Physical Processes: Equilibrium
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This equilibrium is governed by Henry's Law, which states: 'The mass of a gas dissolved in a given amount of a liquid at any temperature is directly proportional to the pressure of the gas present in equilibrium with the liquid'. General characteristics of equilibrium in physical processes as: a) At atmospheric pressure, a solid exists in equilibrium with a liquid only at one temperature, which is called its melting point. Ex: At atmospheric pressure, ice exists in equilibrium at 273 K. b) At a given temperature, vapour pressure remains constant for Liquid - vapor equilibrium. Ex: In equilibrium between water and water vapour, the vapour pressure of water remains constant at a given temperature. c) At a given temperature, solubility remains constant for the dissolution of solids in liquids. Ex: In equilibrium at the given temperature, the solubility of sugar remains constant in a given liquid. d) At a given temperature, the concentration of a gas in a liquid is directly proportional to the pressure of the gas over the liquid. e) Ex: The concentration of carbon dioxide in water at equilibrium is directly proportional to the pressure of the gas over the liquid. f) At a given temperature, equilibrium can only be achieved in a closed system. Ex: No equilibrium is attained when water in a beaker is kept open to the atmosphere. This is because the rate of condensation is much less than the rate of evaporation. g) At equilibrium, both the opposing tendencies occur at the same rate, and hence, a dynamic yet stable status is established. Ex: For water at a given temperature in a closed vessel at equilibrium, the rate of evaporation is equal to the rate of condensation. At a given temperature, the attainment of equilibrium for a physical process is characterised by a constant value of one of its parameters, such as vapour pressure, melting point, solubility or concentration. Ex: During equilibrium between liquid and gaseous water molecules, the vapour pressure of water remains constant at a given temperature. At equilibrium, all the measurable properties of the system remain constant. Ex: For the solubility of solids in liquids, the solubility remains constant at a given temperature.

#### Summary

This equilibrium is governed by Henry's Law, which states: 'The mass of a gas dissolved in a given amount of a liquid at any temperature is directly proportional to the pressure of the gas present in equilibrium with the liquid'. General characteristics of equilibrium in physical processes as: a) At atmospheric pressure, a solid exists in equilibrium with a liquid only at one temperature, which is called its melting point. Ex: At atmospheric pressure, ice exists in equilibrium at 273 K. b) At a given temperature, vapour pressure remains constant for Liquid - vapor equilibrium. Ex: In equilibrium between water and water vapour, the vapour pressure of water remains constant at a given temperature. c) At a given temperature, solubility remains constant for the dissolution of solids in liquids. Ex: In equilibrium at the given temperature, the solubility of sugar remains constant in a given liquid. d) At a given temperature, the concentration of a gas in a liquid is directly proportional to the pressure of the gas over the liquid. e) Ex: The concentration of carbon dioxide in water at equilibrium is directly proportional to the pressure of the gas over the liquid. f) At a given temperature, equilibrium can only be achieved in a closed system. Ex: No equilibrium is attained when water in a beaker is kept open to the atmosphere. This is because the rate of condensation is much less than the rate of evaporation. g) At equilibrium, both the opposing tendencies occur at the same rate, and hence, a dynamic yet stable status is established. Ex: For water at a given temperature in a closed vessel at equilibrium, the rate of evaporation is equal to the rate of condensation. At a given temperature, the attainment of equilibrium for a physical process is characterised by a constant value of one of its parameters, such as vapour pressure, melting point, solubility or concentration. Ex: During equilibrium between liquid and gaseous water molecules, the vapour pressure of water remains constant at a given temperature. At equilibrium, all the measurable properties of the system remain constant. Ex: For the solubility of solids in liquids, the solubility remains constant at a given temperature.

#### References

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