\(\underset { (Lead\quad nitrate) }{ Pb({ { NO }_{ 3 } })_{ 2 } } +\underset { )Potassium\quad iodide) }{ 2KI } \longrightarrow \quad \underset { (Pottassime\quad nitrate) }{ 2KN{ O }_{ 3 } } +\underset { (Lead\quad iodide) }{ Pb{ I }_{ 2 } } \)

In a physical change, no new substance is formed. In a chemical change, new substance is formed.

Precipitation reaction

Magnesium powder particles provide more surface area for contact with the molecules of sulphuric acid as compared to that of magnesium ribbon.

MnO₂ + 4HCl \(\rightarrow\) MnCl₂ + Cl₂ + 2H₂O

Iron reacts with HCl to release hydrogen gas and iron chloride is formed. This reaction is called as displacement reaction.

By applying paint on iron articles, they can be prevented from corrosion (rusting). Paint does not allow oxygen (from air) and water (moisture) to come in contact with the surface of iron.

(a) 3H₂(g) + N₂(g) \(\rightarrow\) 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) \(\rightarrow\) 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂ (aq)  + Al₂(SO₄)₃ (aq)  \(\rightarrow\) 2AICI₃ (aq) + 3BaSO₄ (s)\(\downarrow\)
(d) 2K (s) + 2H₂O (l) \(\rightarrow\) 2KOH (aq)  + H₂ (g)

(i) Substance X is calcium oxide or quicklime. Its formula is CaO
(ii) Quicklime reacts with water as:
CaO(s) + H₂O(l) \(\rightarrow\)Ca(OH)₂ (aq)
    X          Water       Slaked lime (Calcium hydroxide)

(a) A white precipitate of barium sulphite is formed when barium chloride is added to the solution of sodium sulphite.
\(\underset{Barium\\ Chloide}{BaCl_2} +\underset{Sodium\\ sulphite}{ Na2S03} \rightarrow \underset{Baru\ sulphate\\ (white\ ppt)}{ BaS03\downarrow} +\underset{Sodium\\ chloride}{2NaCI}\)
(b) This precipitation reaction is also a double displacement reaction.
(c) Barium chloride, suphur dioxide and water are formed when dilute hydrochloric acid is added to this solution of barium sulphite. Since barium chloride is a soluble substance, thus white precipitate of barium sulphite disappears.
\(\underset { Barium\ sulphate\\ (white\ ppt) }{ BaCl_{ 2 } } +\underset { Hydrochloric\\ acid }{ 2HCl(aq) } \rightarrow \underset { Barium\\ chloride }{ BaC{ l }_{ 2 }(aq) } +\underset { Sulphur\\ dioxde }{ { SO }_{ 2 } } +\underset { Water }{ { H }_{ 2 }O } \)

Hydrogen gas and zinc sulphate are formed when zinc granules are treated with sulphuric acid
\(\underset{Zinc }{Zn(s)}+\underset{Sulphuric\ acid}{H2SO4(l)} \rightarrow\underset{Zinc\ sulphate}{ZnSO_4(aq)}+\underset{Hydrogen}{H_2(g)}\)
Zinc chloride and hydrogen gas are formed, when zinc granules are treated with hydrochloric acid.
\(\underset{Zinc}{Zn(s)} + \underset{Hydrochloric\ acid}{2HCI(l)}\rightarrow \underset{Zinc}{ZnCI2(s)} +\underset{Water}{ H_2(g)}\)
Similarly, hydrogen gas and zinc nitrate are formed when zinc granules are treated with nitric acid.
\(\underset{Zinc}{Zn(s)} + \underset{Nitricic\ acid}{2HNO_3(l)}\rightarrow \underset{Zinc\ nitrate}{Zn(NO_3)_2(s)} +\underset{Hydrogen}{ H_2}\)
Since, zinc is less reactive than sodium, thus no reaction takes place when zinc granules are treated with sodium chloride.
Sodium zinc ate and hydrogen gas are formed when zinc granules are treated with sodium hydroxide and water.
\(Zn + 2H_2O + 2NaOH\rightarrow Na_2Zn(OH) _4 + H_2\)

(a) Carbon dioxide turns lime water milky as it forms calcium carbonate with lime water. This is the characteristics test for carbon dioxide.
Ca(OH)₂ + CO₂ --> CaCO₃ + H₂O
(b) The smell of sulphur dioxide gas is like burning of sulphur, which is the characteristics test for sulphur dioxide. Sulphur dioxide turns moist litmus paper from blue to red.
(c) When a burnt candle or match stick is brought near the oxygen gas, it starts burning brightly. This is the characteristic test for oxygen gas.
(d) When a burnt candle or match stick is brought near the hydrogen gas, it burns with pop sound, which is the characteristic test for hydrogen gas.

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

(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) 

(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.