How Does Dry Ice Work? - Science | HowStuffWorks

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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.

Bubbling cauldron filled with dry ice for smokey fog party effect.
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.

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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.

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