(i) on sider melting of ice in a closed container at 273K. This system reach a state of physical equilibrium in which the amount of water in the solid phase and liquid phase does not change with time.
(ii) In this procedure, the total number of water molecules leaving from and returning to thelid ph at any in tant are equal.
(iii) If m ice cube and water are placed in a thermos flask (at 273K and 1 atm) then there will be no change in the mass of ice and water.
(iv) the equilibrium: Rate of melting of ice ⇌ Rate of freezing of water.
\({ H }_{ 2 }O\rightleftharpoons { H }_{ 2 }O(l)\)

If Q = K_c the reaction is in equilibrium state.
If Q > K_c the reaction will proceed in the reverse direction i.e., formation of reactants.
If Q < K_c the reaction will proceed in the forward direction i.e., formation of products.

Addition of a catalyst does not affect the state of the equilibrium. The catalyst increases the rate of both the forward and the reverse reactions to the same extent. Hence, it does not change the equilibrium composition of the reaction mixture. However, it speeds up the attainment of equilibrium by providing a new pathway having a lower activation energy.
For example, in the synthesis of NH₃ by the Haber's process iron is used as a catalyst. Similarly, in the contact process of manufacturing SO₃ platinum or V₂O_S is used as a catalyst.

The change in pressure has significant eect only on equilibrium systems with gaseous components.
When the pressure on the sysem EMI tI·S· Increased, the volume decreases proportionately and the system responds by shifting the equilibrium in a direction that has fewer moles of gaseous molecules. Let us consider the synthesis of ammonia from nitrogen and hydrogen
\({ N }_{ 2 }\left( g \right) +3{ H }_{ 2 }\left( g \right) \rightleftharpoons { 2NH }_{ 3 }\left( g \right) \)
Let the system, be allowed to attain equilibrium in a cylinder with a piston. If we press the piston down to increase the pressure, the volume decreases.
The system responds to this effect by reducing the number of gas molecules. i.e. it favours the formation of ammonia. If we pull the piston upwards to reduce the pressure, the volume increases. It favours the decomposition of ammonia

However, when the total number of the moles of the gaseous reactants and the gaseous products are equal, the change in pressure has no effect on system at equilibrium.

According to Le Chatelier's principle, the effect of increase in concentration of a substance is to shift the equilibrium in a direction that consumes the added substance.
Let us consider the reaction
\({ H }_{ 2 }\left( g \right) +{ I }_{ 2 }\left( g \right) \rightleftharpoons 2HI\left( g \right) \)
The addition of H₂ or I₂to the equilibrium mixture, disturbs the equilibrium. In order to minimize the stress, the system shifts the reaction in a direction where H₂ and I₂ are consumed i.e., the formation of additional HI would balance the effect of added reactant. Hence, the equilibrium this to the right (forward direction) i.e. the forward reaction takes place until the equilibrium is re established. Similarly, removal of HI (product) also favours the forward reaction.