Question

In most compounds, the solid phase is denser than the liquid phase. Why isn't this true for water?

Short answer

In water, the solid phase (ice) is less dense than the liquid phase due to its molecular structure and hydrogen bonding. While most substances pack more closely together in their solid phase, water molecules form a hexagonal lattice structure in ice, stabilized by hydrogen bonds. This arrangement leaves more open spaces between the molecules, resulting in a lower density for the solid phase compared to the liquid phase.

Step-by-step solution

Understand the molecular structure of water

Water (H2O) is a molecule composed of two hydrogen atoms bonded to a single oxygen atom. Due to the difference in electronegativity between hydrogen and oxygen, the oxygen atom has a partial negative charge while the hydrogen atoms have a partial positive charge. This gives water a bent molecular shape with an angle of approximately 104.5 degrees between the hydrogen-oxygen-hydrogen atoms.

Introduce hydrogen bonding

The partial negative charge on the oxygen atom and the partial positive charge on the hydrogen atoms in a water molecule create a strong electrostatic force known as hydrogen bonding. Hydrogen bonding occurs between adjacent water molecules, where the slightly positively charged hydrogen atoms are attracted to the slightly negatively charged oxygen atoms in the neighboring molecules.

Explain water's density in the liquid phase

In the liquid phase of water, the molecules are in continuous motion due to their thermal energy. This motion allows water molecules to form and break hydrogen bonds rapidly. As a result, the water molecules can pack closely together, creating a relatively high density in the liquid phase.

Explain water's density in the solid phase

When water freezes and turns into ice, its molecules lose some of their thermal energy and are unable to move as freely as they do in the liquid phase. This reduced motion leads to the formation of a more stable hydrogen bonding network, where each water molecule forms four hydrogen bonds with its neighboring molecules. This arrangement causes the water molecules in ice to be organized in a hexagonal lattice structure, leaving more open spaces between the molecules compared to the liquid phase. As a result, the solid phase of water (ice) has a lower density than its liquid phase.

Conclusion

Water is an exception to the general trend that solid phases are denser than liquid phases because of the specific molecular structure of water and the hydrogen bonding between its molecules. In the solid phase, water molecules form a hexagonal lattice structure due to their hydrogen bonding, leaving more open spaces between the molecules and leading to a lower density compared to the liquid phase.