(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) Our body works within the pH range of 7.0 to 7.8.
(ii) Different body fluids have different pH values.
(iii) Any increase or decrease in this value leads to diseases.
(iv) The ideal pH for blood is 7.4.
pH in our digestive system:
(i) HCl present in our stomach helps in digestion.
(ii) During indigestion our stomach produces more acid and this causes pain and irritation.
(iii) pH of the stomach fluid is approximately 2.0.
pH changes as the cause of tooth decay:
(i) pH of the saliva normally ranges between 6.5 to 7.5.
(ii) White enamel coating (calcium phosphate) is a hard substance in our body.
(iii) When the pH value falls below 5.5, it weathers.
(iv) The basic toothpaste neutralises the excess acid and prevents tooth decay.
pH of soil:
(i) Citrus fruits require slightly alkaline soil, rice requires acidic soil and sugarcane requires neutral soil.
pH of rain water:
(i) The pH of rain water is approximately 7.
(ii) If the atmospheric air is polluted with oxides of non-metals, they get dissolved in the rain water and make its pH less than 7.
(iii) As its pH value is less than 7, then it is called acid rain.
(iv) When this rain water reaches river water, the survival of aquatic life becomes difficult.

Important factors that affect rate of a reaction are:
(i) Nature of the reactants
(ii) Concentration of the reactants
(iii) Temperature
(iv) Catalyst
(v) Pressure
(vi) Surface area of the reactants
(i) Nature of the reactants:
a) The reaction of sodium with hydrochloric acid is faster than that with acetic acid.
b) Because Hydrochloric acid is a stronger acid than acetic acid and thus more reactive.
c) \(2 \mathrm{Na}_{(\mathrm{s})}+2 \mathrm{CH}_{3} \mathrm{COOH}_{\text {(aq) }} \rightarrow 2 \mathrm{CH}_{3} \mathrm{COONa}_{\text {(aq) }}+\mathrm{H}_{2(\mathrm{~g})} \rightarrow (slow)\)
d) 2 Na_(S)+ 2HCI_(aq) ⟶ 2NaCI_(aq)+ H_2(g) ⟶ fast
(ii) Concentration of the reactants:
a) The amount of the substance present in a certain volume of the solution is called 'concentration'.
b) More the concentration, more particles per volume exist in it and faster the reaction.
c) Granulated zinc reacts faster with 2 M hydrochloric acid than 1 M hydrochloric acid.
(iii) Temperature:
a) Most of the reactions go faster at higher temperature.
b) Because adding heat to the reactants provides energy to break more bonds and thus speed up the reaction.
c) E.g: Calcium carbonate reacts slowly with hydrochloric acid at room temperature and faster when temperature increases.
(iv) Pressure:
a) Increasing the pressure of reactant gases increases the reaction rate.
b) Because, increasing pressure the reacting particles come closer and collide frequently.
(v) Catalyst:
a) A catalyst is a substance which increases the reaction rate without being consumed in the reaction.
b) E.g: On heating potassium chlorate, it decomposes into potassium chloride and oxygen gas, but at a slower rate.
c) If manganese dioxide is added, it increases the reaction rate.
(vi) Surface area of the reactants:
a) When solid reactants are involved in a reaction, their powdered form reacts more readily.
b) Because, powdering of the reactants increases the surface area and more energy is available on collision of the reactant particles.
c) Thus, the reaction rate is increased.
d) E.g: Powdered calcium carbonate reacts more readily with hydrochloric acid than marble chips.

When two compounds react, if their ions are interchanged, then the reaction is called double displacement reaction.
There are major classes of double displacement reactions. They are:
(i) Precipitation Reactions
(ii) Neutralization Reactions
(i) Precipitation Reactions:
a) When aqueous solutions of two compounds are mixed, if they react to form an insoluble compound and a soluble compound, then it is called precipitation reaction.

b) When the clear aqueous solutions of potassium iodide and lead (II) nitrate are mixed, a double displacement reaction takes place between them.
Pb(NO₃)_2(aq)+ 2KI_(aq) ⟶ PbI_2(S) + 2KNO_3(aq)
PbI₂ form a yellow precipitate
(ii) Neutralization Reactions:
(i) It is a type of displacement reaction in which the acid reacts with the base to form a salt and water.
(ii) It is called 'neutralization reaction' as both acid and base neutralize each other.
Acid + Base ⟶ Salt + Water
(iii) Reaction of sodium hydroxide with hydrochloric acid is a typical neutralization reaction.
(iv) Here, sodium displaces hydrogen from hydrochloric acid forming sodium chloride, a neutral soluble salt.
\(\mathrm{NaOH}_{(\mathrm{aq})}+\mathrm{HCl}_{(\mathrm{aq})} \rightarrow \mathrm{NaCl}_{(\mathrm{aq})}+\mathrm{H}_{2} \mathrm{O}_{(\mathrm{l})}\)
(v) When ammonium hydroxide reacts with nitric acid it forms ammonium nitrate and water.
\(\mathrm{HNO_3}_{(\mathrm{aq})}+\mathrm{NH_4OH}_{(\mathrm{aq})} \rightarrow \mathrm{NH_4NO_3}_{(\mathrm{aq})}+\mathrm{H}_{2} \mathrm{O}_{(\mathrm{l})}\)

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