Advanced #28

Compressibility decreases with rising temperature up to 46.5 degrees Celsius

For most liquids, compressibility increases with temperature -- for water it initially decreases.

Scientific Explanation

When a typical liquid is heated, its molecules become more mobile, spacing increases, and the material becomes easier to compress. Compressibility rises with temperature — a simple, intuitive behavior. Water does the opposite: between 0 and 46.5 degrees Celsius, its isothermal compressibility decreases before rising again above that minimum.

This unusual behavior reflects a competition between two structural tendencies. At low temperatures, an open, ice-like local ordering with many intact hydrogen bonds dominates. This structure contains relatively large void spaces and is therefore more compressible. As the water warms, some bonds break, molecules pack more tightly, and the voids fill in. The result: compressibility drops despite rising temperature.

Only above 46.5 degrees Celsius does the normal thermal effect take over — the increasing kinetic energy loosens the structure faster than the densification from structural rearrangement can compensate. From this point on, water behaves like an ordinary liquid.

Compressibility vs Temperature for Water and a Typical Liquid Line chart showing isothermal compressibility versus temperature. For a typical liquid, compressibility rises steadily with temperature. For water, compressibility decreases from 0 to about 46.5 degrees Celsius before increasing, creating a minimum. Temperature (°C) Compressibility 0 20 46.5 80 Typical liquid Water Minimum Isothermal Compressibility vs Temperature
Isothermal compressibility as a function of temperature. Water reaches a minimum at 46.5 degrees Celsius.

Everyday Relevance

This behavior has practical significance in underwater acoustics and oceanography. The speed of sound in water depends directly on compressibility: the less compressible the medium, the faster sound travels. Since water’s compressibility decreases in the temperature range found in oceans (typically 2 to 30 degrees Celsius), the speed of sound increases with temperature — the exact opposite of what happens in most other liquids.