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) has a lower density than the liquid phase due to its molecular structure and hydrogen bonding properties. When water freezes, its molecules form a stable crystalline structure known as hexagonal ice, where hydrogen bonds arrange water molecules in hexagonal patterns. This lattice occupies more space and creates larger empty spaces between the molecules, resulting in a lower density for the solid phase (ice) than for the liquid phase.

Step-by-step solution

1. Introduction to density and its relation to water's phases

Density is a measure of how much mass is in a unit volume of a substance, usually represented by the Greek letter ρ (rho), with the formula: \[ρ = \frac{m}{V}\] where m is mass, and V is volume. In general, most compounds become denser as they transition from liquid to solid. This is because their molecules arrange themselves in a more compact structure upon solidifying, thus occupying lesser volume for the same amount of mass. However, water is an exception to this trend, as its solid phase (ice) has a lower density than its liquid phase.

2. The structure of water molecules

Water is composed of two hydrogen atoms covalently bonded to an oxygen atom, thus forming a molecule with the formula H2O. Due to the difference in electronegativity between hydrogen and oxygen, the oxygen atom attracts the shared electrons more, resulting in a net negative charge around the oxygen atom and a net positive charge around the two hydrogen atoms. This imbalance creates a dipole moment, making water a polar molecule.

3. Hydrogen bonding

The polarity of water molecules leads to hydrogen bonding, an attractive force between the positively charged hydrogen atoms of one water molecule and the negatively charged oxygen atoms of another water molecule. These relatively strong intermolecular forces create an organized structure in water where each molecule is attracted to its neighbors, affecting the physical properties of water, such as its density.

4. Density differences in the phases of water

In the liquid phase, water molecules are constantly in motion and the hydrogen bonds are continuously forming and breaking. This allows water molecules to be close together, leading to a relatively high density. However, when water freezes and becomes a solid, a more stable crystalline structure forms, known as hexagonal ice. The hydrogen bonds cause the water molecules to arrange themselves in a hexagonal shape, which occupies more space and thus creates larger empty spaces between the molecules. This results in a lower density for ice compared to liquid water as mass is distributed over a larger volume.

5. Conclusion

Water is unique due to its molecular structure and hydrogen bonding properties. Unlike most compounds, water's solid phase (ice) is less dense than its liquid phase because of the hexagonal lattice formed by the hydrogen bonds when it freezes. This ordered structure occupies more space and creates larger empty spaces between the molecules, resulting in a lower density for ice than for liquid water.