Water and Pressure: Resolving the Freezing Paradox with Ice III

Summary

MinutePhysics examines a thought experiment about cooling water inside a perfectly rigid, non-bulging vessel. Since freezing makes water expand and compression tends to melt ice, freezing under high pressure seems paradoxical. The video uses the phase diagram of water to show how temperature and pressure together determine whether water is liquid, solid, or in another ice phase.

• Water expands when it freezes, which can pressurize a tight container and alter freezing behavior.
• Melting under pressure means cooling a confined liquid could produce unexpected results if the solid form cannot accommodate expansion.
• Phase diagrams reveal multiple ice phases, not just Ice Ih, helping to resolve the apparent contradiction.
• At very high pressures and low temperatures a denser Ice III can form, enabling the entire container to freeze solid even under compression.

Introduction

The video presents a thought experiment: what happens if you try to freeze water inside a super stiff vessel that cannot bulge or stretch when the temperature dips below freezing? The intuitive paradox arises because water expands on freezing, while high pressure can melt ice. The discussion uses the water phase diagram to frame the question in terms of temperature and pressure, showing that pressure can alter the solid-liquid boundary in surprising ways.

Water Phase Behavior and the Phase Diagram

Water exhibits complex phase behavior. At normal atmospheric pressure, cooling water causes a liquid-to-solid transition around 0°C. But the phase diagram reveals that when you increase pressure, the solid water can revert to a liquid before it reaches the same low temperature. In other words, the boundary between liquid and solid shifts with pressure. This is the foundation for understanding what happens in a confined, high-pressure scenario where the container cannot expand as water tries to freeze.

The Paradox of Freezing Under Pressure

The central question is whether cooling water below zero in a non-bulging vessel should lead to freezing, or should the expanding ice melt due to the pressure generated by its own expansion. The video clarifies that while the liquid water would freeze in the absence of pressure, the expansion of solid ice within a rigid container increases pressure, which can raise the melting point and complicate the path to complete solidification. The phase diagram helps visualize how pressure and temperature guide the progression from liquid to solid and, under certain conditions, back toward liquid.

Progression Toward Ice III

The discussion explains that at sufficiently low temperatures and high pressures, water can freeze into Ice III, a different crystalline phase with distinct density and volume characteristics. Ice III formation can alter the system’s pressure dynamics, and as more ice forms, the pressure can shift the system further along the phase diagram. This progression demonstrates that the remaining liquid water can eventually transform into a different ice phase, eventually allowing the entire container to become solid under the right conditions.

Resolution: Water Is Not Binary

The video concludes that there is no paradox because water does not switch from liquid to a single, simple solid state. Instead, its phase behavior is non-binary and highly dependent on the exact temperature and pressure. As conditions change, ice Ih can coexist with Ice III, and under extreme confinement the container can become fully solid. The phase diagram makes this clear by showing how the line between solid and liquid moves as pressure increases, guiding which phase is stable at a given combination of temperature and pressure.