Dry ice turns directly from solid to gas at normal atmospheric pressure.

Sublimation is a phase change from solid directly to gas, without a liquid stage in between. A piece of dry ice left at ordinary atmospheric pressure therefore shrinks as carbon dioxide escapes into the air instead of melting into a puddle of liquid carbon dioxide.

The missing liquid stage is a consequence of pressure, not a special exception to thermodynamics. Carbon dioxide has a triple-point pressure of about 5.18 bar. Below that pressure, liquid carbon dioxide is not a stable equilibrium phase, so heating the solid takes it toward gas.

The low sublimation temperature makes dry ice useful for cooling, but also creates hazards. Direct contact can cause cold burns. The gas released during sublimation can accumulate where ventilation is inadequate, creating dangerous carbon dioxide concentrations and reducing the availability of breathable air.

A phase diagram maps the stable forms of a substance against temperature and pressure. Water and carbon dioxide have different diagrams, so everyday experience with melting ice is not a reliable guide to dry ice. At normal atmospheric pressure, water can pass from solid to liquid, whereas carbon dioxide cannot maintain that intermediate liquid phase.

The triple point is the particular pressure and temperature where solid, liquid, and gas can coexist in equilibrium. For carbon dioxide, its pressure is well above the pressure of ordinary open air. Liquid carbon dioxide can exist under suitable higher-pressure conditions, which is why calling dry ice a substance that never forms a liquid would be incorrect.

As dry ice sublimes, the phase change absorbs energy from its surroundings. This cooling is useful in temperature-controlled transport and demonstrations. The rate of disappearance depends on heat transfer, exposed surface area, insulation, and the surrounding conditions, rather than on a single universal time for a block to vanish.

The familiar white cloud around dry ice is not visible carbon dioxide gas. Carbon dioxide itself is colorless. Cold conditions near the subliming solid can condense water vapor into tiny droplets, producing a visible fog. Adding warm water supplies heat and moisture, often making that fog effect much more conspicuous.

Dry ice must not be sealed inside an ordinary airtight container. Continuing gas production can build pressure and rupture the container. Suitable handling includes appropriate insulated protection, tools rather than bare hands, and effective ventilation. A space that seems large can still be unsafe if substantial quantities release gas into poorly ventilated areas.

Dry ice is called dry because it leaves no liquid carbon dioxide puddle at ordinary atmospheric pressure. It can still chill nearby air and surfaces enough to condense or freeze atmospheric water. A wet surface near dry ice therefore does not show that the carbon dioxide itself has melted.