Solved Examples

About This Section

This is a collection of 30+ solved examples covering every topic in Chapter 1. Here you will find —

• From simple equation balancing to tough organic combustion
• All kinds of reactions — combination, decomposition, displacement, double displacement
• Numerical problems (mass, mole, gas volume)
• Identification of redox reactions
• Applications of corrosion and rancidity
• All major NCERT textbook questions

Tips for Board Exam Preparation:

1. Try to solve each example yourself first.
2. Compare your answer with the solution.
3. Memorise the key reactions and definitions.
4. Also do the exercises at the end of the NCERT textbook.

Let's begin!

Example 1: Writing a Balanced Equation (NCERT)

Convert the following statement into a chemical equation and balance it — Nitrogen combines with hydrogen gas to form ammonia.

Solution:

1. Word equation: Nitrogen + Hydrogen → Ammonia
2. With formulae: $N_2 + H_2 \rightarrow NH_3$
3. Atom count: LHS — N=2, H=2 ; RHS — N=1, H=3 → unbalanced
4. Balance N: put coefficient 2 on $NH_3$: $$N_2 + H_2 \rightarrow 2NH_3$$
5. Balance H: put coefficient 3 on $H_2$: $$N_2 + 3H_2 \rightarrow 2NH_3$$
6. Verify: N=2=2 ✓, H=6=6 ✓
7. Final balanced equation: $$N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$$

Example 2: Combustion of Hydrogen Sulphide (NCERT)

When hydrogen sulphide gas burns in air, water and sulphur dioxide are formed. Write the balanced equation.

Solution:

1. Skeletal: $H_2S + O_2 \rightarrow H_2O + SO_2$
2. Atom count: LHS — H=2, S=1, O=2 ; RHS — H=2, O=3, S=1
3. Odd-vs-even trick (O): Multiply $H_2S$ by 2: $$2H_2S + O_2 \rightarrow H_2O + SO_2$$
4. Balance H and S: $H_2O$ × 2, $SO_2$ × 2: $$2H_2S + O_2 \rightarrow 2H_2O + 2SO_2$$
5. Balance O: $O_2$ × 3: $$2H_2S + 3O_2 \rightarrow 2H_2O + 2SO_2$$
6. Verify: H=4=4 ✓, S=2=2 ✓, O=6=6 ✓
7. Final answer: $$2H_2S(g) + 3O_2(g) \rightarrow 2H_2O(l) + 2SO_2(g)$$

Example 3: Precipitation Reaction (NCERT)

React aluminium sulphate with barium chloride to give aluminium chloride and a precipitate of barium sulphate. Write and balance the equation.

Solution:

1. Skeletal: $BaCl_2 + Al_2(SO_4)_3 \rightarrow BaSO_4 + AlCl_3$
2. Start with the complex compound: $Al_2(SO_4)_3$ — Al=2, S=3 → put coefficient 2 on $AlCl_3$, coefficient 3 on $BaSO_4$: $$BaCl_2 + Al_2(SO_4)_3 \rightarrow 3BaSO_4 + 2AlCl_3$$
3. Balance Ba: $BaCl_2$ × 3: $$3BaCl_2 + Al_2(SO_4)_3 \rightarrow 3BaSO_4 + 2AlCl_3$$
4. Verify: Ba=3=3 ✓, Cl=6=6 ✓, Al=2=2 ✓, S=3=3 ✓, O=12=12 ✓
5. Final answer: $$3BaCl_2(aq) + Al_2(SO_4)_3(aq) \rightarrow 3BaSO_4(s)\downarrow + 2AlCl_3(aq)$$
6. Type: Double displacement (precipitation) reaction.

Example 4: Sodium and Water (NCERT)

Potassium reacts with water to give potassium hydroxide and hydrogen gas. Write the balanced equation.

Solution:

1. Skeletal: $K + H_2O \rightarrow KOH + H_2$
2. Bring H to LCM 4: $H_2O$ × 2, $KOH$ × 2: $$K + 2H_2O \rightarrow 2KOH + H_2$$
3. Balance K: $K$ × 2: $$2K + 2H_2O \rightarrow 2KOH + H_2$$
4. Verify: K=2=2 ✓, H=4=4 ✓, O=2=2 ✓
5. Final answer: $$2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g)\uparrow$$
6. Type: Displacement reaction (K displaces H).

Example 5: NCERT — Balanced Equation (HNO₃ + Ca(OH)₂)

Balance: $HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O$

Solution:

1. Atom count: LHS — H=3, N=1, O=5, Ca=1 ; RHS — Ca=1, N=2, O=7, H=2
2. Balance N: $HNO_3$ × 2: $$2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O$$
3. Balance H: $H_2O$ × 2: $$2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O$$
4. Verify: H=4=4 ✓, N=2=2 ✓, O=8=8 ✓, Ca=1=1 ✓
5. Final answer: $2HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l)$
6. Type: Neutralisation reaction (acid + base → salt + water).

Example 6: Conservation of Mass — Numerical

On heating 8 g of $CaCO_3$, 4.48 g of $CaO$ is obtained. Find the mass of $CO_2$ evolved and verify mass conservation.

Solution:

1. Reaction: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$
2. By conservation of mass: $\text{Reactant} = \text{Products}$
3. Given: $CaCO_3 = 8$ g, $CaO = 4.48$ g
4. Calculation: $CO_2 = 8 - 4.48 = 3.52$ g
5. Verification: LHS = 8 g; RHS = 4.48 + 3.52 = 8 g ✓
6. Answer: Mass of $CO_2$ = 3.52 g, and conservation of mass is verified.

Example 7: Identifying Combination Reactions

Select the combination reactions: (i) $C + O_2 \rightarrow CO_2$ (ii) $2HgO \rightarrow 2Hg + O_2$ (iii) $2H_2 + O_2 \rightarrow 2H_2O$ (iv) $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$

Solution:

• (i): Two reactants → one product → Combination ✓
• (ii): One reactant → two products → Decomposition ✗
• (iii): Two → one → Combination ✓
• (iv): Two → two; an element is displaced → Displacement ✗

Answer: (i) and (iii) are combination reactions.

Example 8: Exothermic Reactions

Give three examples of exothermic reactions.

Solution:

(i) Slaking of lime: $$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$$

(ii) Burning of methane (LPG): $$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{Energy}$$

(iii) Respiration: $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$$

In each reaction, heat is released — hence they are exothermic.

Example 9: Three Types of Decomposition (NCERT)

Give one example each of decomposition driven by heat, light, and electricity.

Solution:

(i) Thermal decomposition: $$2Pb(NO_3)_2(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_2(g) + O_2(g)$$

(ii) Electrolytic decomposition: $$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$

(iii) Photochemical decomposition: $$2AgBr(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Br_2(g)$$

Photochemical decomposition is used in black-and-white photography.

Example 10: Combination vs. Decomposition (NCERT)

Why is decomposition called the reverse of combination? Show with paired examples.

Solution:

• Combination: $A + B \rightarrow AB$ (two combine)
• Decomposition: $AB \rightarrow A + B$ (one breaks)

Pair 1 — Lime:

• Combination: $CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$
• Decomposition: $Ca(OH)_2 \xrightarrow{\Delta} CaO + H_2O$

Pair 2 — Mercury:

• Combination: $2Hg + O_2 \rightarrow 2HgO$
• Decomposition: $2HgO \xrightarrow{\Delta} 2Hg + O_2$

The two are clearly opposite of each other.

Example 11: Gas in Electrolysis (NCERT)

In the electrolysis of water, why is the volume of gas in one tube double that in the other? Name that gas.

Solution:

1. Reaction: $2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$
2. Mole ratio: 2 mol $H_2$ : 1 mol $O_2$
3. By Avogadro's law: Volume ∝ number of molecules
4. So: $H_2 : O_2 = 2 : 1$ by volume
5. Answer: The gas with twice the volume is hydrogen ($H_2$) (formed at the cathode).
6. Test: Bring a burning matchstick close — burns with a 'pop' sound.

Example 12: Displacement Reaction (NCERT)

Why does the colour of $CuSO_4$ solution change when an iron nail is dipped in it?

Solution:

1. Reaction: $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$
2. Colour change: Blue $CuSO_4$ → pale green $FeSO_4$
3. Reason: Fe is more reactive than Cu — Fe displaces Cu.
4. Visible signs:

• Solution colour: blue → pale green.
• Brown copper coating on the nail.

1. Type: Displacement reaction (also redox).

Example 13: Refining of Silver (NCERT)

In the refining of silver, copper displaces silver from $AgNO_3$ solution. Write the equation.

Solution:

1. Reaction: $Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$
2. Analysis: Cu is above Ag — Cu displaces Ag.
3. Visible: Colourless $AgNO_3$ → pale blue $Cu(NO_3)_2$ ; silver deposits on the copper plate.
4. Type: Displacement reaction.
5. Industrial use: Refining of silver.

Example 14: NCERT — Equations and Types (a-d)

Balance and identify the type: (a) $KBr + BaI_2 \rightarrow KI + BaBr_2(s)$ (b) $ZnCO_3 \rightarrow ZnO + CO_2$ (c) $H_2 + Cl_2 \rightarrow HCl$ (d) $Mg + HCl \rightarrow MgCl_2 + H_2$

Solution:

(a): $2KBr(aq) + BaI_2(aq) \rightarrow 2KI(aq) + BaBr_2(s)$ — Double displacement

(b): $ZnCO_3(s) \xrightarrow{\Delta} ZnO(s) + CO_2(g)$ — Decomposition (thermal)

(c): $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$ — Combination

(d): $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)\uparrow$ — Displacement

Example 15: Double Displacement Examples (NCERT)

Give an example of double displacement other than Activity 1.10.

Solution:

Example 1 (AgCl): $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s)\downarrow + NaNO_3(aq)$$ White AgCl precipitate — test for $Cl^-$.

Example 2 (PbI₂): $$Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s)\downarrow + 2KNO_3(aq)$$ Yellow PbI₂ precipitate — 'Golden Rain'.

Example 3 (Cu(OH)₂): $$CuSO_4(aq) + 2NaOH(aq) \rightarrow Cu(OH)_2(s)\downarrow + Na_2SO_4(aq)$$ Blue precipitate.

Example 16: NCERT — Identifying Oxidised/Reduced Substances

Identify the oxidised and reduced substances in: (i) $4Na + O_2 \rightarrow 2Na_2O$ (ii) $CuO + H_2 \rightarrow Cu + H_2O$

Solution:

(i):

• Na gained O → Na is oxidised
• $O_2$ gave O → O₂ is reduced

(ii):

• $H_2$ gained O → H₂ is oxidised
• $CuO$ lost O → CuO is reduced

Both are redox reactions.

Example 17: NCERT — Copper Turning Black (Q.17)

A shiny brown-coloured element 'X' becomes black when heated in air. Name 'X' and the black compound.

Solution:

1. Element 'X': Copper (Cu) — shiny brown.
2. Black compound: Copper oxide $CuO$ — black.
3. Reaction: $$2Cu(s) + O_2(g) \xrightarrow{\Delta} 2CuO(s)$$
4. Type: Combination + oxidation (redox).
5. This is the famous reference of Activity 1.11.

Example 18: NCERT — Why Paint? (Q.18)

Why do we paint iron articles?

Solution:

Iron, when in contact with air and moisture, undergoes rusting ($Fe_2O_3 \cdot xH_2O$) — that is corrosion.

Reaction: $4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$

What does paint do? Paint isolates the iron surface from air and moisture. Without the contact of $O_2$ and $H_2O$ — no rust forms.

Therefore iron objects are painted — to protect from rusting.

Example 19: NCERT — Why Nitrogen? (Q.19)

Why are oil and fatty foods flushed with nitrogen?

Solution:

Oil/fat reacts with atmospheric oxygen to become rancid — taste and smell change.

Why use $N_2$?

• $N_2$ is inert — does not react with food.
• Filling with $N_2$ keeps $O_2$ out.
• The food stays fresh.

Application: Chip packets (puffy bags) contain nitrogen.

Therefore nitrogen is used to prevent rancidity.

Example 20: NCERT — Corrosion and Rancidity (Q.20)

Describe (a) Corrosion and (b) Rancidity. Give one example of each.

Solution:

(a) Corrosion: Slow chemical destruction of metals by air, moisture, and acids. Example: Rusting of iron ($4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$).

(b) Rancidity: Oxidation of fat-containing foods, making them go stale. Example: Stale smell of long-stored ghee/oil/butter.

Both are consequences of oxidation reactions.

Example 21: NCERT — True/False Statement (Q.1 Exercise)

In $2PbO + C \rightarrow 2Pb + CO_2$, which statement is incorrect? (a) Lead is reduced. (b) CO₂ is oxidised. (c) Carbon is oxidised. (d) PbO is reduced.

Solution:

• (a) Lead (PbO to Pb) — yes, reduction. True ✓
• (b) $CO_2$ is being formed (not being oxidised); CO₂ is a product, not undergoing oxidation. False ✗
• (c) C → CO₂ (gains O) — yes, oxidation. True ✓
• (d) PbO → Pb (loses O) — yes, reduction. True ✓

Answer: Only (b) is incorrect.

(NCERT options (i)-(iv) — option (i) (a)+(b) — only (b) is incorrect. Answer: (i))

Example 22: NCERT — Fe₂O₃ + 2Al (Q.2 Exercise)

What type of reaction is $Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe$?

Solution:

1. Analysis: Al is a free element, displacing Fe from $Fe_2O_3$.
2. Activity series: Al > Fe — so Al can displace Fe.
3. This is a displacement reaction (correct option — (d)).
4. Special significance: This is the thermite reaction — used in welding railway tracks.
5. Bonus: It is also a highly exothermic and redox reaction.

Example 23: NCERT — Iron Filings + Dilute HCl (Q.3 Exercise)

What happens when iron filings are added to dilute $HCl$?

Solution:

1. Reaction: $Fe + 2HCl \rightarrow FeCl_2 + H_2$
2. Analysis: Fe is above H — displacement happens.
3. Products: Iron chloride ($FeCl_2$) + hydrogen gas.
4. Test: Bubbles of gas (hydrogen).
5. Answer: (a) Hydrogen gas and iron chloride are formed — correct
6. Option (b) chlorine gas — wrong; (c) no reaction — wrong; (d) iron salt and water — wrong (incomplete).

Example 24: NCERT — What is a Balanced Equation? (Q.4)

What is a balanced chemical equation? Why must a chemical equation be balanced?

Solution:

Definition: A chemical equation in which the number of atoms of each element on both sides of the arrow is equal is called a balanced equation.

Why is balancing necessary?

1. By the law of conservation of mass — mass is neither created nor destroyed in a chemical reaction.
2. So the atoms in the reactants and products must be equal.
3. An unbalanced equation violates the law of conservation of mass — and is therefore scientifically wrong.

Example: $2Mg + O_2 \rightarrow 2MgO$ — Mg=2, O=2 on both sides; balanced.

Example 25: NCERT — Balance the Equations (Q.5)

Balance: (a) Nitrogen + Hydrogen → Ammonia (b) $H_2S$ + Air → Water + $SO_2$ (c) $Al_2(SO_4)_3$ + $BaCl_2$ → $AlCl_3$ + $BaSO_4$ (d) Potassium + Water → KOH + $H_2$

Solution:

(a): $N_2 + 3H_2 \rightarrow 2NH_3$

(b): $2H_2S + 3O_2 \rightarrow 2H_2O + 2SO_2$

(c): $Al_2(SO_4)_3 + 3BaCl_2 \rightarrow 2AlCl_3 + 3BaSO_4\downarrow$

(d): $2K + 2H_2O \rightarrow 2KOH + H_2\uparrow$

Example 26: NCERT — Balance (Q.6)

Balance: (a) $HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O$ (b) $NaOH + H_2SO_4 \rightarrow Na_2SO_4 + H_2O$ (c) $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$ (d) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl$

Solution:

(a): $2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O$

(b): $2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O$

(c): $NaCl + AgNO_3 \rightarrow AgCl\downarrow + NaNO_3$ (already balanced)

(d): $BaCl_2 + H_2SO_4 \rightarrow BaSO_4\downarrow + 2HCl$

Example 27: NCERT — Balance with State Symbols (Q.7)

Write balanced equations for: (a) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$ (b) $Zn + AgNO_3 \rightarrow Zn(NO_3)_2 + Ag$ (c) $Al + CuCl_2 \rightarrow AlCl_3 + Cu$ (d) $BaCl_2 + KSO_4 \rightarrow BaSO_4 + KCl$

Solution:

(a): $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s)\downarrow + H_2O(l)$ (already balanced)

(b): $Zn(s) + 2AgNO_3(aq) \rightarrow Zn(NO_3)_2(aq) + 2Ag(s)$

(c): $2Al(s) + 3CuCl_2(aq) \rightarrow 2AlCl_3(aq) + 3Cu(s)$

(d): $BaCl_2(aq) + K_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2KCl(aq)$

Example 28: NCERT — Balance and Type (Q.8)

Write balanced equations and identify the type of each: (a) $KBr(aq) + BaI_2(aq) \rightarrow KI(aq) + BaBr_2(s)$ (b) $ZnCO_3(s) \rightarrow ZnO(s) + CO_2(g)$ (c) $H_2(g) + Cl_2(g) \rightarrow HCl(g)$ (d) $Mg(s) + HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$

Solution:

(a): $2KBr + BaI_2 \rightarrow 2KI + BaBr_2(s)$ — Double displacement (precipitation)

(b): $ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$ — Decomposition (thermal)

(c): $H_2 + Cl_2 \rightarrow 2HCl$ — Combination

(d): $Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$ — Displacement

Example 29: NCERT — Exothermic/Endothermic (Q.9)

What are exothermic and endothermic reactions? Give examples.

Solution:

Exothermic: Reaction in which heat is released. Temperature rises. Example: $CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$ (the beaker becomes hot).

Endothermic: Reaction in which heat is absorbed. Temperature falls. Example: $Ba(OH)_2 + 2NH_4Cl \rightarrow BaCl_2 + 2NH_3 + 2H_2O$ (the test tube becomes cold).

Table:

Aspect	Exothermic	Endothermic
Heat	Released	Absorbed
Temperature	Rises	Falls

Example 30: NCERT — Why is Respiration Exothermic? (Q.10)

Why is respiration considered an exothermic reaction? Explain.

Solution:

Reaction: $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$$

Why exothermic?

1. Chemical reaction: Glucose + $O_2$ form new substances ($CO_2$, $H_2O$).
2. Energy release: Energy is released as ATP.
3. Bodily evidence: The body feels warm during running/exercise.
4. This energy keeps us alive — body temperature, muscle action, brain function.

Therefore respiration is an exothermic chemical reaction.

Example 31: NCERT — Difference Between Displacement and Double Displacement (Q.13)

What is the difference between displacement and double displacement reactions? Give examples.

Solution:

Aspect	Displacement	Double Displacement
Participants	One element + one compound	Two compounds
What?	Element displaces another	Ion exchange
Driver	Activity series	Solubility / precipitation
Example	$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$	$BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$

Simple rule: If a free element is present — displacement. If all are compounds — double displacement.

Example 32: NCERT — Silver Refining Equation (Q.14)

In refining silver, copper displaces silver from $AgNO_3$. Write the equation.

Solution: $$Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$$

Analysis:

• Cu is above Ag in the activity series.
• Cu displaces Ag.
• Pure silver deposits on the copper plate.
• Solution colour goes from colourless to pale blue.

It is a displacement (and redox) reaction.

Example 33: Numerical — Mole and Mass

How much ammonia is formed from 14 g of nitrogen? (N=14, H=1)

Solution:

1. Reaction: $N_2 + 3H_2 \rightarrow 2NH_3$
2. Molecular masses: $N_2 = 28$, $NH_3 = 17$, $2NH_3 = 34$
3. Ratio: 28 g $N_2$ → 34 g $NH_3$
4. Calculation: 14 g $N_2$ → $\frac{34}{28} \times 14 = 17$ g $NH_3$
5. Answer: 17 g of ammonia is formed.

Example 34: NCERT — Explain Precipitation (Q.15)

What is a precipitation reaction? Give examples.

Solution:

Definition: A reaction in which mixing two solutions produces an insoluble solid (precipitate) is called a precipitation reaction.

Example 1: $$Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$$ White $BaSO_4$ precipitate.

Example 2: $$Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$$ Yellow $PbI_2$ precipitate.

Uses: Ion identification, water purification.

Example 35: NCERT — Define Oxidation/Reduction (Q.16)

Define oxidation and reduction in terms of gain or loss of oxygen.

Solution:

Oxidation: Gain of oxygen or loss of hydrogen. Example 1: $2Cu + O_2 \rightarrow 2CuO$ — Cu is oxidised (gains O). Example 2: $H_2S + Cl_2 \rightarrow 2HCl + S$ — $H_2S$ is oxidised (loses H).

Reduction: Loss of oxygen or gain of hydrogen. Example 1: $CuO + H_2 \rightarrow Cu + H_2O$ — CuO is reduced (loses O). Example 2: $Cl_2 + H_2 \rightarrow 2HCl$ — Cl₂ is reduced (gains H).