(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.

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

(i) No, because Nickel is more reactive than Copper.
(ii) So Nickel easily reacts and displaces copper from copper sulphate solution.
(iii) \(\mathrm{Ni}_{(\mathrm{s})}+\mathrm{CuSO_4}_{(\mathrm{aq})} \rightarrow \mathrm{NiSO_4}_{(\mathrm{aq})}+\mathrm{Cu}_{(\mathrm{S})}\)

(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)
\(\mathrm{CaCO}_{3(\mathrm{~g})} \rightarrow \mathrm{CaO}_{(\mathrm{S})}+\mathrm{CO}_{2(\mathrm{~g})} \uparrow\\ \quad \mathrm{A} \quad \quad \quad \quad \mathrm{B} \quad \quad \quad \quad \mathrm{C}\)

\({\left[\mathrm{OH}^{-}\right] } =1 \times 10^{-11}  M\)
\(\mathrm{pOH} =-\log _{10}\left[\mathrm{OH}^{-}\right]  \)
\(=-\log _{10} 1 \times 10^{-11}  \)
\(=-(-11) \log _{10} 1 \times 10  \)
\(=11 \log _{10} 10=11 \times 1 \)
pOH = 11 
pH = 14 - pOH
pH = 14 - 11 
pH = 3
pH of the hydroxide ion concentration of a solution of 1 x 10^-11 M is 3

If the atmospheric air is polluted with oxide gases of sulphur and nitrogen, they get dissolved in the rain water and make its pH less than 7. Thus, if the pH of rain water is less than 7, then it is called acid rain. When acid rain flows into the rivers it lowers the pH of the river water also. The survival of aquatic life in such rivers becomes difficult.

A balanced chemical equation is the simplified representation of a chemical reaction which describes the chemical composition, physical state of the reactants and the products, and the reaction conditions.

The amount of the substance present in certain volume of the solution is called 'concentration'.

The minimum energy with which the reacting particles must collide is called 'Activation Energy' of the reaction.