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Why does dry ice change directly from solid carbon dioxide to gas at atmospheric pressure?

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argon phase diagram — liquid Figure 1: Phase diagram of argon.

Dry ice changes directly from a solid to a gas at atmospheric pressure because of the specific phase properties of carbon dioxide. This process, known as sublimation, occurs because the triple-point pressure of carbon dioxide—the pressure at which the solid, liquid, and gas phases can coexist in equilibrium—is higher than standard atmospheric pressure.1

Because atmospheric pressure is below this triple-point pressure, carbon dioxide cannot exist as a stable liquid under normal conditions. Instead, when the sublimation curve is crossed at atmospheric pressure, the substance transitions directly from a solid to a gas at approximately −78.5 °C (−109.3 °F). This unique behavior is why the substance is referred to as "dry" ice, as it skips the transitional liquid phase entirely.231