Question

Explain what happens to a substance when it is heated in a closed container to its critical temperature.

Short answer

When a substance is heated in a closed container to its critical temperature, it forms a supercritical fluid where the distinction between liquid and gas phases disappears.

Step-by-step solution

Understanding Critical Temperature

The critical temperature of a substance is the highest temperature at which a gas can be converted to a liquid by applying pressure. Beyond this temperature the gas cannot be liquefied regardless of the pressure applied.

Heating in A Closed Container

When a substance is heated in a closed container to its critical temperature, the distinction between the liquid and gas phase disappears. Particles move at such high speeds that the attraction between them becomes negligible compared to their kinetic energy.

Formation of a Supercritical Fluid

As the temperature reaches the critical point, the substance becomes a supercritical fluid. This state has some properties of both liquids and gases and is often denser than a gas but less dense than a liquid at standard conditions.

Key concepts

Supercritical Fluid

When a substance is subjected to heating in a sealed environment, a fascinating transformation occurs at the brink of its critical temperature. At this juncture, the substance transcends traditional boundaries and enters a unique state known as a supercritical fluid. This state embodies a hybrid of sorts, holding properties that are characteristic of both liquids and gases. Supercritical fluids are remarkable for their enhanced solvent abilities, unbound by the constraints seen in either liquid or gas forms. This versatility makes them invaluable in various applications, ranging from critical extractions in the food industry, like decaffeinating coffee, to innovative cleaning processes.

Supercritical fluids exist at conditions above their respective critical points, defined by a critical temperature and pressure. In this state, their molecules have overcome the attractive forces that ordinarily define liquid structures, but have not dispersed as entirely separated entities typical in gases. Instead, the density of a supercritical fluid resembles that of a liquid, while its viscosity is more akin to a gas, allowing it to permeate through materials in a way neither liquid nor gas can achieve alone.

Phase Transition

Phase transition is a pivotal concept in understanding matter's behavior under different environmental conditions. Evoked by fluctuations in temperature or pressure, a phase transition pertains to the transformation from one state of matter to another, such as solid to liquid (melting), liquid to gas (vaporization), or even solid to gas (sublimation). Each substance has distinct points at which these transitions occur, known as melting, boiling, or sublimation points.

When discussing the critical temperature, we touch upon a particularly intriguing phase transition. Beyond this threshold, a gas cannot be liquified by pressure, indicating that the usual gas to liquid transition is nullified. Upon heating to the critical temperature in a closed system, a conflated phase emerges that transcends classical liquid and gas phases— the substance effectively assumes the state of a supercritical fluid. This phase is marked by the unification of the previously separate gas and liquid boundaries, manifesting as a singular, homogeneous phase.

Kinetic Energy of Particles

At the heart of phase transitions lies the concept of the kinetic energy of particles. Kinetic energy encapsulates the energy that an entity possesses due to motion. In the context of particles in a substance, this kinetic energy is significantly influenced by temperature — the greater the temperature, the more vigorous the movement of the particles.

Within a closed system, such as the one described in our exercise, when the substance is heated to its critical temperature, the kinetic energy of the particles intensifies to the point where their incessant motion disrupts and dilutes the attractive forces that would normally maintain the particles close together in a liquid state. This escalation in the kinetic energy of particles is the force majeure that propels the substance into the supercritical realm, where the particles are too energetic to settle into a distinct liquid phase under the influence of pressure, rendering the liquid and gas phases indistinguishable and creating a supercritical fluid.