Dry Ice is frozen carbon dioxide, a normal part of our earth's atmosphere. It is the gas that we exhale during breathing and the gas that plants use in photosynthesis. It is also the same gas commonly added to water to make soda water. This gas is often captured during industrial processes and recycled to make Dry Ice.
Dry Ice is particularly useful for freezing, and keeping things frozen because of its very cold temperature: -109.3°F or -78.5°C. Dry Ice is widely used because it is simple to freeze and easy to handle using insulated gloves. Dry Ice changes directly from a solid to a gas -sublimation- in normal atmospheric conditions without going through a wet liquid stage. Therefore it gets the name "dry ice."
As a general rule, Dry Ice will sublimate at a rate of five to ten pounds every 24 hours in a typical ice chest. This sublimation continues from the time of purchase; therefore, pick up Dry Ice as close to the time needed as possible. Bring an ice chest or some other insulated container to hold the Dry Ice and slow the sublimation rate. Dry Ice sublimates faster than regular ice melts but will extend the life of regular ice.
It is best not to store Dry Ice in your freezer because your freezer's thermostat will shut off the freezer due to the extreme cold of the Dry Ice! Of course if the freezer is broken, Dry Ice will save all your frozen goods.
Commercial shippers of perishables often use dry ice even for non frozen goods. Dry ice gives more than twice the cooling energy per pound of weight and three times the cooling energy per volume than regular water ice (H2O). It is often mixed with regular ice to save shipping weight and extend the cooling energy of water ice. Sometimes dry ice is made on the spot from liquid CO2. The resulting dry ice snow is packed in the top of a shipping container offering extended cooling without electrical refrigeration equipment and connections.
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How Does It Work
When you drop a piece of dry ice in a bowl of water, the gas that you see is a combination of carbon dioxide and water vapor. So, the gas that you see is actually a cloud of tiny water droplets. The thin layer of soap film stretched across the rim of the bowl traps the expanding cloud to create a giant bubble. When the water gets colder than 50°F, the dry ice stops making fog, but continues to sublimate and bubble. Just replace the cold water with warm water and you’re back in business.
Take It Further
If you accidentally get soap in the bowl of water, you’ll notice that zillions of bubbles filled with fog will start to emerge from the bowl. This, too, produces a great effect. Place a waterproof flashlight in the bowl along with the dry ice so that the light shines up through the fog. Draw the cloth across the rim to create the soap film lid and, if you are inside, turn off the lights. The crystal bubbles will emit an eerie glow and you’ll be able to see the fog churning inside the transparent bubble walls. When the giant bubble bursts, the cloud falls to the floor, followed by an outburst of ooohs and ahhhs from your audience!
NOTE: Whenever you use dry ice, always be aware of the rules for handling it safely.
- This is not a toy. It’s for demonstration purposes only.
- Use dry ice only with adult supervision.
- Dry ice must be handled using heavy gloves or tongs. It will cause severe burns if it comes in contact with bare or unprotected skin.
- Always wear safety goggles when handling dry ice. The debris and shards are extremely dangerous to your eyes. When tapping dry ice with a hammer, first cover it with a towel to keep the pieces in one place.
- Never put dry ice in your mouth.
- Never store dry ice in an airtight container. As the dry ice sublimates, gas pressure will build and the container will explode. Make sure your container is ventilated or has a loose-fitting lid.
- Do not store dry ice in your freezer. It will cause your freezer to become too cold and the freezer may shut off. On the other hand, if you lose power for an extended period, dry ice is a good way to keep things cold if you can get it.
- In the unlikely event of a dry ice burn, treat it the same as you would a heat burn. See a doctor if the skin blisters or comes off. Apply antibiotic ointment to prevent infection and bandage mild burns.
What is Dry Ice?
Dry ice is not frozen water – it’s frozen carbon dioxide (CO2). Unlike most solids, dry ice does not melt into a liquid as the temperature rises, but instead, changes directly into a gas. This process is called sublimation. The temperature of dry ice is –109.3°F (-78.5°C). Dry ice is particularly useful for keeping things cold because of its temperature. Dry ice does not last very long, however, so it’s important to purchase the dry ice you need for these science activities as close as possible to the time you need it. The best place to store dry ice is in a Styrofoam ice chest with a loose fitting lid that allows the CO2 to escape as the ice sublimates.
Some grocery stores and ice companies will sell dry ice to the public especially around Halloween. Dry ice is typically sold as flat, square slabs a few inches thick or as cylinders that are about three inches long and about a half-inch thick. Either size will work fine for these experiments.
Remember the science when purchasing dry ice. Dry ice in a grocery bag will vanish in about a day. The experts tell us that, depending on weather conditions, dry ice will sublimate at a rate of 5 to 10 pounds (2.3 to 4.5 kg) every 24 hours even in a typical Styrofoam chest. So, again, it’s best to purchase the dry ice as close to the time you need it as possible. Last minute shopping is necessary. If you are planning to perform a number of dry ice demonstrations or have a lot of people involved, purchase 5 to 10 pounds (2.3 to 4.5 kg). A little dry ice does go a long way in these activities.
How is Dry Ice Made?
The first step in making dry ice is to compress carbon dioxide gas (CO2) until it liquefies while at the same time removing excess heat. The CO2 will liquefy at a pressure of approximately 870 pounds per square inch (4500 cmHg) at room temperature. Once liquid CO2 is formed, the CO2 is sent through an expansion valve and enters a pressure chamber. This pressure change causes the liquid to flash into a solid and causes the temperature to drop quickly. About 46% of the gas will freeze into “dry ice snow.” The rest of the CO2, about 54%, is released into the atmosphere or recovered to be used again. The dry ice snow is collected in a chamber where it is compressed into block, pellet, or rice-sized pieces using hydraulics. It’s complicated but really cool science – really cool.
Can you make your own dry ice? Sure, anything is possible, but it’s not practical (unless you have a huge tank of compressed CO2 sitting around and lots of extra time and equipment on your hands). For around $2 US a pound, it’s hard to beat the convenience of just purchasing it at the store.