Applying an external force to a gas to reduce the space it occupies is known as compression. This process is possible because gas molecules are widely spaced, allowing a significant reduction in volume when pressure is applied. Compression fundamentally alters the physical characteristics of the gas, changing how its molecules behave and interact with their surroundings. Understanding this process is key to various technologies, from refrigeration and energy storage to internal combustion engines.
The Immediate Increase in Pressure and Density
The most immediate consequence of compressing a gas is a sharp increase in its internal pressure. Gas pressure is defined by the constant, random collisions of its molecules with the walls of the container. When the volume is forcibly reduced, the molecules are confined to a much smaller space. Consequently, the molecules travel a shorter distance before striking a wall, significantly increasing the frequency of these impacts. This increased collision rate rapidly elevates the measured pressure. Simultaneously, the gas density increases directly in proportion to the volume reduction, because packing the same mass of gas into a smaller container results in a denser substance.
How Compression Generates Heat
When a gas is compressed, mechanical work is done on the system, and that energy is converted into the internal kinetic energy of the gas molecules, resulting in a rise in temperature. This phenomenon is known as adiabatic heating, where the compression occurs so quickly that there is no time for the resulting heat to escape into the surroundings. If the compression were slow, the heat would dissipate, maintaining a constant temperature (an isothermal process). In rapid compression, the process is effectively adiabatic, trapping the energy and dramatically raising the temperature. A common example is a bicycle pump, where the rapid stroke makes the body of the pump noticeably hot. In a diesel engine, this intense heating is deliberately used to ignite the fuel mixture without the need for a spark plug.
Defining the Relationships of Volume and Pressure
The behavior of a gas under compression is governed by precise scientific rules. The primary relationship is described by Boyle’s Law, which states that for a fixed amount of gas at a constant temperature, pressure and volume are inversely related. If the volume is halved, the pressure exerted by the gas will double, meaning the product of pressure and volume remains constant under isothermal conditions. This relationship is a simplified component of the overarching Ideal Gas Law. The Ideal Gas Law links all four measurable characteristics of a gas: pressure, volume, temperature, and the amount of gas, providing the framework for predicting gas behavior when conditions are altered.
The Ultimate Result: Changing Gas to Liquid
If the process of compression is continued to an extreme degree, it can force a phase transition, changing the gas into a liquid. This occurs when the molecules are pushed so close together that the weak attractive forces between them overcome the kinetic energy that keeps them apart. The resulting liquid state occupies a vastly smaller volume, making it much more efficient for storage and transport.
Critical Temperature
Liquefaction is highly dependent on temperature, which must be below a specific point known as the critical temperature. Above this critical temperature, no amount of compression alone can turn the gas into a liquid, regardless of the pressure applied.
Practical Applications
For gases like propane and butane, the critical temperature is well above room temperature, meaning they can be liquefied and stored in tanks simply by pressurization. Other substances, such as natural gas (methane), require cooling to extremely low temperatures (around -162°C) in addition to compression to create Liquefied Natural Gas (LNG).