Water Can Boil and Freeze at the Same Time

At a very precise combination of temperature and pressure, water achieves a remarkable state of equilibrium where all three of its phases—solid, liquid, and gas—exist in perfect harmony. In this delicate balance, molecules are simultaneously escaping the liquid surface to become vapor, while others are condensing back into liquid. At the same time, ice is melting into water, and water is freezing into ice. This unique condition, known as the triple point, occurs for pure water at exactly 0.01 degrees Celsius and a pressure that is a tiny fraction of our normal atmosphere. It is a point so precise and reproducible that it served as a fundamental standard in science for decades.

The significance of the triple point extends beyond a simple scientific curiosity. For many years, it was used to define the kelvin, the base unit of temperature in the International System of Units. By setting the triple point of water to exactly 273.16 K, scientists created a universal benchmark for temperature measurement, ensuring that a degree Celsius or a kelvin meant the same thing in any laboratory around the world. This foundational role has made the triple point indispensable for calibrating thermometers and other sensitive scientific instruments where accuracy is paramount.

While these specific conditions are not found in our daily environment, the triple point is regularly demonstrated in physics laboratories. Inside a vacuum chamber, as air is pumped out to dramatically lower the pressure, water will begin to boil even at room temperature. This rapid boiling uses energy, causing the water (Review)'s temperature to plummet until it begins to freeze, resulting in a fascinating display of water that is simultaneously boiling and turning to ice.