Problem 88 - Chemistry

Question

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

Step-by-Step Solution

Answer

Water's phase diagram is unusual because the line between solid and liquid has a negative slope, indicating that increased pressure can cause a transition from solid to liquid, contrary to most substances. This occurs because water expands when it freezes due to the unique molecular structure of ice, which is less dense than liquid water.

Step 1: Understanding Phase Diagrams

A phase diagram displays different states of a substance (solid, liquid, gas) at varying pressures and temperatures. It typically shows areas representing each state, separated by lines that indicate phase changes (e.g., solid to liquid). Crucially, the slope of the line between solid and liquid phases is typically positive.

Step 2: Water’s Phase Diagram and Anomalous Behavior

Water’s phase diagram has one significant difference from most substances: the line between solid (ice) and liquid (water) slopes leftwards, meaning its slope is negative. This is highly unusual since for most substances, increasing pressure leads to the solid phase, but for water, more pressure can lead to the liquid state.

Step 3: Identifying Cause of Difference

This unique behavior results from one of water's peculiar properties. At typical pressures, ice is less dense than water because of its molecular structure, which allows for plenty of empty space between water molecules. When pressure increases, these spaces collapse, leading to the liquefaction of ice. In essence, water expands when frozen, unusual for most substances that typically contract upon freezing.