Water has an unusually low compressibility
Despite being a liquid, water is harder to compress than many other liquids.
Scientific Explanation
Most liquids are relatively easy to compress because their molecules lack rigid structure and leave gaps between one another. Water is a striking exception: its isothermal compressibility at 25 degrees Celsius is only about 4.6 times 10 to the minus 10 per pascal — less than half the value of ethanol or acetone.
The reason lies in the three-dimensional network of hydrogen bonds. Each molecule is connected to up to four neighbors through hydrogen bonds, creating an open, tetrahedral local ordering. This structure acts like an internal scaffold that resists external pressure. Although hydrogen bonds constantly break and reform, a large fraction of the network is intact at any moment, keeping the volume remarkably stable.
By comparison, ethanol and acetone are held together by much weaker van der Waals forces, which pressure can overcome far more easily. Mercury, with its metallic bonds, is even less compressible than water, but among molecular liquids, water stands nearly alone.
Everyday Relevance
Water’s low compressibility has far-reaching consequences. Hydraulic systems — car brakes, hydraulic presses, and excavators — rely on this property: the working fluid barely compresses, transmitting pressure almost without loss.
In nature, the oceans maintain their volume despite the enormous pressures in the deep sea. If water were as compressible as typical organic liquids, sea levels would differ significantly, and pressure conditions at the ocean floor would be fundamentally altered.