Solid-Liquid Equilibrium

Solid solubility in liquids is critical for the design of crystallizers, and reliable data on this topic is required. The expression that is produced serves as a foundation for the calculation of the solubility of a solid in a solvent as well as the freezing point depression of a solvent, among other things. 

Physical Equilibrium

Physical equilibrium is defined as the state of equilibrium that develops between two or more phases or physical properties of the same substance. There is no change in the chemical composition of the material during these processes. A substance can exist in two different physical states at the same time, which is represented by this symbol. 

Equilibrium 

Equilibrium refers to the consistency of the content and composition (as measured by colour, pressure, or temperature) of an item of interest in a system, regardless of the time period over which it is observed. It is possible to achieve equilibrium in a system by having a reaction rate that is equal to or greater than the rate of the reverse reaction.

Equilibrium can be represented by a book on a table, a liquid in a closed container, a saturated solution, ionic substances in polar solvents, and the production of ammonia, to name a few examples.

Forms of Physical Equilibrium

Equilibrium in the first phase:

At 0°C, the number of water molecules that turn into ice equals the number of water molecules that turn into liquid water when ice melts. The rate at which water freezes is exactly the same as the rate at which ice melts. As a result, there is a state of equilibrium between solid ice and flowing water.

Water (l) ←→ Ice (s)

In a closed container, the number of molecules of a liquid that condensate into vapour will be equal to the number of molecules that condense back into liquid. In a closed system, the rate of liquid water evaporation is equal to the rate of water vapour condensation. The liquid phase is in equilibrium with the vapour phase that it is surrounded by.

A solute in a saturated solution comes into contact with an undissolved solute, and the number of molecules going out of the solution (depositing) equals the number of molecules entering (dissolving) from the solid into the liquid (Solute-Solute Equilibrium). As a result, the solute in a solution is in equilibrium with the solid that has not dissolved.

Gas-Liquid Equilibrium

‘Gas-Liquid Equilibrium’ refers to the state in which gases do not react with liquids but may dissolve as a function of the pressure in the liquid. In a closed container, there is a state of equilibrium between the gas contained within the liquid and the gas present above and around the container. In soft drinks, for example, the carbon dioxide gas contained within the liquid is in equilibrium with the gas contained within the container’s empty space.

Physical Equilibrium Exemplifications

Examples of Solid-Liquid Equilibria include the following:

Consider the following scenario: ice and water in a perfectly insulated thermos flask at 0­­­­0C in the open environment. Water levels and ice quantities will not change, implying that the rate of transfer of molecules from water to ice is the same as the rate of transfer of molecules from ice to water. Because of this, we can say that the system is in a steady state. The following equation can be used to represent this situation:

H2O (s) ←→ H2O (l)

The rate of melting is equal to the rate of freezing.

Examples of Liquid-Gas Equilibria 

Take distilled water and place it in a closed container. Once the water is heated, it turns into steam. After a period of time, we will notice that the level of water remains constant, indicating that there is no longer any conversion of water to vapour and vice versa.

In technical terms, we can say that the rate of evaporation (from liquid to vapour) equals the rate of condensation (from vapour to liquid), resulting in the achievement of a steady state. 

Examples of Solid-Vapour Equilibria:

 This type of equilibrium can only exist in the case of sublimates, and it is the most common type of equilibrium (solid directly converts to vapour). Consider the following scenario: solid iodine is heated in a closed container, and the vessel slowly fills with violet-colored vapour, with the intensity of the colour increasing over time.

After a certain period of time, the intensity of the colour does not change with the passage of time. This implies that a steady state has been reached in which the rate of sublimation of solid iodine is equal to the rate of deposition of iodine vapour.

I2 (s) → I2 (s) (vapour)

The rate of sublimation equals the rate of deposition (or the reverse).

Conclusion

Therefore, we can finally conclude that the expression that is produced serves as a foundation for the calculation of the solubility of a solid in a solvent as well as the freezing point depression of a solvent, among other things. Also discussed are systems that exhibit eutectic behaviour and are capable of forming solid solutions. In order to obtain solid-liquid equilibrium data, it is necessary to cool a liquid mixture of known composition while continuously recording the temperature as a function of time.