If you've ever received a special shipment of meat or seafood at home, it probably arrived surrounded by blocks of dry ice. When you place the blocks in your sink, they slowly disappear, but no water streams from the sides as you'd expect from melting ice. How does dry ice perform this disappearing act? Why is dry ice dry?
The secret is in the chemical composition of dry ice, which is carbon dioxide in solid form. At room temperature, carbon dioxide exists a gas, the very gas we exhale every time we breath. However, this familiar compound has some unusual physical properties. Instead of melting, at room temperature, into a liquid, and then evaporating into gas, dry ice passes directly from solid to gas by a process called sublimation.
That disappearing block actually sublimates into the air as colorless carbon dioxide gas. The reverse process can also occur at extremely cold temperatures, such that carbon dioxide gas changes directly into a solid.
Does carbon dioxide ever exist as a liquid? In other words, is dry ice always dry? Carbon dioxide does have a liquid form in fact, when it is stored as a liquid in tanks for the manufacture of dry ice.
However, changing into a liquid is one trick the compound won't perform in your kitchen sink. At room temperature, the only way to bring carbon dioxide to liquid state is to drastically increase the pressure, to at least about 55 times normal atmospheric pressure. That's why dry ice, under everyday household conditions, is dry.