Concept of Change of State

The three states of matter are solid, liquid, and gaseous, and any physical change in their condition is referred to as a change of state of matter. These modifications are reversible in nature, which means they can return to any state.

A change in state of matter refers to a physical change in a substance. They are reversible modifications that do not necessitate a change in the chemical composition of the matter. Common state transitions include melting, freezing, sublimation, deposition, condensation, and vaporisation.

Thermal Equilibrium

When two physical systems are connected by a heat-permeable route, they are in thermal equilibrium if there is no net movement of thermal energy between them. The zeroth law of thermodynamics governs thermal equilibrium. If the temperature within a system is spatially uniform and temporally constant, it is said to be in thermal equilibrium with itself.

Thermodynamically equilibrium systems are always thermally balanced, whereas the opposite is not necessarily true. The two systems may reach thermal equilibrium without reaching thermodynamic equilibrium if the connection between them allows transfer of energy as ‘change in internal energy’ but not transfer of matter or energy as work.

Two Varieties of Thermal Equilibrium

Relation of thermal Equilibrium between two thermally connected Bodies

Thermal equilibrium is an example of equilibrium between two bodies, referring to transmission through a selectively permeable partition of matter or work; it is referred to as a diathermal connection. The basic meaning of the thermal equilibrium connection, according to Lieb and Yngvason, is that it is reflexive and symmetric.

Whether it is transitive or not is not included in the core definition. After exploring the semantics of the term, they postulate a substantial physical assumption, that they call the “zeroth law of thermodynamics”, that thermal equilibrium is a transitive relation. Isotherms are the equivalence classes of systems that have been constructed.

Internal thermal equilibrium of an isolated body

The term “thermal equilibrium of a body” refers to the state of a body when it is isolated. The background is that no heat enters or leaves it, and it is given an endless amount of time to settle into its natural state. It is in its own thermal equilibrium when it has entirely settled to the point where macroscopic change is no longer detectable.

It does not imply that it is in any other type of internal equilibrium. For example, a body may achieve internal temperature equilibrium but not internal chemical equilibrium; glass is an example of this.

The state of a body when it is isolated is referred to as “thermal equilibrium of a body”.  The background is that it receives no heat and is allowed an unlimited amount of time to return to its native state. When it has completely settled to the point where macroscopic change is no longer evident, it is in its own thermal equilibrium. It doesn’t mean it’s in any other kind of internal equilibrium. Glass is an example of a body that achieves internal temperature equilibrium but not internal chemical equilibrium.

Regelation

The phenomena of ice melting under pressure and then refreezing when the pressure is released is known as regelation. This can be illustrated by wrapping a fine wire around a block of ice and securing it with a heavy weight. The ice is gently melted by the pressure applied to it, allowing the wire to penetrate through the entire block. The ice block will remain intact even after the wire has passed entirely through because the wire’s track will refill as soon as pressure is relieved.

Since latent heat of fusion from the top side must be transported to the lower side to supply latent heat of melting, the phenomena works best with high thermal conductivity materials like copper. Regulation is the process by which ice transforms to liquid when pressure is applied and then returns to ice when the pressure is removed.

Vaporisation

A phase shift from liquid to vapour occurs when an element or compound evaporates (or vaporises). Evaporation and boiling are the two kinds of vaporisation. Boiling is a bulk phenomenon, whereas evaporation is a surface phenomenon.

At temperatures below the boiling point at a given pressure, evaporation is a phase transition from the liquid phase to vapour (a state of substance below critical temperature). On the surface, evaporation mainly occurs.

Boiling is a phase transition from the liquid to the gas phase, but it involves the formation of vapour as bubbles under the liquid’s surface. Boiling happens when the substance’s equilibrium vapour pressure is greater than or equal to the atmospheric temperature. The boiling temperature, often known as the boiling point, is the temperature at which water boils. The boiling point of water varies depending on the surrounding pressure.

Sublimation

Sublimation is the process of a substance changing directly from a solid to a gas state without passing through a liquid stage. Sublimation is an endothermic process that occurs whenever a substance’s triple point on its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid, is reached. Deposition or de-sublimation is the reversal of sublimation, in which a substance changes from a gas to a solid state.

Conclusion

In this article we have studied about change in states of matter. We have discussed different types of changes and also studied vaporisation and sublimation.

The term “changing state” refers to the transition from one state of matter (solid, liquid, or gas) to another. Ice melting is a good example of this. This is a solid (ice) transforming into a liquid (water). When particles gain or lose energy, states of matter change; this might be caused by a change in temperature. Particles gain energy as the temperature rises; they lose energy as the temperature falls. It’s crucial to keep in mind that matter cannot be destroyed; it can only change.