Sublima

Meet Sublima, a chunk of dry ice who skips the liquid stage and goes straight from solid to gas.

Most solids melt into a liquid before they boil. But whether a liquid can exist depends on pressure. For each substance, a phase diagram shows which phase is stable at each temperature and pressure. Below the triple point pressure, a solid heated up turns directly into gas: sublimation.

Carbon dioxide's triple point is at about 5.1 atmospheres and −56.6 °C. Normal air pressure is only 1 atmosphere, so liquid CO₂ cannot form. Instead, dry ice sublimes at about −78.5 °C, which makes it a handy cooling agent that leaves no puddle behind.

Dry ice is used to keep things cold, to make stage fog (the CO₂ gas chills the air and condenses water droplets), and in blast cleaning. Water ice sublimes too, much more slowly: freezer burn and snow disappearing on cold, dry days are examples. Freeze-drying food uses sublimation at low pressure.

Dry ice releases a lot of CO₂ gas, which can build up in closed spaces, and it can cause frostbite if handled bare.