A Teacher's Guide to Dry Ice Experiments in Chemistry

What is Dry Ice?

Where does dry ice come from? Students probably do not know, so it will be fun to have them hypothesize.

After allowing a few guesses, explain to them the following:

• Dry ice is made out of carbon dioxide.
• The carbon dioxide gas is put into a compressor that squeezes the gas molecules together until they liquefy.
• While the gas is compressed, it is also cooled.
• The cooled liquid carbon dioxide forms at about 870 psi at room temperature.

Now for the tricky part:

• The liquid carbon dioxide is pumped through an expansion valve into an empty container (that means a vacuum…there is no gas in it).
  • Some of the CO liquid evaporates into the gas phase. This change requires energy, called the Energy of Transformation.
  • The remaining CO particles supply the energy needed for evaporation. As a result, these particles have less energy and some will freeze into a dry ice “snow.”
  • This “snow” is pressed together into blocks or pellets, producing dry ice as we know it.

Pre-Activity Questions

1. What is it called when a solid changes directly into a gas?

Answer - sublimation.

2. For this phase change to occur, energy must be added to the substance. Where do you think the energy is coming from?

Answer - from everything surrounding the dry ice that is warmer than the dry ice.

3. When some of the CO2 particles turn into gas it causes the remainder to cool. How can this be explained on the particle level?

Answer - Changing a liquid into a gas requires energy to break attractive forces between liquid molecules. The energy comes from other carbon dioxide particles, thus lowering their kinetic energy and causing them to get colder.

4. Dry ice is about twice as heavy as regular ice. Why do you think dry ice is so much heavier than “regular” ice?

Answer - Dry ice is heavier because it has more matter packed into the same volume. It has a higher density. One mole of dry ice is 44 grams and one mole of regular ice is 18 grams.

5. Draw a picture of how molecules are arranged as a solid in dry ice. Draw another showing how the molecules look when they are in the gas phase.

Start the Experiment!

Put on goggles and gloves.

• Get a 250-ml beaker
• a straw,
• soap solution
• a coin
• a NaOH pipette
• and pipette of universal indicator

For each of the following situations, write down what you observed and a possible explanation for what happened.

1. Take a small piece of dry ice and place it on the table-top. Gently nudge it.

2. Place the dry ice in the beaker. Get the bubble solution and practice blowing bubbles with the straw. When all of the dry ice has changed to vapor, gently blow bubbles and get them to land in the beaker. DON’T blow directly into the beaker.

3. Get another small piece of dry ice from your teacher. Place it on your table. Insert a coin’s thin edge into the ice so that it sticks up in the air. Remove the coin.

4. Place the dry ice in a film canister and put on the lid. QUICKLY, point it away from yourself and others!!

5. Clean out your beaker, then add 25-ml of warm tap water to it. Add 3 drops of Universal Indicator, and then 3 drops of NaOH. Get another small piece of dry ice from your teacher. Add the dry ice to the beaker and observe.

Observations & Explanations for the Teacher

1. For this situation, the dry ice will float across the table like an air-hockey puck. This is because the “warm” table is causing the dry ice to sublime and produce a “cushion” of carbon dioxide gas for the dry ice to float on.

2. The density of the dry ice vapor is higher than the density of regular air, so the bubbles will float on top of the vapor. Since the vapor is invisible, it will appear that the bubbles are floating in the middle of the beaker, unsupported.

3. Placing a coin on edge in the dry ice causes the dry ice to sublime where the “hot” coin touches the ice, causes a burst of vapors to push the coin away. This happens on both sides of the coin in rapid succession, making the coin vibrate.

4. Sublimation inside a closed container builds up gas pressure, which eventually pops the top off of the film canister.

5. Universal indicator will be a violet color in a basic solution of NaOH. As the carbon dioxide interacts with the water, it forms carbonic acid, thus acidifying the solution and causing the indicated color to shift to green and finally to pink.