Chemistry

Question Analysis
This question involves explaining water’s unique phase diagram feature. The main focus is on the link between boundary slope and relative phase densities.

Key Concept Explanation
Phase diagram boundary slopes reflect the effect of pressure on phase stability: higher pressure favors the denser phase.
Water’s density anomaly arises from hydrogen bonding in ice, which creates an open, less dense lattice structure.

Step-by-Step Solution
1. State the general rule for most substances: Positive slope = solid is denser than liquid.
2. Identify water’s exception: Liquid water is denser than ice.
3. Connect density to slope: Negative slope means increasing pressure favors the denser liquid phase (melts ice).
4. Answer: Water’s solid-liquid boundary has a negative slope because liquid water is denser than solid ice. In most substances, the so...