Matter is defined as anything that has mass and occupies space. Compressibility is a fundamental physical property that describes a substance’s ability to decrease in volume when external pressure is applied. Among the three common phases of matter, the gas state is the only one considered highly compressible. This characteristic is directly related to the vast amount of empty space separating their constituent particles.
Defining Compressibility at a Molecular Level
The extent to which any substance can be compressed depends entirely on the distance between its particles. Applying pressure is essentially an attempt to reduce the total volume by forcing these particles closer together. The more empty space that exists between the particles, the greater the volume reduction that can be achieved. This means that compressibility is inversely proportional to the initial density of the substance.
The particles within a substance naturally resist being pushed too close due to repulsive forces that arise when their electron clouds overlap. This inherent resistance is why any compression stops once the particles are forced into close proximity. Therefore, the physical arrangement of a substance’s particles determines its capacity for volume change under pressure.
Why Gases are Highly Compressible
Gases exhibit high compressibility because their structure is primarily empty space. Gas particles are in constant, rapid, and random motion, and the average distance between them is significant. At standard temperature and pressure, the distance between gas molecules can be approximately ten times the diameter of the molecules themselves.
This wide separation means the actual volume occupied by the gas particles is negligible compared to the total volume of the container. When pressure is applied, the gas molecules are simply pushed closer together, decreasing the vast empty space without significantly increasing the repulsive forces between them. This allows the gas to drastically reduce its volume.
High compressibility has many practical applications. For instance, the air inside a scuba tank is compressed to extremely high pressures, often between 200 and 300 atmospheres. If the air from a typical scuba tank were released at normal atmospheric pressure, it would expand to fill a volume of over 2,500 liters.
The air used to inflate a car tire is compressed to a pressure much higher than the surrounding atmosphere, allowing a large quantity of gas to be stored in a relatively small space. This same principle is used for storing liquefied petroleum gas (LPG) and compressed natural gas (CNG) by forcing the gaseous molecules into a much smaller volume for transport and use.
Why Solids and Liquids Are Considered Incompressible
Solids and liquids are considered incompressible because their particles are already packed closely together. The resistance to compression in these states is due to the strong repulsive forces that develop immediately upon attempting to force the particles any nearer.
In solids, particles are held in fixed positions by strong intermolecular forces. These particles are essentially touching, leaving virtually no empty space that can be collapsed by external pressure. Attempting to reduce the volume of a solid requires immense force, which typically results in breaking the material rather than compressing it significantly.
Liquids also have particles that are closely packed, similar to solids, though they are free to move and slide past one another. Their lack of substantial empty space means they resist compression almost as much as solids do. The particles are already in contact, and any attempt to reduce the volume is met with the strong repulsive forces between the electron shells of the neighboring molecules.
The compressibility of liquids is exceedingly low, making the change in volume negligible for most engineering and practical purposes. For example, water will compress by only about 46 parts per million for every single bar of pressure increase. This near-zero compressibility is why liquids are highly effective in hydraulic systems, where an applied force is transmitted through the fluid without any significant loss of volume.