Dry Ice: Simply Sublime!
Physical Science
Phases of Matter
Grades 4 - 10
Objective: Student
inquiry into the nature
of matter and changes in its state.
Background:
Dry Ice is frozen carbon dioxide,
a gas which makes up about .04% of our atmosphere. As a gas,
it is colorless and odorless, and is released when living organisms
respire. It is also 1.5 times the density of air, (which is 78%
nitrogen, 21% oxygen, and less than 1% argon.) Carbon dioxide
(CO2 ) gas turns to dry ice when the temperature drops below
minus 78.5° C. It turns to vapor directly from the solid
state, never passing through a liquid phase. This change, from
a solid to a gas, is called "sublimation."
Dry ice is largely used for refrigeration.
Look in your Yellow Pages to find a local vendor. A couple of
blocks about 2 inches by 10 inches by 7 inches cost about $10
in my area. Place it in an ice chest and keep it there, taking
out only what you need. It is a good idea to wear gloves when
handling the dry ice.
Students are familiar with water
ice. Dry ice gives you a chance to challenge their expectations,
and force them to create some new models to account for their
observations. Observation and experimentation of dry ice sheds
light on two concepts: changes of state, and the behavior (or
even existence!) of gases. Here are some steps you might follow
to guide your students in uncovering the properties of dry ice
in particular, and matter in general.
Step One: What is a Solid?
Experiment with water ice to establish
melting point and boiling point. It is useful for students to
graph the temperature as ice is heated, melted, then boiled.
They will observe a graph which looks levell for a while, then
reflects the temperature of the water rising, and then levels
off again when it reaches the boiling point.
The water stays at 0° C as long
as there is ice left to absorb the heat by melting. The liquid
water warms to boiling, then again holds steady, so long as water
remains in the liquid phase, absorbing heat by turning to vapor.
Activities which do this are widely available, so I will not
repeat them. It might be useful to relate the idea of solids
melting to other materials they are familiar with. Water ice
is one solid which melts. Others include metals, chocolate, wax,
and plastic. Water melts at 0° C, mercury melts at minus
39° C, gold at 1064°C.
Step Two: Discovering Sublimation
Give the kids a chunk of dry ice,
one to two inches square. Warn about possibledanger from contact.
(Since dry ice is so cold, it will quickly freeze skin if it
is in contact for more than a few seconds. The result is similar
to a burn, though no heat is involved) Give them also a styrofoam
cup, and a piece of plastic wrap. Have them cover the cup, and
observe. As the ice sublimes, it will swell the plastic, and
may even pop it, if they can maintain a tight enough seal. They
should record their observations.
Note: This will look more dramatic
with water in the cup, but I have found that the kids often mistake
the vapor for steam, and get the idea that the ice is actually
"boiling" the water, a wrong idea, since the bubbles
coming up are mainly CO2, not water vapor. If students blow into
the cup, they will see even more white vapor. Why is that? Isn't
it the same as hot steam? No, it isn't. The dry ice vaporizes
into a clear, colorless gas. The white vapor we see is actually
water in the air which is condensing. Our warm, moist breath
cools quickly when in contact with the dry ice, and as it cools,
the moisture condenses into tiny, fog-like droplets. It is like
steam, in that it is water vapor, but it is not the product of
heated water.
Water ice will sublime as well.
Ice cubes will shrink when left in the freezer, frozen foods
will become desiccated.
Step 3: What is this gas?
After they have experienced sublimation
without water, and you have established that the dry ice is indeed
turning into a gas, you could explore what the gas is. They may
notice that the vapor coming off their cup goes down, unlike
hot steam, which rises. This is due both to the temperature difference,
and the inherent density of CO2 gas. If the kids place a lit
candle on the table, they can then "pour" the CO2 gas
from their styrofoam cup onto the flame. It should go out. This
is evidence that you have a gas, heavier than air, and that it
is a gas which does not provide oxygen for combustion. CO2 fire
extinguishers are quite common.
You might point out that this gas
is the same one which living things respire (breathe out). In
high concentrations, it is poisonous to life, as well as to fire.
It is also the gas released by Alka Seltzer and the gas in the
bubbles of soda pop.
Note: The kids have to "pour"
very cautiously, as a slight breeze will blow the CO2 away
Step 4: Experimenting with Dry
Ice and Water.
Give them water and a thermometer.
Tell them not to stick the thermometer right on the dry ice,
and not to get it stuck in the water ice which forms.
You have established that the dry
ice is shrinking, and turning into a gas. What happens when you
add water? They will see a high rate of bubbling at first. They
should note the rate of bubbling, because they should discover
that it slows as the water cools. When the water cools enough,
water ice will form a covering on the dry ice. This is interesting
to observe. The ice will even encapsulate the chunk of dry ice,
then pop, as further sublimation builds up inside the capsule
of ice. They might experiment by switching the water after it
has gotten very cold.
After students have experimented
in an open-ended way, ask them to measure temperature in a systematic
way. They should begin with a new chunk of dry ice, and water
at room temperature. They should measure the temperature of the
water before they add the dry ice, then record the temperature
every minute after the dry ice is added. They should create a
graph similar but opposite to the one they created of the ice
melting and boiling. These graphs illustrate that heat is being
transferred through the process of the phase change. (So long
as there is liquid water with the dry ice, it can transfer heat
to the dry ice as it freezes. So long as there is liquid water
being heated, it can transfer heat to the gas it forms as steam.
Thus the temperatures remain at the freezing point or boiling
point as long as liquid remains)
Remind them that the water is not
"boiling," even though it is bubbling. Remind them
also that the "steam" they see, is actually CO2 gas
made white by water vapor.
A question you might pose to the
students is: Does the CO2 sublime more quickly in water than
air? Why?
[Answer: Water transfers heat much
more quickly than does air at the same temperature. Drawings
illustrating the kinetic model of gas and liquid would be useful
to see why this is so. ]
Bubble Chamber Fun
At the Exploratorium in San Francisco
there is The Bubble Chamber. It is a large, four foot high plexiglass
cylinder, about two feet in diameter. At the bottom of this cylinder
there is a large chunk of dry ice. After it has been subliming
there for a while, a layer of CO2 gas builds up in the lower
part of the cylinder. If you blow soap bubbles into the cylinder,
they drop down to the level of the CO2 gas, and float there,
since the air within them is less dense than the CO2 gas. Large
plexiglass cylinders are hard to find, so just blow the bubbles
into the ice chest where you have your dry ice. You will need
to be careful to avoid breezes which would displace the gas,
and you will need a fairly big chunk of dry ice to do this.
Get the students to develop hypotheses
and conduct research to find out why the bubbles float. They
should be able to find out in the library that CO2 gas is denser
than air. Their earlier experiment, where they "poured"
the CO2 gas onto the candle also supports the idea that CO2 gas
is denser than air.
Student Directions: You might
wish to transfer this onto a worksheet with lines for student
writing.
Dry Ice: Simply Sublime: Part One
Warning: Handle Dry Ice with caution.
It can quickly freeze your skin!
Anyone playing during the lab activity
will not be allowed to participate.
I. Take a chunk of dry ice, no bigger
than 4 cm across. Place it in a styrofoam cup and experiment
with it. Record what you see.
2. Break the chunk of dry ice up
into smaller pieces by poking it with a pen or pencil. Cover
the opening of the cup with a piece of plastic wrap. Record what
you observe.
3. How do you explain your observations
so far? What is "Dry Ice"? What is it doing?
4. Light a candle, and carefully
pour the "air" from your cup onto the flame. Do not
let any solid dry ice come out. What happens?
5. How would you explain this?
6. Add water to your cup. What do
you observe?
The following questions should
get students to critically reflect on their observations.
Dry Ice Puzzlers
1. When you pour the gas from the
dry ice onto the candle, why does it go down?
2. If the gas from dry ice is colorless
and clear, why does it form clouds of white vapor, especially
when you blow on it?
3. When you add dry ice to water,
it seems to boil. Why is that?
4. The dry ice in water "pops"
every once in a while. Explain what is happening.
5. Does the dry ice disappear more
quickly in air or in water? Can you explain why?
6. While the dry ice was in the
water, the temperature fell to 0° but no farther. Why do
you think that is?
7. What temperature do you think
the dry ice is?
8. In the Bubble Chamber, air bubbles
floated. Can you explain why?
9. Is Dry Ice a good name for this
material? Why or why not?
DRY ICE RESOURCES
An excellent GEMS Guide is available
titled Dry Ice Investigations. You can read details here
An outstanding web site has LOTS
of information about Dry Ice: www.dryiceInfo.com
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