(i) Chemical Equilibrium: It is state of a reversible chemical reaction in which no change in the amount of reactants and products takes place.
(ii) At equilibrium, Rate of forward reaction = Rate of backward reaction
Characteristics of equilibrium:
(i) In chemical equilibrium, the rates of forward and backward reactions are equal.
(ii) The observable properties such as pressure, concentration, color, density, viscosity etc., of the system remain unchanged with time.
(iii) The chemical equilibrium is a dynamic equilibrium, because both the forward and backward reactions continue to occur even though it appears static externally.
(iv) In physical equilibrium, the volume of all the phases remain constant.

(i) In a decomposition reaction, a single compound splits into two or more simpler substances by the help of heat. It is called 'Thermolysis'.
(ii) In this type of reaction, the reactant is decomposed by applying heat.
(iii) For example, It is a class of compound to element/element decomposition. i.e. a compound (HgO) is decomposed into two elements (Hg and Oxygen).

(iv) When calcium carbonate is heated, it breaks down in to calcium oxide and carbon dioxide.
(v) It is a type of compound to compound/compound decomposition.
(vi) In thermal decomposition reaction, heat is supplied to break the bonds.
(vii) Such reactions, in which heat is absorbed, are called 'Endothermic reactions'.
\(\mathrm{CaCO}_{3(\mathrm{~s})} \stackrel{\text { heat }}{\longrightarrow} \mathrm{CaO}_{(\mathrm{s})}+\mathrm{CO}_{2}(\mathrm{~g})\)

Concentration of hydrogen ion H⁺ = 10^-pH M
Concentration of hydrogen ion in lemon juice = 10^-2 M
= 0.01 M
Concentration of lemon juice is 0.01 M

S. No Reversible Reaction
Irreversible reaction
(i) Reversible reaction can be reversed under suitable conditions. Reversible reaction cannot be reversed. (ii) Both forward and backward reactions take place simultaneously. It is unidirectional It proceeds only in a forward direction. (iii) It attains equilibrium. Equilibrium is not attained. (iv) The reactants cannot be converted completely into products. The reactants can be completely converted into products. (v) It is relatively slow. It is fast.

(i) The temperature of a reaction increases, the adding heat provides energy to break more bonds of the reactant molecules.
(ii) So more reactants ions increases, produce more products.
(iii) Thus speed of the reaction increases.

KCI_(aq) + AgNO_3(aq) ⟶ AgCl_(s) ↓+ KNO_3(aq)
white precipitate of AgCl is formed in aqueous solution of potassium nitrate.