Advanced #13

Water molecules shrink with rising temperature and expand with rising pressure

Counterintuitively, individual water molecules become smaller when heated.

Scientific Explanation

Most molecules become larger when heated: higher temperatures mean stronger vibrations, which on average push atoms further apart. In water, the opposite occurs — the O-H bond length in individual water molecules decreases as temperature rises. The molecule shrinks.

This effect has been precisely measured through neutron diffraction experiments. The O-H bond length in liquid water is approximately 0.991 angstroms at 4 degrees Celsius and decreases to about 0.985 angstroms at 77 degrees Celsius. Although this change appears tiny (less than 1 percent), it is statistically significant and contradicts normal behavior.

The cause lies in the interplay between the covalent O-H bonds within the molecule and the hydrogen bonds to neighboring molecules. At low temperatures, hydrogen bonds are stronger: they “pull” on the hydrogen atom, thereby elongating the O-H bond within the molecule. At higher temperatures, hydrogen bonds weaken, the pull diminishes, and the covalent O-H bond can contract to its natural, shorter length.

Conversely, the molecules expand under increasing pressure — also counterintuitive. High pressure pushes neighboring molecules closer together, strengthening the hydrogen bonds and thereby elongating the O-H bond. It is a fascinating interplay: the environment of the molecule (the hydrogen bond network) influences the internal geometry of the molecule itself.

Water Molecule O-H Bond Length vs Temperature Diagram showing that the O-H bond length in individual water molecules decreases as temperature increases, contrary to the typical thermal expansion behavior. The left side shows a larger molecule at low temperature and the right side shows a smaller molecule at high temperature, with a graph showing the declining bond length. Temperature (°C) O–H Bond Length 0 50 100 Shrinking! Cold O H H Warm O H H O–H Bond Length Decreases with Temperature
The O-H bond length in water molecules decreases with rising temperature — the molecule shrinks.

Everyday Relevance

Although the shrinking of individual molecules is not directly noticeable in everyday life, it has fundamental implications for water’s behavior as a solvent. The length and strength of the O-H bond determine how effectively water can dissolve and react with other molecules. In analytical chemistry, the temperature-dependent shift of the O-H stretching frequency (which directly correlates with bond length) is routinely used as an “internal thermometer” for hydrogen bond strength.

For materials science, this phenomenon demonstrates how powerfully the local environment can influence the properties of individual molecules — an insight also relevant to the design of new materials and catalysts where water serves as the reaction medium.