Equilibrium in Physical Processes

Just the way chemical reactions attain a state of equilibrium, there exists equilibrium in physical processes too. This refers to the equilibrium that develops between different states or phases of a substance such as solid, liquid and gas. Let’s try and understand equilibrium in physical processes in more detail. Substances undergo different phase transformation processes such as –

solid ⇌ liquid

liquid ⇌ gas

solid ⇌ gas

Let’s understand how they attain equilibrium during each of these transformations.

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Solid-Liquid Equilibrium

What happens if you keep ice and water in a perfectly insulated manner, such as in a thermos flask at a temperature of 273K and atmospheric pressure? We see that the mass of ice and water do not change and that the temperature remains constant, indicating a state of equilibrium.

However, the equilibrium is not static because there is intense activity at the boundary between ice and water. Some ice molecules escape into liquid water and some molecules of water collide with ice and adhere to it. Despite this exchange, there is no change in mass of ice and water. This is because the rates of transfer of ice molecules to water and the reverse process are equal at 273K and atmospheric pressure.

It is evident that ice and water are in equilibrium only at a particular pressure and temperature. Therefore, for any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of the substance.

The system of ice and water is in dynamic equilibrium and we can conclude the following –

  • Both opposing processes occur at the same time.
  • The two processes occur at the same rate such that the amount of ice and water remain constant.


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Liquid-vapour Equilibrium

Equilibrium Vapor Pressure

To understand this concept, let’s perform the following experiment.


  • Place a drying agent like anhydrous calcium chloride for a few hours in a transparent box with a U-tube containing mercury i.e. manometer. This will soak up all the moisture in the box.
  • Remove the drying agent by tilting the box to one side and quickly place a petri dish containing water.


  • The mercury in the manometer rises slowly and then attains a constant value. This is because the pressure inside the manometer increases due to the addition of water molecules into the gaseous phase.
  • Initially, there is no water vapour in the box. As the water in the petri dish evaporates, the volume of water in the petri dish decreases and the pressure in the box increases.
  • The rate of increase in pressure decreases with time because of condensation of vapour into water.
  • Finally, it reaches an equilibrium where there is no net evaporation or condensation.

Conclusion: Equilibrium is reached when the

rate of evaporation = rate of condensation

H2O(l) ⇌ H2O (vap)

At equilibrium, the pressure that the water molecules exert at a given temperature remains constant and is called the equilibrium vapour pressure of water. The vapour pressure of water increases with temperature.

Boiling Point

At the same temperature, different liquids have different equilibrium vapour pressures. The liquid with a higher vapour pressure is more volatile and has a lower boiling point. Let’s understand this concept with the following experiment.


  • Expose three Petri dishes containing 1ml each of acetone, water and ethyl alcohol to the atmosphere.
  • Repeat the experiment with different liquid volumes in a warmer room.


  • In all cases, the liquid eventually disappears.
  • The time taken for complete evaporation of each liquid differs.


  • The time taken for complete evaporation of the liquid depends on – the nature of the liquid, the amount of liquid and the temperature.
  • In an open system i.e. when the petri dish is kept open, the rate of evaporation remains constant but the molecules of the liquid are dispersed into a larger volume of the room. Consequently, the rate of evaporation is much higher than the rate of condensation from vapour to liquid. Therefore, open systems do not reach an equilibrium.

On the other hand, in a closed vessel or system, water and water vapour are in equilibrium at atmospheric pressure (1.013 bar) and 100°C.

For any pure liquid at one atmospheric pressure, the temperature at which the liquid and vapour are at equilibrium is called the normal boiling point of the liquid. 

For water, the boiling point is 100°C at atmospheric pressure. The boiling point of liquids depends on atmospheric pressure i.e. the altitude of the place. Boiling point decreases at higher altitudes.

Solid-Vapor Equilibrium

Have you ever observed what happens to solid iodine placed in a closed jar? The jar gets filled up with violet coloured vapour and the colour intensity increases with time! The intensity of the colour becomes constant after a certain time i.e. equilibrium is reached. In this way, solid iodine sublimes to give iodine vapour while the vapour condenses to form solid iodine. The equilibrium in this process is given as –

I2(solid) ⇌ I2(vapour)

equilibrium in physical processes

Sublimation of Iodine [Source: Wikimedia Commons]

Other substances that show this kind of equilibrium are –

Camphor (solid) ⇌ Camphor (vapour)
NH4Cl (solid) ⇌ NH4Cl (vapour)

Equilibrium Involving Dissolution of Solid or Gases in Liquids

Solids In Liquids

What happens when you make a thick sugar solution by dissolving sugar at high temperature, then allow it to cool at room temperature. Yes, the sugar crystals separate out.

In this case, the thick sugar solution is a saturated solution because no more solute i.e. sugar can be dissolved in it at a given temperature. The concentration of solute in a saturated solution depends on the temperature. A dynamic equilibrium exists between the solute molecules in the solid state and in solution in a saturated solution.

Sugar (solution) ⇌ Sugar (solid)

Also, the rate of dissolution of sugar = rate of crystallization of sugar. Let’s understand this further, using an example.  What happens when you add radioactive sugar to a saturated solution of non-radioactive sugar? You will see radioactivity both in the solution and solid sugar after some time. Initially, there are no radioactive sugar molecules in the solution.

But, due to the dynamic nature of equilibrium, there is an exchange between the radioactive and non-radioactive sugar molecules from the two phases. Thus, the ratio of radioactive to non-radioactive sugar molecules in the solution increases till it reaches a constant value.

Gases In Liquids

Why do we see fizz and hear a sound when we open soda bottles? This happens because some of the CO2 dissolved in it fizzes out rapidly due to the difference in solubility of CO2 at different pressures. The equilibrium between the CO2 molecules in the gaseous state and those dissolved in liquid under pressure is given as –

CO2(gas) ⇌ CO2(in solution)

This equilibrium is governed by Henry’s law. It states that the mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent. This amount decreases with increase in temperature.

The soda bottle is sealed under the pressure of the gas where its solubility in water is high. When the bottle is opened, some of the CO2 escapes trying to reach a new equilibrium or its partial pressure in the atmosphere. This is why soda water turns flat when the bottle is left open for too long.

equilibrium in physical processes

Fizz in soda water [Source: pxhere]

Features of Equilibrium in Physical Processes

Process Conclusion
Solid ⇌ Liquid
H2O(s) ⇌ H2O(l)
Melting point is fixed at constant pressure.
Liquid⇌ Vapour
H2O(l)⇌ H2O(g)
pH2O constant at given temperature.
Solute(s) ⇌ Solute (solution)
Sugar(s) ⇌ Sugar (solution)
Concentration of solute in solution is constant at a given temperature.
Gas(g) ⇌ Gas (aq)
CO2(g) ⇌ CO2(aq)
[gas(aq)]/[gas(g)] is constant at a given temperature.

[CO2(aq)]/[CO2(g)] is constant at a given temperature.

General Characteristics Of Equilibrium In Physical Processes

The following characteristics are common to the state of equilibrium in physical processes.

  • At a given temperature, equilibrium in physical processes is achieved only in a closed system.
  • The opposing processes occur at the same rate and there exists a dynamic but stable condition during equilibrium in physical processes.
  • All properties of the system that are measurable remain constant.
  • Equilibrium in physical processes is characterized by a constant value of one of its parameters at a given temperature.
  • The extent to which a physical process has progressed before reaching equilibrium is indicated by the magnitude of the abovementioned parameter at any stage.

Solved Example For You

Question: The fizz observed when you open a bottle of soda water is governed by which of the following laws?

  1. Murphy’s law
  2. Henry’s law
  3. Raoult’s law
  4. Avogadro law

Solution: The answer is option ‘b’. Henry’s law explains the phenomenon of fizz i.e. the escape of some CO2 molecules due to the difference in solubility of CO2 at different pressures.

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One response to “Acids, Bases and Salts”

  1. sylvester says:

    Thanks for this. It’s fantastic

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