Question

what is different about the general structure of the phase diagram for water compared to most other substances? a the slope of the line between solid and liquid is positive. b the slope of the line between solid and liquid is negative. c there is more than one triple point. d the line between liquid and vapor is straight instead of curved.

Explanation:

For most substances, the solid - liquid phase boundary has a positive slope. This is because increasing pressure favors the denser phase (usually the solid). However, for water, ice is less dense than liquid water. According to Le Chatelier's principle, when pressure is increased, the system will shift to the phase that can relieve the pressure change. Since ice is less dense than liquid water, increasing pressure on ice - water equilibrium will cause the ice to melt (shift to the liquid phase). Mathematically, from the Clapeyron equation \(\frac{dP}{dT}=\frac{\Delta H_{fus}}{T\Delta V_{fus}}\), for water \(\Delta V_{fus}=V_{liquid}-V_{solid}<0\) (because \(V_{solid}>V_{liquid}\) for water), so \(\frac{dP}{dT}<0\) (negative slope for the solid - liquid line).

Answer:

B. The slope of the line between solid and liquid is negative.