Water is frequently assumed to be completely unyielding to external forces, but the question of its compressibility is nuanced. Compressibility is defined as a measure of how much the volume of a substance decreases when pressure is applied. While water is highly resistant to this volume reduction under normal conditions, it is technically a compressible fluid, meaning its density increases slightly with increasing pressure. The perception of water as incompressible stems from the fact that liquids, unlike gases, have molecules that are already closely packed, leaving little empty space to reduce. This near-incompressibility gives water many of its unique properties.
Why Water Resists Squeezing
The physical structure of the water molecule (H₂O) and the forces between them explain water’s exceptional resistance to compression. Water is a polar molecule; the oxygen atom attracts electrons more strongly than the hydrogen atoms, giving the molecule partial negative and positive charges. This polarity leads to the formation of hydrogen bonds, which are electrostatic attractions between neighboring molecules.
These hydrogen bonds create a dynamic, three-dimensional network in liquid water, holding the molecules together in a relatively stable, dense arrangement. Unlike gases, where molecules are far apart, liquid water molecules are already in close proximity. The molecules are packed tightly enough that external pressure cannot significantly reduce the overall volume.
Applying pressure only manages to force the water molecules slightly closer together by bending or briefly breaking some of the hydrogen bonds. Even though hydrogen bonds are weak compared to the covalent bonds within the molecule, their sheer number provides a strong collective resistance to external force.
Quantifying Compressibility
The scientific measurement used to quantify a substance’s resistance to uniform compression is the Bulk Modulus (\(K\)). The Bulk Modulus indicates how much pressure is needed to cause a fractional change in volume; a higher value signifies greater resistance to compression. The inverse of the Bulk Modulus is the coefficient of compressibility (\(beta\)).
For liquid water at standard temperature, the Bulk Modulus is approximately 2.2 Gigapascals (GPa). This high value means that an immense amount of pressure is required to compress water. For example, a pressure of about 22 Megapascals (MPa), or 220 times the atmospheric pressure at sea level, is needed to reduce the volume of water by just 1%.
To put this resistance into perspective, the Bulk Modulus of water is roughly 20,000 times greater than that of air, a highly compressible gas. On the other hand, it is significantly lower than the Bulk Modulus of a rigid solid like steel, which is around 160 GPa. Water is often treated as incompressible in many engineering calculations due to this high resistance.
Real-World Impacts of Near-Incompressibility
Water’s near-incompressibility is the foundation for the entire field of hydraulics, which involves the use of fluid power to generate massive mechanical force. In a hydraulic system, a piston applies a small force to a volume of fluid, which is then transmitted through the fluid to a larger piston. Because the fluid does not compress, almost all the input force is immediately and efficiently transferred, enabling heavy machinery like car brakes, construction cranes, and excavators to lift enormous loads.
The slight compressibility of water becomes significant in the immense depths of the ocean. At the bottom of the Mariana Trench, the pressure can exceed 100 MPa, causing a measurable reduction in water’s volume. Over the vast depths of the ocean, this small effect accumulates, causing seawater to compress by about 1.8% in volume at 4 kilometers deep. This slight increase in density due to compression influences ocean circulation patterns and sound propagation.