Liquids and gases are both considered fluids, meaning they flow and conform to the shape of their containers. However, a key difference between liquids and gases is that gases are highly compressible, while liquids are essentially incompressible. This means that applying pressure to a gas will cause a significant decrease in its volume, while applying pressure to a liquid results in only a slight decrease in volume.
What does it mean for a substance to be compressible?
Compressibility refers to the relative volume change of a substance in response to applied pressure. Substances that experience a significant decrease in volume under applied pressure are considered compressible. Substances that experience little volume change under applied pressure are considered incompressible.
Gases are highly compressible because their molecules have large spaces between them, allowing the molecules to move closer together when pressure is applied. Liquids, on the other hand, are nearly incompressible because their molecules are closely packed together, leaving little free space.
Intermolecular forces in liquids
The reason liquids are so difficult to compress compared to gases is that the molecules are held together by strong intermolecular attractive forces. The main intermolecular forces in liquids are:
- Dispersion forces – caused by temporary uneven charge distributions between molecules
- Dipole-dipole forces – between polar molecules
- Hydrogen bonding – a very strong dipole-dipole force when hydrogen is bonded to small, highly electronegative atoms like nitrogen, oxygen, and fluorine
These intermolecular forces pull liquid molecules close together so they adopt positions that maximize interactions between molecules. This close packing makes liquids resistant to compression.
How much are liquids compressed under high pressure?
While nowhere near as compressible as gases, liquids do experience a slight amount of compression under extremely high pressures. For example, at a pressure of 1 atmosphere (atm), water is compressed by only 5 x 10-4 % of its original volume. This is a miniscule amount of compression.
Even up to pressures of 1000 atm, water only compresses by 3%. To put this in perspective, air compresses by over 80% under the same conditions. Other liquids behave similarly to water. This illustrates just how incompressible liquids are compared to compressible gases.
What causes the slight compression in liquids under high pressure?
As external pressure is applied to a liquid, the intermolecular forces resisting compression do allow for a tiny bit of volume reduction. This slight compression occurs because the pressure can bring molecules infinitesimally closer together when it exceeds the opposing intermolecular forces.
The pressure causes a very slight decrease in the average distance between the molecules. It may also cause minor rearrangements of molecular positions that allow them to pack more tightly together. These subtle changes in intermolecular distances and molecular arrangements account for the minute compressibility observed in liquids under extreme pressures.
How does compression affect liquids vs. gases differently?
| Factor | Liquids | Gases |
|---|---|---|
| Intermolecular forces | Strong intermolecular attractive forces keep molecules close together | Weak intermolecular forces allow molecules to move freely |
| Response to pressure | Very slight decrease in volume | Large decrease in volume |
| Percent compression at 1 atm | 0.0005% (water) | Over 80% (air) |
As this table summarizes, liquids and gases respond very differently to compression due to differences in intermolecular forces. Strong attractive forces in liquids resist compression, while weak forces in gases allow molecules to be squeezed much closer together when pressure is applied.
Real-World Examples of Liquid Incompressibility
The fact that liquids are essentially incompressible has important practical consequences and applications:
- Hydraulic machines – Hydraulic presses, lifts, brakes, and other hydraulic machines rely on the incompressibility of liquids to transmit force and motion. Applying a small force to a master cylinder filled with liquid transmits that force to slave cylinders at high levels of force due to the incompressibility of the liquid.
- Fluid dynamics – Equations describing fluid flow and hydrodynamics treat liquids as incompressible. Allowing slight liquid compressibility makes the equations tremendously more complex with only tiny gains in accuracy.
- Water hammer – When valves suddenly close in water supply pipes it can cause a loud hammering noise due to the incompressibility of water propagating a pressure wave through the pipe. Compressibility would dampen the effect.
- Scuba diving – The pressure effects of water when scuba diving are based on the fact that water is incompressible. The water pressure underwater only increases due to the weight of water above rather than compression of water.
Engineers frequently rely on and design around the incompressibility of liquids for technologies like these. Compressing liquids sufficiently would require extraordinary pressures that are not practical in real-world systems and devices.
Unusual Cases of Liquid Compressibility
While liquids are generally assumed to be incompressible, there are some unusual situations where they display higher levels of compressibility:
- Ultrahigh pressures – At extremes of millions of atm pressure, such as in the core of gas giant planets, liquids become significantly more compressible. However, these are not conditions encountered in typical applications on Earth.
- Microscale confinement – Studies have found that liquids becomes more compressible when confined in very small nanometer-sized spaces. This may result from surface effects becoming more important at these small scales.
- Associating liquids – Liquids composed of large chain-like molecules can sometimes compress more easily because the chains can bend and fold to fit in smaller volumes. An example is ethanediol, a liquid with hydrogen bonding between chains.
However, these examples represent highly uncommon and specialized situations. Under normal conditions on human scales, the incompressibility of liquids can still be assumed in most engineering applications and models.
Conclusion
In summary, liquids are considered essentially incompressible because the strong attractive forces between their molecules make them highly resistant to compression. Applying high levels of pressure results in only a miniscule amount of volume reduction for a liquid. Gases, on the other hand, are highly compressible due to weak intermolecular forces that allow molecules to move much closer together when compressed.
The negligible compressibility of liquids makes them ideal for transmitting forces and fluids flows efficiently in technologies like hydraulic machines. Engineers frequently rely on the incompressible nature of liquids in their designs and analyses. While liquids can deviate slightly from perfect incompressibility under certain uncommon conditions, they are effectively incompressible for most practical purposes. This explains why liquids cannot be significantly compressed, unlike compressible gases.
