Physical Equilibrium is a state of balance in which all forces acting upon an object are equal and opposite, and the object remains at rest or in uniform motion.

The physical equilibrium can be:

• A state of balance between two or more physical states or properties
• A situation where no change in chemical composition occurs
• The existence of the same substance in two different physical states

Phase Equilibrium

Solute-Solid Equilibrium

Gas-Liquid Equilibrium

Types of Physical Equilibrium

Table of Contents

Types of Physical Equilibrium

Solute-Solid Equilibrium

Examples of Physical Equilibrium

Liquid-Gas Equilibria

Solid-Vapor Equilibria

Equilibrium indicates that the content and composition (as measured by colour, pressure or temperature) of an item of interest in a system remain constant, regardless of the time period. In the Equilibrium state, the rate of the forward reaction is equal to the rate of the backward reaction.

Some examples of equilibrium are:

• A book on a table
• Liquid in a closed container
• Saturated solution
• Ionic substances in polar solvents
• Manufacture of Ammonia

Also Read:

Chemical Equilibrium

Le Chatelier’s Principle

Equilibrium Constant

Let’s Discuss the Different Types of Physical Equilibrium in Detail

Types of Physical Equilibrium

Phase Equilibrium:

At 0°C, the number of water molecules becoming ice is equal to the number of water molecules as ice melting to form liquid water. The rate of freezing of water is equal to the rate of melting of ice, creating an equilibrium between solid ice and liquid water.

Water (l) ⇌ Ice (s)

The number of molecules of a liquid becoming vapour will be equal to the number of molecules condensing into liquid in a closed container. The rate of evaporation of liquid water is equal to the rate of condensation of water vapour. The liquid phase is in equilibrium with its own vapour phase.

Water (g) ⇌ Water (l)

Solubility of Solid Equilibria

When a solute in a saturated solution is in contact with an undissolved solute, the number of molecules leaving the solution is equal to the number of molecules entering the solution from the solid. Thus, the solute in the solution is in equilibrium with the undissolved solid.

Solute (s) ⇄ Solute (aq)

Gas-Liquid Equilibrium:

In a closed container, there is an equilibrium between the gas inside the liquid and the gas present above the liquid. Gases that do not react with the liquid may dissolve depending on the pressure in the liquid. For example, in soft drinks, carbon dioxide gas in the liquid is in equilibrium with the gas in the empty space of the container.

Gas (g) ⇌ Gas (solution)

Examples of Physical Equilibrium

1. A pendulum at rest
2. A book balanced on a shelf
3. A balloon floating in the air
4. A seesaw balanced in the middle

Examples of Solid-Liquid Equilibria:

• Ice and liquid water
• Salt and water
• Sugar and water

The level of water and quantity of ice in a perfectly insulated thermos flask at 0°C in an open atmosphere will remain unchanged, indicating that the rate of transfer of molecules from water to ice is equal to the rate of transfer of molecules from ice to water.

Therefore, we can conclude that this system is in a steady state, which can be represented by the equation:

H2O (l) ⇄ H2O (s)

The rate of melting = Rate of freezing

Examples of Liquid-Gas Equilibria:

After heating distilled water in a closed container, the water will convert to vapour. After a certain period of time, the level of water will remain constant, indicating that no more water is converting to vapour or vice-versa.

We can technically say that the rate of evaporation (liquid to vapour) is equal to the rate of condensation (vapour to liquid) thus achieving a steady state. This equation can be represented by the following equation:

$$Evaporation = Condensation$$

H2O (g) ⇄ H2O (l)

The rate of evaporation = Rate of condensation

Examples of Solid-Vapor Equilibria:

The equilibrium of a solid directly converting to a vapour can be observed when heating solid iodine in a closed container. As the temperature increases, the vessel is filled with a violet coloured vapour and the intensity of the colour increases over time.

The intensity of colour remains constant after a certain period of time, suggesting that a steady state has been reached where the rate of sublimation of solid iodine is equal to the rate of deposition of iodine vapour.

I2 (vapor) ⇄ I2 (s)

The rate of sublimation = Rate of deposition.

Equilibrium Solved Questions

![JEE Chemistry: Equilibrium Solved Questions]()

Also Read:

Ionic Equilibrium

Dynamic Equilibrium

Equilibrium By Changing The Concentration

Shift in Equilibrium by Increasing/Decreasing the Concentration