Properties of Liquid Nitrogen
and Various Gasses
Introduction:
This demonstration was developed as part of the Outreach Program
at the University of Minnesota. This demonstration shows the
cryogenic properties of liquid nitrogen and it effect on common
items (rubber tubing, rubber balls, bananas). It will also compare
properties such as the melting points and boiling points of various
gasses. Typical gasses used for this demo are hydrogen, helium,
and nitrogen. By having a volunteer blow up a balloon, one can
also show how a combination of gasses (carbon dioxide, oxygen,
nitrogen) will respond. The presence of the heavier gasses will
cause this balloon to expand slower than the balloons filled
with lighter gasses such as hydrogen and helium. Because oxygen
is present in ones breath, when this balloon is cooled by the
liquid nitrogen, the oxygen gas condenses and liquid oxygen is
visible if a clear balloon is used. Another interesting gas is
argon which has a very small difference between its m.p. and
b.p. It is possible that argon can be made available with prior
notice. This property causes the cold balloon to expand much
more rapidly than the other gasses. Lastly, the balloons can
be ignited to illustrate the reactivity and/or flammability.
Materials:
- Liquid Nitrogen (approximately 3 liters)
- Styrofoam bucket
- Large tongs
- Insulating gloves
- Gas filled balloons (H2, He, N2, possibly
Ar)
- Flowers
- Banana, nail and board
- Racket ball
- Matches and meter stick
Procedure:
There are many things that can be done with the liquid nitrogen.
Many times you will not have enough time to do all of these,
you may need to choose only two or three.
Various things can be frozen in the liquid nitrogen. Explain
how most thing tend to become very brittle when they are frozen.
This can be illustrated with rubber tubing, racket balls or
a flower. A banana may also be frozen so that it becomes hard
enough to pound a nail into a board. It should be noted that
the banana will usually fracture when you try this so for better
success, you may want to start the nail in a previously started
hole.
A great visual demonstration that can
be done is to add liquid nitrogen to a small amount of soapy
water. As the liquid nitrogen boils, soap bubbles containing
gas will be produced. As the bubbles pop, they will "smoke".
One may also add food coloring to the water to give the bubbles
color.
There are several properties of gasses that can easily be
demonstrated using liquid nitrogen. These properties include
phase changes (gas to liquid, liquid to gas, and visa versa),
and the temperature dependence of volume. If you have a volunteer
blow up a clear balloon, you can show how a gas (oxygen) can
go from gas to liquid. Oxygen has a boiling point of -180o
C, so when the balloon containing oxygen from someone's breath
is submerged in the liquid nitrogen the oxygen is cooled to
below its boiling point and it begins to condense. There is
usually some water vapor present in this balloon also- under
good conditions, the water vapor will condense to liquid and
then freeze, thus going through two phase changes. You can
also discuss the kinetics involved in the balloons expanding
when they are removed from the liquid nitrogen and begin to
return to room temperature. Notice how the lighter gasses do
not constrict as much since the boiling points of hydrogen
and helium are both lower than that of nitrogen. You may also
notice though that they will also expand a bit faster than
the heavier gasses. The differences in the expansion rate becomes
even more obvious if argon is available. Argon has a very small
difference between the freezing point and boiling point (4o
C) thus an argon filled balloon will expand very rapidly. Compare
this to a breath filled balloon or a balloon filled with a
gas such as ethane (95o C difference between freezing and boiling
points).
Helpful Hints:
Visually compare the difference
in expansion rates of the various gasses.
Dumping liquid nitrogen on the floor is a great visual demonstration
and gives the students a good idea of what the liquid is like.
When something is being frozen in the liquid nitrogen, be sure
to leave it submerged until the nitrogen no longer boils and
be sure to completely submerges the item.
Have the students help break the flowers, rubber tubing or
racket ball.
Talk about the practical use of liquid nitrogen such as wart
removal, flash freezing of fish, uses in sperm banks, etc.
Safety:
When igniting balloons, use a match taped to a meter stick.
This will save the hair on your hands and arms.
Younger children are often frightened by the explosion of
the hydrogen balloon, use caution.
Always use the metal tongs when removing something submerged
in the liquid nitrogen. The insulating gloves alone do not
provide sufficient protection from the cold temperatures.
Always use safety goggles when handling these chemicals and
be sure to provide safety goggles for any volunteers that
may be helping you. |
