Are Liquids Truly Incompressible? A Comparison Between Gas and Liquid Pressure Responses

We often learn that gases are compressible, while liquids are not. But in reality, liquids do compress—just extremely slightly. This post compares the compressibility of gases and liquids in terms of physical properties, experimental evidence, and industrial applications.     Structural Differences Between Gases and Liquids Gases have molecules that are far apart and free to move, so they respond readily to external pressure and decrease in volume. In contrast, liquids have tightly packed molecules, so even with high pressure, their volume changes very little. This is the essence of the difference in compressibility. Comparing Compressibility Coefficients Compressibility of air (gas): approx. 10 -5 Pa -1 Compressibility of water (liquid): approx. 4.6 × 10 -10 Pa -1 A compressibility coefficient indicates how much a substance's volume decreases per unit pressure. Water is about 20,000 times harder to compress than air.    ...