In this demonstration, students will see up close how the process of sublimation works with dry ice as an example.
Dry ice is solid. It sublimates or changes states from a solid to a gas at temperatures of -78 degrees Celsius under normal atmospheric pressure of 1 atm. Because of its low temperature at normal atmospheric pressure, it is useful as a coolant.
When dry ice is placed into warm water, a cloud forms. This cloud is similar to the clouds we see in the sky. The cloud consists of water droplets that are trapped inside the carbon dioxide gas and eventually flow out. It forms because the dry ice is cold enough to make water from the air condense.
Safety tip: Dry ice is cold to the touch and can cause severe burns from freezing if it comes in contact with skin. Be very careful when handling dry ice and ensure to wear the proper safety gear and use the correct safety equipment.
| Why doesn't dry ice melt? |
| Question Date: 2016-10-17 |
| Answer 1: Normally, a solid will melt into a liquid, and then the liquid will evaporate into a gas. Dry ice is special because it skips the liquid phase: it is a solid that sublimates into a gas. Why does dry ice sublimate instead of melting? It's because at room temperature and normal pressure (atmospheric pressure), carbon dioxide is usually a gas. So when you take dry ice (solid carbon dioxide) and expose it to this temperature and pressure, it will try to return to the gas phase. If you specify a temperature and a pressure, we can tell what phase of a substance is stable there (solid, liquid, or gas) using a phase diagram, like this one for carbon dioxide . From the diagram you can see that at room temperature (T = 273 Kelvin) and atmospheric pressure (P = 1 bar), carbon dioxide is a gas. But if we went to a much higher pressure, say P = 100 bar, carbon dioxide would be a liquid. At that pressure, dry ice would melt! Click Here to return to the search form. |
What is Dry Ice?
Dry ice is frozen carbon dioxide. A block of dry ice has a surface temperature of -109.3 degrees Fahrenheit (-78.5 degrees C). Dry ice also has the very nice feature of sublimation -- as it breaks down, it turns directly into carbon dioxide gas rather than a liquid. The super-cold temperature and the sublimation feature make dry ice great for refrigeration. For example, if you want to send something frozen across the country, you can pack it in dry ice. It will be frozen when it reaches its destination, and there will be no messy liquid left over like you would have with normal ice.
Many people are familiar with liquid nitrogen, which boils at -320 degrees F (-196 degrees C). Liquid nitrogen is fairly messy and difficult to handle. So why is nitrogen a liquid while carbon dioxide is a solid? This difference is caused by the solid-liquid-gas features of nitrogen and carbon dioxide.
When you put dry ice with water it will produce smoky fog like seen here. Great for party tricks and concerts.Jenny Dettrick / Getty Images
We are all familiar with the solid-liquid-gas behavior of water. We know that at sea level, water freezes at 32 degrees F (0 degrees C) and boils at 212 degrees F (100 degrees C). Water behaves differently as you change the pressure, however. As you lower the pressure, the boiling point falls. If you lower the pressure enough, water will boil at room temperature. If you plot out the solid-liquid-gas behavior of a substance like water on a graph showing both temperature and pressure, you create what's called a phase diagram for the substance. The phase diagram shows the temperatures and pressures at which a substance changes between solid, liquid and gas.
At normal pressures, carbon dioxide moves straight between gas and solid. It is only at much higher pressures that you find liquid carbon dioxide. For example, a high-pressure tank of carbon dioxide or a carbon-dioxide fire extinguisher contains liquid carbon dioxide.
The Temperature of Dry Ice
To make dry ice, you start with a high-pressure container full of liquid carbon dioxide. When you release the liquid carbon dioxide from the tank, the expansion of the liquid and the high-speed evaporation of carbon dioxide gas cools the remainder of the liquid down to the freezing point, where it turns directly into a solid. If you have ever seen a carbon dioxide fire extinguisher in action, you have seen this carbon dioxide snow form in the nozzle. You compress the carbon dioxide snow to create a block of dry ice. Dry ice sublimates at temperatures higher than −109.2 °F so you will need to use it quick or store it at temperatures lower than -109.2 °F as unlike regular ice it turns into a gas rather than a liquid.
How to Store Dry Ice
Dry ice should be stored in a well-insulated container outside like a cooler with the lid only slightly closed. The thicker the insulation the better as it will turn back into a gas at a slower rate. Never store it in an sealed container as the sublimated CO2 gas will sink and can be dangerous. If airtight it can make a dry ice bomb causing the container to explode. You might think the freezer is a great place to keep it cool but it actually too warm. Due to the low temperature of dry ice it could cause your fridge to turn off.
The Many Uses of Dry Ice
Dry ice pellets used by wine growers for maceration and conservation.Instants / Getty Images
- Food Industry: Dry ice pellets are added to prevent bacterial growth during large scale blending or grinding processes such as hamburger production.
- Deep Cleaning: Dry ice blasting machines are used to remove mold, glue, paint, oil, grease and other tough to remove adhesive materials.
- Medical Industry: Dry ice is used frequently to store organs for transplant and also in minor dermatology surgeries like mole removals, warts, and other skin imperfections.
Originally Published: Apr 1, 2000