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 has a negative slope for the solid-liquid line due to hydrogen bonding making ice less dense than liquid water.

Step-by-step solution

Understanding a Phase Diagram

A phase diagram represents the different states of matter (solid, liquid, gas) of a substance under various conditions of temperature and pressure. It shows the equilibrium between different phases.

Analyzing Water's Phase Diagram

In a typical substance's phase diagram, the slope of the line dividing the solid and liquid phases is positive. However, in water's phase diagram, the line between the solid and liquid phases has a negative slope, indicating that increasing pressure at a constant temperature can convert ice (solid) into liquid water.

Identifying the Cause of the Anomalous Behavior

The unusual property of water that causes this difference is its hydrogen bonding. Water molecules form a unique crystalline structure when frozen, making ice less dense than liquid water. This leads to the negative slope in the phase diagram.

Key concepts

Phase Diagram

Phase diagrams are essential tools in understanding how different substances behave under varying conditions of temperature and pressure. They depict the state of a substance—whether it's solid, liquid, or gas—as conditions change. The diagram is a map that indicates the pressure (y-axis) against the temperature (x-axis) where these different phases coexist. Typically, the line separating the solid and liquid phases slopes positively. This means that increasing pressure at a constant temperature usually favors the solid phase over the liquid one.
In most substances, such an increase in pressure makes the solid phase more stable, pushing the equilibrium towards forming solid from liquid. However, water behaves differently. It's a classic example of an exception due to its unique molecular structure.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction occurring between molecules that have a hydrogen atom bonded to a highly electronegative atom, like oxygen. In water, this happens between the hydrogen atom of one water molecule and the oxygen atom of another.
This bonding is significant because it causes water molecules to organize themselves into a crystal lattice when frozen. This crystal structure is less dense than when the molecules are in the liquid state, which is why ice floats on water. The presence of hydrogen bonds explains the unusual behavior observed in water's phase diagram, particularly the negative slope between the solid and liquid phases.

• Hydrogen bonds are strong compared to other forms of intermolecular forces, which leads to high boiling and melting points relative to molecular weight.
• These bonds are responsible for many of water's unique properties, including its high specific heat capacity, surface tension, and solvent abilities.

Anomalous Properties of Water

Water exhibits several unusual properties compared to most substances, primarily due to its hydrogen bonding. One of the most notable anomalies is its behavior under temperature and pressure changes as seen in its phase diagram.
The negative slope of the line dividing the solid and liquid phases indicates that increasing pressure can change ice into liquid water. This is contrary to most materials, where increased pressure would solidify the liquid. Another anomaly is that ice is less dense than liquid water, allowing it to float.

• This seemingly small detail has enormous ecological implications, such as the insulation of water bodies in winter, providing a habitat for aquatic life.
• Water's high heat capacity acts as a universal temperature buffer, influencing weather and climate.

These anomalous properties stem primarily from hydrogen bonding, making water essential for life as we know it.