With the summer, we’ve been seeing a lot of dry ice in frozen food deliveries lately. As I observed dry ice melting, I wondered why dry ice changes from a solid to a gas without going through a liquid. Let’s take a look at dry ice.

Dry ice is frozen carbon dioxide (CO2). Carbon dioxide comes from burning fossil fuels and is one of the main causes of global warming. Dry ice is a colourless solid, and at atmospheric pressure, it sublimates directly into a gas, bypassing the liquid phase. Dry ice sublimates from gas to solid at around minus 78°C.

Dry ice has excellent properties as a refrigerant and is used in frozen products and ice cream packaging. It has a lower temperature than ice, is two to three times more efficient at cooling, and produces no byproducts during sublimation. Dry ice sublimates 3 to 5 kg in a typical ice box in 24 hours.

So why does dry ice sublimate without going through a liquid state like water?
This is due to the triple point, which is the temperature and pressure at which all three states – gas, liquid, and solid – can exist together in parallel. There is only one temperature and pressure at which these three states can coexist.
Example) The triple point of water has a temperature of 0.01°C and a pressure of 0.6117 kPa.

If the pressure is lower than this triple point, the substance cannot exist as a liquid, so dry ice sublimates. The triple point of carbon dioxide is 5.13 atm and -56.4°C. Therefore, if the pressure is lower than 5.13 atm, dry ice will sublimate into a gas. And when the pressure of dry ice is 1 atm (atmospheric pressure), dry ice will sublimate when it reaches minus 78.5°C.

In real life, dry ice is used on stage when smoke is needed, to create smoke when dry ice sublimates, and to deliver food or other items that need to be kept frozen.