Question

Explain how water's phase diagram differs from those of most substances. What property of water causes the difference?

Short answer

Water's phase diagram is unique because the line dividing its solid and liquid phases has a negative slope. This means that applying more pressure actually lowers the freezing point, opposite of most substances. The underlying reason is water's hydrogen bonding, which causes an increase in volume when transitioning from liquid to solid - thus, water expands upon freezing.

Step-by-step solution

Understanding phase diagrams

A phase diagram is a graphical representation that shows the phase of a substance that would be stable at any given pair of temperature and pressure conditions. The three common phases considered are solid, liquid, and gas. In general, an increase in temperature or a decrease in pressure transforms a substance from the solid phase to the liquid phase, and ultimately to the gaseous phase. However, the phase diagram of water, unlike that of most substances, has a slope to the left.

The unique place of water

On most phase diagrams, the line between the solid and liquid phases slopes to the right, indicating that as pressure increases, temperature must also increase for the substance to transition from solid to liquid. Water's phase diagram, on the other hand, has a line with a negative slope between the solid and liquid phases. This means that for water, as pressure increases, the temperature for it to transform from solid (ice) to liquid (water) actually decreases.

Identifying the property that causes the difference

The property of water that results in this variance is its unique hydrogen bonding that leads to an increase in volume when it transitions from the liquid phase to the solid phase. When water freezes into ice, it becomes less dense due to the formation of a crystalline structure maintained by hydrogen bonds. Hence, water expands upon freezing, and pressure has the opposite effect that it does on most substances: increasing pressure lowers the freezing point of water rather than raising it.