Analyzing a Few Examples of Physical Equilibrium

Equilibrium refers to a state, attained during a process when properties such as pressure, temperature, and concentration of the substance have nil effect on them concerning a change in time. When such an equilibrium state of process results in physical changes, it is called Physical Equilibrium.

Characteristics of Physical Equilibrium

•  Properties such as temperature, pressure, and concentration which are measurable remain constant.
•  Equilibrium has a dynamic nature which means that there is a continuation of the reaction, however, the rate remains constant.
• At equilibrium, the contrary forces are equal.

Equilibrium state between ice and water

The equilibrium between ice, a solid substance, and water, a liquid substance attains physical equilibrium as it does not involve any chemical reaction.

The temperature at which both solid and liquid substances can exist side by side provided that the substances are pure, at a particular atmospheric pressure is known as the normal melting point of the solid substance. The state changes from solid to liquid and at the specific melting point, there exists equilibrium. The temperature at which a substance in a liquid state can convert into a solid state, provided that the substances are pure, at a particular atmospheric pressure, is known as the normal freezing point of the liquid.

When ice is heated at a constant temperature, it converts into water. The ice starts absorbing the heat of the surrounding atmosphere due to which the molecules that were rigidly enclosed to each other become loose which causes a decrease in the volume. In other words, when the substance is in a solid form the volume is larger as compared to the volume of the substance in a liquid form.

                      V_ice > V_water

However, the number of molecules that were present in the solid substance is always equal to the number of molecules in the liquid substance.

If pressure is increased, the substance/body with the largest volume will be affected due to its large surface area. The substance is affected on a larger scale where its density is low. 

Hence, when pressure is increased, the reaction will travel in a direction where volume is less which means that when pressure is applied to the ice at a certain temperature, the ice starts melting into water and thus the melting point increases vice-versa.

 This exists a dynamic equilibrium between ice and water.

Physical Equilibrium examples are as follows:

Liquid- Vapor Equilibrium

A closed vessel that is internally fitted with a mercury gauge is being used for the evaporation of water. The vapors in the vessel start increasing and gradually a pressure is built up which constantly increases as indicated by a manometer.

The manometer shows a persistent reading of the vapour pressure, which indicates that no more evaporation is taking place within the vessel. Here, the rate of evaporation of water becomes equal to the rate of condensation i.e., 

Rate of Evaporation = Rate of Condensation.

In the case where the vessel is an open container, the vapours will withdraw from the vessel and will get dispersed due to which the rate of evaporation will never become equal to the rate of condensation.

 The volume of the vapour increases which causes the vapour pressure to decrease initially. As a result of a larger presence of volume, there is an increase in the rate of evaporation whereas the rate of condensation decreases. It cannot be assumed that the vapour pressure depends on the size of the jar/container. It however remains constant at a given specific temperature. Therefore, at the equilibrium stage, the initial vapour pressure is stored.

Solid-Vapour Equilibrium

We will take solid iodine for the purpose, which is kept in a closed jar/vessel. We will observe violet vapours emerging in the container and the intensity of which increases over time and gradually becomes constant. Here, such solid substances are to be taken that undergo sublimation.

Solid-Solution Equilibrium

 Let’s take the example of sugar. As more and more quantities of sugar are added to an allotted quantity of water at a certain room temperature and agitated vigorously with a glass rod, the sugar keeps on dissolving. However, a stage comes, when no more of such sugar is dissolved and they settle at the bottom of the container. The solution becomes saturated and attains the stage of equilibrium. The molecules of the undissolved sugar lying at the bottom start getting mixed in the formed saturated solution are the equal number of molecules from the solution that rest at the bottom. Resultantly, the quantity and the concentration of the sugar in the solution are constant.

Conclusion

Thus, physical equilibrium examples are widely practically applicable and are a common phenomenon in chemical industries and labs. Other Examples of Physical Equilibrium include the manufacturing of Ammonia, ionic substances in polar solvents and a book on a table. Equilibrium helps to ascertain the extent, direction and concentration of a reaction.