When the rate of the forward reaction becomes equal to the rate of backward reaction, then no more product is formed. This stage of the reaction is called 'equilibrium state'.

The reaction that cannot be reversed is called irreversible reaction.

(i) Greater the surface area, greater is the rate of the reaction
(ii) Powdered calcium carbonate offers large surface area for the reaction to occur at a faster rate.

Universal Indicator/pH paper: It is a mixture of a number of indicators. It shows a specific colour at a given pH.

The pH is the negative logarithm of the hydrogen ion concentration
i.e., pH = -log₁₀ [H⁺]

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) A combination reaction is a reaction in which two or more reactants combine to form a compound.
(ii) It is also called as synthesis reaction (or) composition reaction (a single product).
(iii) S_(s) + O_2(g) → SO_2(g)
(iv) Sulphur reacts with oxygen it produce sulphur dioxide.
(v) Most of combination reactions are exothermic in nature, because they involve the formation of new bonds, which releases a huge amount of energy in the form of heat.

(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.

i) Since the metal X is used for galvanisation, it is most probably Zn.
ii) Since the metal Y is used in electric cable, it is likely to be Cu.
iii) The salt is CuSO_4.
iv) It is an example of metal displacement reaction. Zn lies above Cu in the activity series and has therefore, displaced Cu from the blue CuSO solution.