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Equilibrium in Physical Processes

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  • Last Updated : 25 May, 2022
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Equilibrium exists in physical processes, just as it does in chemical reactions. The equilibrium that arises between different states or phases of a substance, such as solid, liquid, and gas, is referred to as this. Let’s take a closer look at how equilibrium works in physical processes. Substances go through a variety of phase change processes, including Solid ⇌ Liquid, Liquid ⇌ Gaseous, Solid ⇌ Gaseous. Let’s look at how they achieve equilibrium during each of these changes.

Equilibrium in Physical Processes

Equilibrium is the state of a process in which the properties of the process, such as pressure, temperature, and so on, do not change throughout time.

Processes of Equilibrium :

  • Chemical Process
  • Physical Process

The equilibrium is called physical equilibrium when the opposing processes simply involve physical changes.

Physical equilibrium refers to a state of balance that can be observed in physical processes. It is the state of equilibrium between the same chemical species in several stages. Physical equilibrium is the state of being in balance between two or more phases or physical qualities. Chemical composition and characteristics are not altered in these operations. It also denotes the presence of the same substance in two or more physical states. Furthermore, the physical equilibrium can be divided as follows:

  • Solute-Solid Equilibrium
  • Phase Equilibrium
  • Gas-Liquid Equilibrium

Types of Physical Equilibrium

Equilibrium depicts the content as well as the composition of a system’s item of interest. It is unaffected by the passage of time. In addition, the forward and backward reaction rates are always identical in the equilibrium condition. Many examples can be found in our surroundings, such as a book on a table, a saturated solution, and ionic compounds in polar solvents, among others.

Let us now go over the many types of physical balance in detail for a better understanding.

  • Solute-Solid Equilibrium

When a solute in a saturated solution interacts with an undissolved solute, the number of molecules that deposit out of the solution equals the number of molecules that dissolve from the solid into the liquid. As a result, the undissolved solids and the solute in a solution are in equilibrium. Thus,

Solute(aq) ⇌ Solute(s)

  • Phase Equilibrium

At 0°C, the amount of water molecules that turn into ice equals the total quantity of water molecules. This is due to the fact that ice is melting to generate liquid water. The rate at which water freezes will be equivalent to the rate at which ice melts. As a result, a balance between solid ice and liquid water will exist. Thus,

Ice (s) ⇌ Water (l)

Chemistry’s phase equilibrium is an extremely active equilibrium. In a closed container, the number of molecules turning to vapour will be equal to the number of molecules condensing into the liquid. As a result, we may state that the rate of liquid water evaporation is equal to the rate of water vapour condensation. The liquid phase is in equilibrium with its vapour phase in this fashion. Hence,

Water (l) ⇌ Water (g)

  • Gas-Liquid Equilibrium

Gases that cannot react with liquid but can dissolve in direct proportion to the liquid’s pressure. The gas inside the liquid and the gas above it are in equilibrium in a closed container. In soft drinks, for example, the carbon dioxide gas in the liquid will be in balance with the gas present in the container’s empty space. Therefore,

Gas (solution) ⇌ Gas (g)

Solid-Liquid Equilibrium

What happens if you maintain ice and water fully insulated in a thermos flask at 273 degrees Fahrenheit and atmospheric pressure? We can observe that the mass of ice and water does not change, and the temperature does not change, showing that the system is in equilibrium. The equilibrium is not static, though, because there is a lot of activity at the ice-water interface. Some ice molecules dissolve in liquid water, whereas others smash with ice and stick to it. Despite this exchange, the mass of ice and water remains unchanged. This is because, at 273K and atmospheric pressure, the speeds of ice molecules transferring to water and the converse process are equivalent.

Ice and water are clearly only in balance at a specific pressure and temperature. As a result, the normal melting point or normal freezing point of any pure substance at atmospheric pressure is defined as the temperature at which the solid and liquid phases are in balance.

Because the ice and water system is in dynamic equilibrium, we can deduce the following:

  1. Both procedures take place at the same time.
  2. Both processes take place at the same time, resulting in an equal amount of ice and water.

Liquid-Vapour Equilibrium

Let’s try the following experiment to better comprehend this notion.

Experiment: In a clear box with a U-tube containing mercury, such as a manometer, place a drying agent such as anhydrous calcium chloride for a few hours. All of the moisture in the box will be absorbed by this. By tilting the box to one side, you may rapidly remove the drying agent and replace it with a petri dish filled with water.


  1. The mercury in the manometer slowly rises until it reaches a constant value. Because water molecules enter the gaseous phase, the pressure inside the manometer rises.
  2. The box is initially devoid of water vapour. The volume of water in the petri dish reduces as it evaporates, while the pressure in the box rises.
  3. Due to the condensation of vapour into water, the rate of increase in pressure reduces over time.
  4. Finally, it reaches a point where neither net evaporation or condensation occurs.


  1. When the rate of evaporation equals the rate of condensation, equilibrium is established.
  2. The equilibria vapour pressure of water is defined as the pressure exerted by water molecules at a given temperature when they are in equilibrium. With increasing temperature, water’s vapour pressure rises.

The temperature at which water begins to boil 

Different liquids have different equilibrium vapour pressures at the same temperature. The volatile liquid with a higher vapour pressure has a lower boiling point. Let’s use the following experiment to better comprehend this notion.

Experiment: Three Petri dishes containing 1ml each of acetone, water, and ethyl alcohol should be exposed to the environment. In a warmer room, repeat the experiment with varying liquid quantities.

Observation :

  1. The liquid eventually evaporates in every situation.
  2. Each liquid takes a different amount of time to evaporate completely.

Solid-Vapour Equilibrium

For solids that sublimate, this form of equilibrium is reached. When solid iodine is placed in a closed vessel, violet vapours begin to form in the vessel, increasing in intensity over time until it reaches a constant intensity. Equilibrium has been reached at this point, i.e.

Sublimation rate of solid I2 into vapour ⇌ Condensation rate of I2 vapour into solid I2

Solid-Solution Equilibrium

Assume that a constant amount of sugar is introduced to a fixed volume of room-temperature water and thoroughly swirled with a glass rod. The sugar will continue to dissolve, but eventually, there will be no more sugar to dissolve. Instead, it sinks to the very bottom. The solution has now reached saturation and is in a state of equilibrium. At this point, as many sugar molecules from the undissolved sugar’s surface enter the solution, the solution returns the same number of sugar molecules to the undissolved sugar’s surface. As a result, the amount of dissolved sugar and the sugar concentration in the solution stays constant.

Dissolution rate ⇌ Precipitation rate

The solubility of a solid in a given solvent at a given temperature is defined as the quantity of solid in grams that dissolves in 100 gram of the solvent to form a saturated solution at that temperature.

Gas-Solution Equilibrium

In a soda water bottle, this form of equilibrium can be found. Within the bottle, there is a state of equilibrium that is:

CO2(g) ⇌ CO2(solution)

Henry’s law asserts that the mass of a gas dissolved in a given quantity of a solvent at any temperature is directly proportional to the pressure of the gas above the solvent, i.e. m α p or m=Kp

where K is a proportionality constant known as Henry’s constant. Its worth is determined by the nature of the gas, the type of the liquid, and the temperature.

The pressure of the gas above the liquid in a sealed soda water bottle is quite high, hence the mass of the gas dissolved is also very high. As soon as the bottle is opened, the pressure drops to atmospheric levels, lowering the solubility and allowing the dissolved gas to escape.

Dissolution of Solids or Gases in Liquids Equilibrium

  • Solids in Liquids: What happens when you make a thick sugar solution by dissolving sugar at a high temperature and then cooling it to room temperature? The sugar granules do, indeed, separate. Because no additional solute, i.e. sugar, can be dissolved in it at a given temperature, the thick sugar solution is a saturated solution. The temperature influences the solute concentration in a saturated solution. Solute molecules in the solid-state and in solution in a saturated solution are in a dynamic equilibrium. Also, the rate of sugar dissolving equals the rate of sugar crystallization.
    • Example: What happens when you mix radioactive sugar with non-radioactive sugar in a saturated solution? After some time, you will notice radioactivity in both the solution and the solid sugar. The solution contains no radioactive sugar molecules at first.
  • Gases in Liquids: When we open soda bottles, why does it fizz and make a noise? Because of the differential in solubility of CO2 at different pressures, some of the CO2 dissolved in it fizzes out quickly. – is the equilibrium between CO2 molecules in a gaseous form and those dissolved in a liquid under pressure.

CO2 (gas) ⇌ CO2 (solution)

Henry’s law is in charge of this equilibrium. It asserts that at any temperature, the mass of a gas dissolved in a given quantity of a solvent is proportional to the gas’s pressure above the solvent. As the temperature rises, this amount decreases. The soda bottle is sealed under pressure from the gas, which has high water solubility. When the bottle is opened, some CO2 escapes in an attempt to reestablish equilibrium or partial pressure in the environment. When a bottle of soda water is left open for too long, it flattens out.

Characteristics of Equilibrium In Physical Processes

  1. During equilibrium in physical processes, opposing processes occur at the same pace, and there is a dynamic yet stable condition.
  2. Only a closed system can establish equilibrium in physical processes at a given temperature.
  3. At a particular temperature, equilibrium in physical processes is defined by a constant value of one of its parameters.
  4. The size of the above-mentioned parameter at any stage indicates how far a physical process has advanced before reaching equilibrium.
  5. All quantifiable properties of the system stay the same.

Sample Questions

Question 1: What is the Equilibrium Constant, and what does it mean?


The value of a chemical reaction quotient at the point of chemical equilibrium is known as the equilibrium constant. The composition will have no discernible propensity to shift at this time.

Question 2: Give four characteristics of Equilibrium in a physical process.


Characteristics are:

  1. Properties of system constant.
  2. Two opposite process will be equal.
  3. It attain only close vessels.
  4. The size of the above-mentioned parameter at any stage indicates how far a physical process has advanced before reaching equilibrium.

Question 3: During equilibrium, what are two physical processes that oppose one other?


Equilibrium is the state of a process in which characteristics such as temperature, pressure, and reactant and product concentrations in the system do not change with the passage of time. Every chemical or physical process has two opposing forces: a driving force and an opposing force.

Question 4: In chemistry, what does phase equilibrium mean?


The amount of water molecules that convert into ice at 0°C is equal to the entire amount of water molecules. This is due to the melting of ice, which produces liquid water. The rate at which water freezes will be the same as how quickly ice melts. As a result, there will be a balance between solid ice and liquid water. Thus,

Ice(s) ⇌ Water(l)

The phase equilibrium in chemistry is a very dynamic equilibrium. The number of molecules that transform to vapour in a closed container is equal to the number of molecules that condense into liquid. As a result, we may say that the rate of evaporation of liquid water is equal to the rate of condensation of water vapour. In this way, the liquid phase and its vapour phase are in equilibrium. Therefore,

Water(l) ⇌ Water(g)

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