Suppose that I fill a rigid container with water and that this container will not expand or contract as its temperature changes. Will the water turn to ice when I cool it below 0° C? — PL, Taikoo Shing, Hong Kong
Since water normally expands as it forms ice, the rigid container will prevent it from freezing at 0° C. If the container was completely filled with water at room temperature, then an “empty” region will appear inside the container when you first begin to cool it toward freezing. That’s because water contracts as you cool it toward 4° C. The “empty” region isn’t really empty, it contains gaseous water vapor. But once the water’s temperature drops below 4° C, the water begins to expand as it cools. It will first expand into the “empty” region, but when that region becomes full the water will no longer be able to expand. Instead, its pressure will begin to rise dramatically. This elevated pressure is what will ultimately prevent the water from freezing at 0° C—high pressure depresses water’s freezing temperature. Although the water will eventually freeze, you’ll have to cool it far below 0° C for that to occur.