube and a dry ice cube side by side on a cafeteria tray. Watch as the ice cube melts and the dry
ice disappears. As this is happening, let the students talk about what’s going on.
Observe the volume change
Put some dry ice into a can with a sealable lid (a small coffee can). As the dry ice sublimes it
expands. After a short time the lid will pop off, because the pressure inside the can increases as
carbon dioxide gas is produced. This can also be demonstrated using a balloon. Put some dry
ice into a plastic pop bottle. Cover the opening with a balloon. As the carbon dioxide changes to
its gas phase, the balloon will inflate. Demonstrate that the balloon doesn’t contain a normal
amount of oxygen (as found in air) by using the balloon as a fire (candle) extinguisher. You
have to convince the students that you’re not just blowing the candle out. Put the candle in a
transparent glass. Use the balloon to “pour” carbon dioxide into the glass without disturbing the
flame. The candle will go out when the carbon dioxide level rises to the level of the candle wick.
Dry ice in water: Student lab activity
As a class activity have students predict what will happen if you add some dry ice to ordinary
water. Have each student (or student pair) write down their predictions on a piece of paper.
Perform the experiment as a demonstration (or have the students do it, if they are old enough).
The thermal energy in the water accelerates the sublimation of the dry ice. This creates bubbles
that are filled with very cold carbon dioxide and some water vapor. The water vapor is
sufficiently cold that it condenses into tiny water droplets (fog). As the bubbles rise (because
they are much less dense than the surrounding water, they burst at the surface, releasing the fog.
The fog cold, so it’s less dense than the room air. This causes it to flow over and down the sides
of the container. The fog disappears as the tiny water droplets evaporate. The generation of
ground fog by this mechanism is so efficient that it’s used in theaters as a special effect.
Temperature Scale
Use the data in the second or the fifth column (depending on whether you have meter sticks or
yard sticks) to make the paper tape thermometers.