Section: Chemical and Physical Foundations of Biological Systems

26) Which of the following best accounts for the negative slope of the liquid-solid equilibrium line in the phase diagram for water?

1. H₂O(s) has a greater density than H₂O(l), which causes the solid to form liquid under high pressure conditions.
2. H₂O(s) has a greater density than H₂O(l), which results from the hydrogen bonds formed between water molecules.
3. H₂O(s) has a lower density than H₂O(l) which results from the crystalline framework that forms due to hydrogen bonds.
4. H₂O(s) has a lower density than H₂O(l) which causes the solid to form liquid under low pressure conditions.

This question asks the examinee to identify the correct explanation for the negatively sloped phase diagram for water. During the crystallization of water, each molecule forms four hydrogen bonds with adjacent water molecules resulting in three-dimensional hexagonal lattice structure. Thus, the density of solid water is less than the density of liquid water eliminating choices A and B. The negatively sloped liquid-solid equilibrium line in the phase diagram means that an increase in pressure at a constant temperature can cause water to change phases from solid to liquid. The increase in pressure causes water to convert to the more compact liquid phase in order to relieve the increased pressure. Therefore, answer choice C is the best answer.