(i) CaCO₃(s) + 2HCl(aq) \(\longrightarrow \) CaCl₂(s)+ H₂O(I) + CO₂(g)
(ii) 2Al(s) + 6HCl(aq)\(\longrightarrow \) 2AlCI₃(aq) + 3H₂(g)
(iii) MnO₂ + 4HCl(aq) \(\longrightarrow \) MnCl₂ + 2H₂0(I) +Cl₂(g)

(i) Change in colour: Reaction between lead nitrate solution and potassium iodide solution.
Pb(NO₃)₂(aq) + 2KI \(\longrightarrow \) PbI₂(s) + 2KNO₃(aq)
In this reaction, colour changes from colourless to yellow.
(ii) Change in temperature: Action of dilute sulphuric acid on zinc.
Zn(s) + H₂SO₄(aq) \(\longrightarrow \) ZnSO₄(aq) + H₂
In this reaction, heat is evolved
(iii) Formation of precipitate : Action of barium chloride on sodium sulphate.
BaCl₂(aq)+ Na₂SO₄(aq) \(\longrightarrow \) BaSO₄(s) + 2NaCl(aq)

(i) Na₂CO₃(s) + 2HCl(aq) \(\longrightarrow \) 2NaCI(aq) + H₂O(I) + CO₂(g)
(ii) CaO(s) + H₂O(I) \(\longrightarrow \)Ca(OH)₂(aq) + Heat
(iii) Pb(NO₃)₂(aq) + NaCl(aq) \(\longrightarrow \) PbCI₂(s) + 2NaNO₃(aq)

(i) Substance X-Calcium Hydroxide.
Ca(OH)₂(aq) + CO₂(g)\(\longrightarrow \) CaCO₃(s) + H₂Cl
                                             (White ppt.)
(ii) Calcium hydroxide is obtained by reaction of calcium oxide and water.
CaO(s) + H₂O(l) \(\longrightarrow \)Ca(OH)₂(aq) + Heat

(i) Exothermic Reaction
CaO + H₂O \(\longrightarrow \)Ca(OH}₂
(ii) Double displacement reaction.
BaCl₂ + Na₂SO₄ \(\longrightarrow \) BaSO₄ + 2NaCl

(a) (i) Ca(OH)₂ + CO₂\(\longrightarrow \) CaCO₃ + H₂O
(ii) Ca(OH)₂(aq) + CO₂(g)\(\longrightarrow \) CaC0₃(s) +H₂O(I)
CaCO₃(s) + H₂O(I) + CO₂(g) \(\longrightarrow \) Ca(HCO₃)₂ (aq)
(b) Aragonite and calcite

(a) (i) The reactants are copper oxide and hydrogen. Copper oxide undergoes reaction and hydrogen (H₂) undergoe oxidation.
(ii) The reactants are copper oxide and zinc. Zinc undergoes oxidation and copper oxide undergoes reduction.
(b) Calcium carbonate is reduced in industries to get CaO and CO₂.

(i) Before heating, it is pale green.
After heating, it is brown or reddish brown.
Two observations are:
(a) Change in state and colour
(b) Evolution of gas
(ii) Decomposition reaction
2FeSO₄(s) \(\overset { Heat }{ \rightarrow } \) Fe₂O₃(s) + S0₂(g) + SO₃(g) 

(i) Copper is more reactive than silver. Hence, when copper wire is dipped in silver nitrate solution, it displaces silver from AgNO₃ solution forming copper nitrate which is bluish green in colour.
\((ii)\quad Cu+2AgN{ O }_{ 3 }\longrightarrow \underset { Copperr\quad (II)\quad nitrate\\ bluish\quad green }{ Cu{ (N{ O }_{ 3 }) }_{ 2 } } +\underset { Silver }{ 2Ag } \)

(a) (i) Physical state of reactants and products.
(ii) Conditions such as temperature, pressure, heat etc.
(iii) Catalyst involved.
(iv) Change in state. 
(b) Total mass of the elements present in the products in a chemical reaction has to be equal to the total mass of elements present in the reactants or Mass can neither be created nor. destroyed in a chemical reaction.

N₂ + H₂ \(\rightarrow \)  NH₃ (Unbalanced equation).
(i) Examine the number of atoms of different elements present in the unbalanced equation.
(ii) In the above reaction, left hand N₂ and H₂ of both the sides are unbalanced.
(iii) To balance hydrogen, H₂ is multiplied by 3 on side. It makes 6H-atoms on the left hand side.
(iv) Now to balance hydrogen atoms on the right hand side, NH₃ should be multiplied by 2. It makes 6H-atoms on this side.
(v) Now to balance nitrogen atoms, they are counted separately for both the sides and we will find that, nitrogen atoms are 2 on both the sides.
(vi) Balanced chemical equation will be :
N₂ + H₂ \(\rightarrow \)  NH₃.

Reaction in which a single reactant breaks down to give simpler products.
Thermal decomposition-
\(CaCO_{ 2 }\overset { \triangle }{ \longrightarrow } CaO+C{ O }_{ 2 }\)
(Or any other)
Electrolytic decomposition-
2H₂O\(\overset { Electric\quad current }{ \rightarrow } \) 2H₂+O₂
(or)
In a decomposition reaction, a single reactant breaks down into two or more simpler products.
When a decomposition reaction is carried out by heating, it is called thermal decomposition reaction.
\(\underset { (Calucium\quad carbonate) }{ CaCO_{ 3 }(s) } \overset { Heat }{ \rightarrow } \underset { Calcium\quad oxide) }{ CaO(s) } +\underset { (Carbon\quad dioxide) }{ { CO }_{ 2 }(g) } \)
When a decomposition reaction is carried out with the help of electric current, the process is called electrolysis.
\(\underset { Water }{ 2{ H }_{ 2 }O(I) } \overset { Electric\quad current }{ \longrightarrow } \underset { (Hydrogen\quad gas) }{ 2{ H }_{ 2 }(g) } +\underset { (Oxygen\quad gas) }{ { O }_{ 2 }(g) } \)