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

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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.

A comparison of gas and liquid compressibility: the left side shows gas molecules widely spaced and easily compressed under pressure, while the right side shows tightly packed liquid molecules barely changing volume under the same pressure.

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.

 

 

Examples and Experiments in Real Life

  • Syringe experiment: A syringe filled with air compresses easily, but one filled with water hardly moves.
  • Submarine pressure: In deep oceans, water remains nearly unchanged in volume despite immense pressure.
  • Hydraulic systems: Used in construction and machinery due to the minimal volume change under pressure.

Liquids Are Not Perfectly Incompressible

No material is completely incompressible. It’s just that extremely high pressure is needed to notice volume changes. For example, under 1,000 atmospheres, water's volume decreases by only about 0.5%.

Relationship Between Pressure and Volume in Liquids

ΔV = -κ × V₀ × ΔP (κ: compressibility coefficient, V₀: original volume, ΔP: pressure change)
According to this formula, the "incompressible" nature of liquids is really a very small but measurable volume change under pressure.

 

 

Industrial Applications: Hydraulic vs. Pneumatic Systems

  • Hydraulic systems: Use liquid’s near-incompressibility for precise, consistent force transmission (e.g., excavators, elevators).
  • Pneumatic systems: Use gas’s compressibility for cushioning or energy absorption (e.g., air hammers, tires).

Each system is chosen based on its purpose, and the compressibility properties significantly affect design and operation.

 

 

Conclusion

Liquids are not truly incompressible—they just compress very little. This subtle property isn’t just a scientific detail; it's crucial in many real-world systems. Understanding the differences between gas and liquid behavior enhances not only your scientific literacy but also your appreciation for the technologies around you.