HCl is a strong acid and is completely dissociated in its solutions according to the process:
HCl_(aq) → H+_(aq) + Cl^–_(aq)
From this process it is clear that one mole of HCl would give one mole of H⁺ ions. Therefore, the concentration of H⁺ ions would be equal to that of HCl, i.e., 0.001 molar or 1.0 × 10^–3 mol litre^–1
Thus, [H⁺] = 1 × 10^-3 mol litre^–1.
pH = –log₁₀[H⁺] = –log₁₀ 10^–3
= –(–3 × log₁₀) = –(3 × 1) = 3
Thus, pH = 3

(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'.
\(\mathrm{CaCO}_{3(\mathrm{~s})} \stackrel{\text { heat }}{\longrightarrow} \mathrm{CaO}_{(\mathrm{s})}+\mathrm{CO}_{2}(\mathrm{~g})\)

(i) On passing ' C ' through water it becomes acidic.
(ii) Therefore the gas ' C ' must be a non-metal oxide \(\left(\mathrm{CO}_{2}\right)\).
(iii) So a solid compound must be a calcium carbonate.
(iv) It decomposes into calcium oxide and carbon dioxide. (C)
\(\mathrm{CaCO}_{3(\mathrm{~g})} \rightarrow \mathrm{CaO}_{(\mathrm{S})}+\mathrm{CO}_{2(\mathrm{~g})} \uparrow\\ \quad \mathrm{A} \quad \quad \quad \quad \mathrm{B} \quad \quad \quad \quad \mathrm{C}\)

A	CaCo3	Calcium carbonate
B	CaO	Calcium oxide
C	CO2	Carbon di oxide

Liquefied Petroleum Gas is a mixture of hydrocarbon gases like propane, butane, propylene etc..

Decomposition caused by light is called photolysis.

"Rate of a reaction is the change in the amount or concentration of anyone of the reactants or products per unit time".
Consider the following reaction
A ⟶ B
The rate of this reaction is given by
Rate = \(\frac{d[A]}{dt}=+\frac{d[B]}{dt}\)
Where,
[A] - Concentration of A
[B] - Concentration of B

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.

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