Advanced #26

The number of nearest neighbors increases with temperature

With increasing temperature, the number of nearest neighbors in liquid water increases.

Scientific Explanation

In ordinary liquids, the number of nearest neighbors decreases upon heating because molecules vibrate more strongly and drift further apart on average. In liquid water, the opposite occurs: the coordination number grows from about 4.4 at 0 degrees Celsius to approximately 5.0 at 80 degrees Celsius.

This behavior reflects the progressive breakdown of ice-like structures. At low temperatures, liquid water retains many tetrahedrally coordinated regions where each molecule has roughly four neighbors, similar to ice. This open arrangement leaves gaps in the local order. As temperature rises, hydrogen bonds are increasingly disrupted and distorted, tetrahedral order is lost, and molecules from more distant positions can move into the gaps.

X-ray and neutron diffraction measurements confirm this trend. The radial distribution function of water shows a flattening of structural peaks at higher temperatures, indicating more but less ordered neighbors.

Nearest Neighbors Increase with Temperature Line chart showing the number of nearest neighbors in liquid water increasing from about 4.4 at 0 degrees C to about 5 at 80 degrees C as hydrogen bonds break and molecules pack more closely at higher temperatures. Nearest Neighbors Temperature (°C) 4.0 4.5 5.0 0 20 40 60 80 Ice: 4 ~4.4 ~5.0 Nearest Neighbors in Liquid Water
The coordination number in liquid water increases with temperature — the opposite of normal liquid behavior.

Everyday Relevance

This phenomenon explains why the physical properties of water change with temperature in unexpected ways. The increasing number of neighbors upon heating means that hot water becomes “more normal” compared to other liquids, while cold water is particularly anomalous due to its ice-like structure. This understanding is important for oceanography, climate science, and any field where the temperature dependence of water properties plays a role.