We all know that ice keeps things cold. You put food and drinks in a cooler, pour a bag of ice over them, and they stay cold, but they also get wet. Sometimes too wet. Dry ice sounds like a good way to keep things cold and dry, but only if it works.
Dry ice keeps things cold because it’s carbon dioxide that has been frozen, or sublimated, to -109.3˚ F (-78.5˚ C). Because it is much colder than frozen water, dry ice is great for freezing food or keeping most things cold.
Although dry ice can be used to keep food and drinks cold, it is not an easy fix. Keep reading, and we will explain why, how you can use dry ice, and alert you to some of the dangers of this substance.
What is dry ice?
As we all learned in middle school chemistry, there are three primary states of matter—solid, liquid, and gas. (Plasma is a fourth state, but we will leave that to the scientists—the same ones who decided Pluto isn’t a planet anymore). Water starts as a gas—hydrogen, and oxygen turns into a liquid and then becomes a solid when it freezes.
Dry ice skips a stage. It starts off as a gas—carbon dioxide (which we exhale), but it skips the liquid stage and goes directly to a solid—a cold solid. In other words, it sublimates.
Sublimation works in both ways—a gas can turn to a solid, or a solid can turn to gas.
A few other solids that sublimate (turn directly to a gas) are:
Other solids can be sublimated through pressure or a vacuum. Freeze-dried coffee, anyone?
How does dry ice keep things cold?
There is no magic in how it keeps things cold—dry ice keeps objects cold the same way ice does—by lowering the temperature of the air surrounding it. In contrast, ice takes on the temperature of the air surrounding it.
As meteorologist Tom Skilling says, “An ice cube sitting in a freezer with an air temperature of -20˚ (degrees) will also chill down to -20˚.”
Do you know anyone who keeps their freezer at -109.3˚˚ F (-78.5˚ C)? Of course not. Even if you are keeping your freezer at the ideal temperature, which is not 32˚˚ F (0˚ C), but 0˚ F (-18˚C), that is nowhere close to the temperature of cold ice.
Where can I buy dry ice?
Since most people don’t keep their freezers below the temperature that dry ice sublimates at, they have to purchase it. If you want to buy some, you have three options:
- Grocery stores. Many, but not all, grocery stores carry dry ice. Even if a chain claims to sell it online, you should still call ahead before you head to your local store.
- Online. Several suppliers sell dry ice for delivery. Dry Ice Delivered is one such company. Be forewarned—shipping is expensive.
- Local Suppliers. Many areas have suppliers that make and sell dry ice. A quick Google search will let you know if there is one in your area. You will have to call for prices and quantities.
Your best bet, if you’re planning on buying enough for an outing, is a local grocery store that sells it. You will be able to get smaller amounts appropriate for camping trips. Larger amounts are used for theatrical productions where fog is needed or school experiments.
Is dry ice safe?
Yes, if you take proper precautions. Temperature and gas are two ways that dry ice poses some risks.
Dry ice needs to be handled with extreme care. It will freeze your skin quickly unless you handle it with care. Use the following tools to protect yourself:
- Oven mitts
- Leather gloves
- Tongs for small pieces
Should it freeze your skin, it will injure the cells and cause something the same type of damage that a burn would. Frostbite is highly unlikely unless you somehow manage to hold onto it long enough for the ice to freeze the tissue under your skin.
As dry ice warms up, it sublimates into a gas. In doing so, it will cause a container to expand. If there is too much dry ice in a container, or if there is no ventilation, the container has the potential to explode. For that reason, you should not pack a container with dry ice. Here are some other don’ts:
- Avoid storing in the freezer. It will turn off your refrigerator’s thermometer. The exception is when the power goes out—then dry ice is an excellent way to keep your freezer cold.
- Avoid storing in unventilated rooms, basements, and cars. The carbon dioxide will sink and replace the oxygen. If you are entering an unventilated space where dry ice is being stored, wait about 10 minutes before entering to let the carbon dioxide to dissipate.
- Do not stay if you develop a headache. Other signs of carbon dioxide poisoning include dizziness, nausea, lips, or fingernails turning blue.
If you treat dry ice appropriately—handle it carefully and have it in a ventilated area—you will be safe.
How can I pack a cooler with dry ice?
First, since dry ice will burn your hands, and you probably don’t plan to put on oven mitts when you reach in your cooler, you should wrap your dry ice. Newspaper does double-duty here—it keeps your fingers safe, and it insulates the ice, making it last longer.
Some sources suggest that you put the dry ice on top of your food/drinks. The reasoning behind this is that the cold air will sink. But as Dry Ice Info points out, it gets in the way when you’re trying to get things out of the cooler. Most people, therefore, prefer to put the dry ice (properly wrapped, of course) on the bottom of the cooler.
- Stuff some crumpled newspaper in air gaps. Air pockets make the dry ice melt more quickly.
- To prevent food freezing, you can place a piece of cardboard on top. A small hole cut into the cardboard will let the carbon dioxide gas escape.
- Then arrange the food on top of the dry ice. Remember: the closer it is to the ice, the colder the food will be.
Although some folks will recommend that you put ice cubes on top, once the ice melts, the water will cause the dry ice to warm up more quickly.
Some people put the items they want frozen on the bottom of the container, then add the dry ice, and that the food they want cold on top.
If you are wondering how much dry ice to get, Penguin Dry Ice has a table you can check out.
How should I get rid of dry ice?
The best way to get rid of it is to allow it to dissolve. To do this, you should:
- Unwrap it from the newspaper
- Place it outside and let it dissolve
- Or, place it in a container, leave the lid off, and leave it alone. If you are doing this inside, keep the area well-ventilated.
Five to ten pounds of dry ice will take about 24 hours to dissolve.
A few other safety tips.
- If you are camping, don’t put your cooler with dry ice inside your tent. Remember that the carbon dioxide will sink, depriving you of oxygen.
- Don’t pour it down sinks—it can freeze pipes.
- Avoid placing it directly on a solid surface, such as your countertop—there is a likelihood it will crack the counter.
- Store a maximum of 10 pounds of dry ice in your freezer.
If you just want to keep food and drinks cold while taking a trip or on an outing, then ice is a simpler solution. You will wind up with condensation, but if you store food carefully, you can avoid soggy sandwiches, or whatever other food you want to keep cold. However, if you need to keep food cold for longer—such as a camping trip, then dry ice will do a better job of keeping your food frozen.