Matter exists in various phases—solid, liquid, and gas—and the transition between these states is known as a phase change. Freezing is the specific physical process where a substance transitions into its solid state, typically from a liquid. The reason gases appear stable and resistant to this transition lies entirely within the physics of their constituent particles.
Defining the States of Matter
The core difference between the states of matter is the arrangement and energy of the constituent particles. In a solid, particles are tightly packed and fixed in an orderly, rigid structure, allowing only for slight vibrations. In a liquid, particles are still close together, but the weak forces allow them to slide past one another, giving the liquid its fluid nature and ability to conform to a container’s shape.
Gases represent the most disordered state, characterized by particles that are extremely far apart. A gas has no fixed shape or volume and will expand to fill any container entirely. While the transition from a liquid to a solid is called freezing, a gas can bypass the liquid phase entirely, transitioning directly into a solid state through a process known as deposition.
The Role of Kinetic Energy
A gas maintains its non-frozen state because its particles possess a high amount of kinetic energy, which is the energy of motion. According to the Kinetic Molecular Theory, the temperature of a gas is directly proportional to the average kinetic energy of its particles. At room temperature, gas molecules move at very high speeds, often hundreds of meters per second.
This high-speed, random motion causes the particles to continuously overcome the weak attractive forces, known as intermolecular forces, that exist between them. For a gas to transition into a liquid or a solid, these intermolecular forces must be strong enough to pull and hold the particles close together. The constant movement of the gas particles prevents this close association, keeping them scattered in the gaseous phase. To force a phase change, this internal kinetic energy must be significantly reduced, allowing the weak attractive forces to dominate the motion.
Conditions Required for Gas Freezing
To force a gas to freeze, two physical conditions must be manipulated: temperature and pressure. The first requirement is extremely low temperature, which dramatically reduces the particles’ kinetic energy. When the temperature drops, the gas molecules slow down, allowing intermolecular forces to draw them close enough to form a solid structure.
The second condition, pressure, aids the process by physically forcing the widely dispersed particles closer together. Increasing the pressure reduces the volume available to the gas, increasing the particle density, and bringing the molecules within the effective range of their weak attractive forces. For most atmospheric gases, such as nitrogen and oxygen, the temperatures required for freezing are hundreds of degrees below zero on the Celsius scale, with nitrogen freezing at about -210 degrees Celsius at standard atmospheric pressure.
The precise combination of temperature and pressure at which a substance can exist in equilibrium as a solid, liquid, and gas is called the triple point. For a gas to deposit directly into a solid, the pressure must often be below the triple point pressure. The unique physical properties of each gas dictate the exact temperature and pressure combination needed to achieve the frozen state.
Examples of Frozen Gases
The most common example of a frozen gas is solid carbon dioxide, widely known as dry ice. Carbon dioxide gas undergoes deposition at standard atmospheric pressure when the temperature drops to approximately -78.5 degrees Celsius (194.7 Kelvin). At this temperature, the gas transforms directly into a solid without ever becoming liquid.
This process is utilized industrially to produce dry ice by first liquefying the carbon dioxide gas under high pressure. Allowing the liquid to rapidly expand causes an extreme drop in temperature that flash-freezes a portion of it. Laboratory settings also routinely freeze atmospheric gases like oxygen and nitrogen; oxygen forms a pale blue solid when cooled below -218.8 degrees Celsius.