Audience-Pleasing Physical Models to Support CO2 Outreach




Materials & Supplies






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Demo 4 — CO2 is a Gas [See materials and supplies]

Is CO2 dangerous? Does it explode? Can it be transported safely?

This set of experiments is used in introductory physics and chemistry classes to examine properties of gasses. Our motive here is to increase public understanding of the basic properties of CO2 so that people can be informed about safe handling and so that fears arising from absence of information will be reduced.

Set-up: Freeze caution: warn participants not to touch dry ice with bare skin.

Using hot pads or gloves, place the dry-ice block into a shallow plastic tub.

The plastic will provide insulation so that the cold from the dry ice will not crack the glass. Use the ice pick to break off a number of chunks of dry ice, ice-cube size or smaller, for the experiments. Place the dry ice in the tub into the bottom of a 10-gallon fish tank.

In a turbulent or breezy setting it is helpful to cover the tank with a piece of newspaper to allow the CO2 to build up and to break the dry-ice block into more pieces to increase the rate of sublimation. This experiment is not really suitable for outside demos.

Dry ice is frozen CO2. Ask: Does anyone know where the CO2 goes as the dry ice sits in the room and warms up? Do you see any liquid CO2 drips? (No, liquid CO2 does not exist at atmospheric pressure. Frozen CO2 “thaws” or sublimes directly to gas.) Can anyone see the CO2?

Even though CO2 gas is invisible (transparent to light), we can test for it. CO2 gas is heavier than air (air is mostly nitrogen and oxygen). So if we blow bubbles full of regular air, they will float on CO2 . Have participants try it (figure 4). Participants should blow soap bubbles gently into the tank and watch them “float” on CO2. Too vigorous swooshing of the bubble wand or blowing of bubbles will displace the CO2 and no effect will be seen.

Figure 4. Using the property of density to test for CO2 gas that collects in the tank as dry ice sublimes.

Another way to see the gas is to collect it (figure 5). Fill a small water bottle nearly to the top. Drop an ice-cube-size piece of dry ice (broken in several pieces) into the water. Put a balloon over the top of the bottle, and watch the CO2 gas blow up the balloon.

Figure 5. As dry ice sublimes, gas expands and fills the balloon.

Ask: Is CO2 dangerous? Is it explosive? CO2 collects in low places and displaces lighter oxygen. If a hamster were put in a tankful of CO2, it would die. If we light a candle and put it in the tank, we will see that there is not enough oxygen for the candle to burn. Have participants try putting a lit candle into the tank (figure 6).

CO2 is not explosive; it is used to put out fires. Slowly fill a plastic cup with CO2 gas from the bottom of the aquarium and pour it onto a lighted candle (short, fat candles are easiest to hit) (figure 6).

Figure 6. Testing for CO2 by extinguishing flame.

In low and moderate concentrations, CO2 is not dangerous to people. Have participants pour a transparent cup half full of drinking water. Add a small cube of frozen CO2. The water warms the frozen CO2 causing it to form gas, which fizzes (figure 7) (CO2 makes the fizz in carbonated beverages). As some of it dissolves in the water, it forms a weak acid. If you drink the water, it tastes slightly tangy (like lemon), which is the taste of acid. (This part of the experiment has proven to be very popular and brings home the message. Don’t let people touch the dry ice, though.)

Figure 7. Making carbonated water with dry ice.