The State Where Water Boils and Freezes Simultaneously

Water doesn't have to choose between being ice or steam. In a specific vacuum at a temperature of 0.01 degrees Celsius, water reaches a state where it can boil and freeze in the same container.

This happens because the boiling point of water drops as pressure decreases. When the pressure is lowered to exactly 0.006 atmospheres, the vapor, liquid, and solid phases all reach a state of thermodynamic equilibrium.

At this moment, the water is a chaotic slurry. You can see ice crystals forming as bubbles of steam erupt through the liquid. It looks like a physical impossibility, but the molecules are transitioning between all three states at an equal rate.

This isn't just a lab trick. The Triple Point is the foundation of modern temperature measurement. For decades, the kelvin scale was defined specifically by this exact point of water.

Even more confusingly, water can be chilled past its freezing point without turning to ice—a state called 'supercooling'—as long as it remains perfectly still and pure.