Science Chapter 1 – Chemical Reactions & Equations

🔍 How to Identify a Chemical Reaction?

When we observe changes around us — food getting cooked, iron rusting, or grapes fermenting — a chemical reaction has taken place. Atoms of one element do not change into another; only bonds break and new ones form.

🌡️Change in Temperature

🎨Change in Colour

💨Evolution of Gas

🧊Change in State

Remember with: TCGS → Temperature · Colour · Gas · State!

🧠 Quick Practice

Define chemical reaction and give 2 examples from daily life.
List all 4 indicators that a chemical reaction has occurred.
Which of these is a chemical change: melting ice OR burning paper? Why?

⚡ 30 Sec Test ⭐ Basic

Q: Iron left in humid air gets a reddish-brown coating. Which indicator of chemical reaction is this?

✅ Change in Colour (and also change in state — solid iron → solid rust). This is corrosion.

✍️ Chemical Equations

Word Equation

The simplest way to represent a reaction in words:

Example: Burning of Magnesium

Magnesium + Oxygen → Magnesium Oxide

Symbolic (Chemical) Equation

Use chemical formulae for conciseness. A chemical equation represents a chemical reaction.

Skeletal (Unbalanced):

Mg + O₂→MgO

📝 Rules for Writing Equations

Reactants on LHS, Products on RHS, arrow (→) in middle
Plus (+) sign between multiple reactants or products
Physical states: (s) solid · (l) liquid · (g) gas · (aq) aqueous
Conditions (heat, light, pressure, catalyst) written above/below arrow

Balancing Equations — Hit and Trial Method

Based on Law of Conservation of Mass: atoms are neither created nor destroyed. Count must be equal on both sides.

⚖️ Steps to Balance Fe + H₂O → Fe₃O₄ + H₂

Draw boxes around each formula. Never change formula inside.
Count atoms: Fe=1 vs 3 · H=2 vs 2 · O=1 vs 4 (unequal!)
Balance O first (most atoms): put 4H₂O on LHS → 4 oxygens ✓
Balance H: 8H on LHS → put 4H₂ on RHS ✓
Balance Fe: 1 on LHS vs 3 on RHS → put 3Fe on LHS ✓
Verify all atoms, then add physical state symbols.

✅ Balanced:

3Fe(s) + 4H₂O(g)→Fe₃O₄(s) + 4H₂(g)

Photosynthesis (conditions above arrow):

6CO₂ + 12H₂O → Sunlight + Chlorophyll C₆H₁₂O₆ + 6O₂ + 6H₂O

🧠 Quick Practice

Balance: Zn + H₂SO₄ → ZnSO₄ + H₂ (Verify atom count for each element)
Write a balanced equation for: Sodium + Water → Sodium Hydroxide + Hydrogen
What do symbols (s), (l), (g) and (aq) represent? Give one example each.

⚡ 30 Sec Test ⭐ Basic

Q: In a balanced equation, what is the coefficient of H₂ in:
H₂ + Cl₂ → HCl ?

✅ H₂ + Cl₂ → 2HCl — Coefficient of H₂ = 1. But HCl gets coefficient 2 to balance H and Cl atoms.

⚗️ Types of Chemical Reactions

🔗

1. Combination

Two or more substances combine to form a single product. Often exothermic.

A + B → AB
CaO + H₂O → Ca(OH)₂ + Heat

💥

2. Decomposition

A single substance breaks down into two or more simpler substances.

AB → A + B
2FeSO₄ →^Heat Fe₂O₃ + SO₂ + SO₃

🔄

3. Displacement

A more reactive element displaces a less reactive element from its compound.

Fe + CuSO₄ → FeSO₄ + Cu
(Fe displaces Cu)

⇄

4. Double Displacement

Ions exchange between two reactants — often forms a precipitate (↓).

Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

🔥

5. Exothermic

Reactions that release heat/energy along with products.

Burning, Respiration, CaO + H₂O

❄️

6. Endothermic

Reactions that absorb energy from surroundings.

2AgCl →^Light 2Ag + Cl₂
Electrolysis of water

🔁 Redox Reactions — OIL RIG Rule

OIL = Oxidation Is Loss (of electrons / hydrogen, or gain of oxygen)
RIG = Reduction Is Gain (of electrons / hydrogen, or loss of oxygen)
➜ Both happen simultaneously in the same reaction!

CuO + H₂→^HeatCu + H₂O

CuO → Cu: Reduced (lost O) | H₂ → H₂O: Oxidised (gained O)

🧠 Quick Practice

Classify: CaCO₃ → CaO + CO₂ — which type of reaction?
Why is decomposition the opposite of combination? Give one example of each.
In Fe₂O₃ + 2Al → Al₂O₃ + 2Fe — identify which is oxidised and which is reduced.
What is a precipitate? Name the precipitate in: BaCl₂ + Na₂SO₄ → ?

⚡ 30 Sec Test ⭐⭐ Moderate

Q: Which gas is evolved when Zn reacts with dilute HCl? What type of reaction is it?

✅ Hydrogen (H₂) is evolved. Reaction: Zn + 2HCl → ZnCl₂ + H₂↑
Type: Displacement reaction (Zn displaces H from HCl)

🎨 Key Diagrams

⚡ Electrolysis of Water — H₂ at cathode is DOUBLE the volume of O₂ at anode (ratio 2:1)

🔄 Iron (more reactive) displaces Copper — blue colour fades, nail turns brownish

📐 All Important Equations ⭐ Must Know

#	Reaction	Balanced Equation	Type
1	Burning of Mg	2Mg + O₂ → 2MgO	Combination + Exothermic
2	Quick Lime + Water	CaO + H₂O → Ca(OH)₂ + Heat	Combination + Exothermic
3	Decomp. of FeSO₄	2FeSO₄ → Fe₂O₃ + SO₂ + SO₃	Thermal Decomposition
4	Decomp. of Pb(NO₃)₂	2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂	Thermal Decomposition
5	Electrolysis of Water	2H₂O → 2H₂ + O₂	Electrolytic Decomposition
6	AgCl in Sunlight	2AgCl → 2Ag + Cl₂	Photolytic Decomposition
7	Fe + CuSO₄	Fe + CuSO₄ → FeSO₄ + Cu	Displacement
8	Zn + CuSO₄	Zn + CuSO₄ → ZnSO₄ + Cu	Displacement
9	BaCl₂ + Na₂SO₄	BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl	Double Displacement
10	CuO + H₂ (Redox)	CuO + H₂ → Cu + H₂O	Redox
11	Respiration	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy	Exothermic
12	Zn + H₂SO₄	Zn + H₂SO₄ → ZnSO₄ + H₂↑	Displacement
13	Whitewash reaction	Ca(OH)₂ + CO₂ → CaCO₃ + H₂O	Combination
14	Burning natural gas	CH₄ + 2O₂ → CO₂ + 2H₂O	Exothermic

🧠 Quick Practice

Write 3 equations from the table above from memory — close the page and try!
Pick any 2 reactions and write their type with reason.
Which reactions release brown fumes of NO₂? Write the equation.

⚡ 30 Sec Test ⭐⭐ Moderate

Q: What is the balanced equation for reaction of Calcium Hydroxide with Carbon Dioxide? What is it used for?

✅ Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Used in whitewashing walls — thin CaCO₃ layer gives shiny finish after 2–3 days.

🌍 Real Life Applications

Chemistry is everywhere around you! Connect your chapter to real life:

🍳

Cooking Food

Food changes chemically — new substances form. Can't be reversed.

Chemical Change

🔩

Rusting of Iron

Fe + O₂ + H₂O → Fe₂O₃·xH₂O. Slow oxidation by moisture & oxygen.

Corrosion

🍟

Chips go Stale

Oils oxidise → bad smell & taste. Solved by flushing N₂ in packets.

Rancidity

🫁

Respiration

Glucose + O₂ → CO₂ + H₂O + Energy. Keeps us alive every second!

Exothermic

📸

Photography

AgCl/AgBr decompose in light → forms Ag deposits on film. B&W photos!

Photolytic Decomp.

🏗️

Cement Making

CaCO₃ → CaO + CO₂ at high temp. CaO (quick lime) is used in cement.

Thermal Decomp.

🍇

Fermentation

Yeast converts glucose → ethanol + CO₂. Used in bread, wine making.

Chemical Change

🌱

Photosynthesis

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. Plants use sunlight to make food!

Endothermic

🧱

Whitewashing

Ca(OH)₂ + CO₂ → CaCO₃ — shiny finish on walls after 2–3 days.

Combination

🚫 Mistakes Students Make Avoid These!

❌

Mistake 1: Changing the formula while balancing

Writing H₂O₄ or (H₂O)₄ to balance oxygen

✅ Always use ONLY coefficients: 4H₂O — never change the formula inside!

❌

Mistake 2: Confusing Oxidation and Reduction

Saying "CuO is oxidised because it reacts with H₂"

✅ CuO loses oxygen → it is REDUCED. H₂ gains oxygen → it is OXIDISED.

❌

Mistake 3: Forgetting ↓ (precipitate) symbol

Writing BaSO₄ without ↓ in double displacement reactions

✅ Always write BaSO₄↓ to show it is an insoluble precipitate. Marks deducted!

❌

Mistake 4: Mixing up Exothermic and Endothermic

Saying "decomposition is always exothermic"

✅ Decomposition reactions ABSORB energy — they are ENDOTHERMIC (heat, light, or electricity needed).

❌

Mistake 5: Not writing physical state symbols

Writing 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (incomplete)

✅ Full answer: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) — states matter in board exams!

❌

Mistake 6: Calling Fe₂O₃ + 2Al → Al₂O₃ + 2Fe a double displacement

Students say "two compounds react = double displacement"

✅ No ion exchange here — it is a Displacement reaction. Al (more reactive) displaces Fe.

📘 NCERT Exercise — Full Solutions

⭐ Basic Q1. Which statements about 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) are incorrect?

(a) Lead is getting reduced. (b) CO₂ is getting oxidised. (c) C is getting oxidised. (d) PbO is getting reduced. → Options: (i)(a&b) (ii)(a&c) (iii)(a,b,c) (iv) all

✅ Answer: (i) (a) and (b)
• PbO loses oxygen → PbO is REDUCED (not lead). So (a) is wrong.
• CO₂ is the product — it cannot be oxidised further. So (b) is wrong.
• C gains oxygen → becomes CO₂ → C is OXIDISED. So (c) is CORRECT.
• PbO loses oxygen → PbO is REDUCED. So (d) is CORRECT.

⭐ Basic Q2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. This reaction is an example of:

(a) Combination (b) Double displacement (c) Decomposition (d) Displacement

✅ Answer: (d) Displacement reaction
Aluminium (more reactive) displaces Iron from Fe₂O₃. This is also a Redox reaction (thermite reaction — used in welding railway tracks).

⭐ Basic Q3. What happens when dilute HCl is added to iron fillings?

(a) H₂ gas + iron chloride (b) Cl₂ gas + iron hydroxide (c) No reaction (d) Iron salt + water

✅ Answer: (a) Hydrogen gas and iron chloride are produced
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑
Iron displaces hydrogen from HCl — displacement reaction.

⭐⭐ Moderate Q4. What is a balanced chemical equation? Why should chemical equations be balanced?

✅ A balanced chemical equation has equal number of atoms of each element on both the LHS (reactants) and RHS (products).

Why balance? Based on the Law of Conservation of Mass — matter can neither be created nor destroyed. So the total mass of reactants must equal total mass of products. Balancing ensures this holds true.

⭐⭐ Moderate Q5. Translate and balance: (a) H₂ + N₂ → NH₃ (b) H₂S burns in air (c) BaCl₂ + Al₂(SO₄)₃ → (d) K + H₂O →

(a) 3H₂ + N₂ → 2NH₃
(b) 2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄↓ + 2AlCl₃
(d) 2K + 2H₂O → 2KOH + H₂↑

⭐⭐ Moderate Q6. Balance the following chemical equations:

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl↓ + NaNO₃ (already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl

⭐⭐ Moderate Q7. Write balanced equations for:

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄↓ + 2KCl

⭐⭐ Moderate Q8. Write balanced equations and identify reaction type:

(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(aq) — Double Displacement
(b) ZnCO₃(s) → ZnO(s) + CO₂(g) — Thermal Decomposition
(c) H₂(g) + Cl₂(g) → 2HCl(g) — Combination
(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — Displacement

⭐⭐ Moderate Q9. What are exothermic and endothermic reactions? Give examples.

Exothermic: Reactions that release heat/energy along with products.
Examples: Burning of CH₄, Respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy), CaO + H₂O → Ca(OH)₂ + Heat

Endothermic: Reactions that absorb energy from surroundings.
Examples: Photosynthesis, 2AgCl →(Light) 2Ag + Cl₂, Electrolysis of water

⭐⭐ Moderate Q10. Why is respiration an exothermic reaction?

During respiration, glucose (food) is broken down in cells using oxygen, releasing energy which keeps our body warm and functional.
C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) + Energy
Since energy (heat) is released — it is an exothermic reaction.

⭐⭐⭐ Board Level Q11. Why are decomposition reactions called opposite of combination?

Combination: Two or more substances → ONE product: A + B → AB
Example: CaO + H₂O → Ca(OH)₂

Decomposition: ONE substance → two or more products: AB → A + B
Example: CaCO₃ → CaO + CO₂

They are exactly opposite in terms of direction of reaction.

⭐⭐⭐ Board Level Q12. Decomposition with heat, light, electricity:

Heat: CaCO₃(s) →(Heat) CaO(s) + CO₂(g)
Light: 2AgCl(s) →(Sunlight) 2Ag(s) + Cl₂(g)
Electricity: 2H₂O(l) →(Electricity) 2H₂(g) + O₂(g)

⭐⭐⭐ Board Level Q13. Difference between Displacement and Double Displacement:

Displacement: One element replaces another from its compound.
Zn + CuSO₄ → ZnSO₄ + Cu

Double Displacement: Exchange of ions between two compounds — often forms precipitate.
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

⭐⭐⭐ Board Level Q14. Recovery of silver from AgNO₃ by copper displacement:

✅ Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Copper (more reactive) displaces Silver from silver nitrate solution.

⭐⭐⭐ Board Level Q15. What is a precipitation reaction? Give examples.

A reaction in which an insoluble solid (precipitate) is formed when two solutions are mixed.

Example 1: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄↓(s) + 2NaCl(aq) — white precipitate
Example 2: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂↓(s) + 2KNO₃(aq) — yellow precipitate

⭐⭐⭐ Board Level Q16. Explain oxidation and reduction with 2 examples each:

Oxidation (gain of O / loss of H):
(i) 2Cu + O₂ → 2CuO (Cu gains oxygen)
(ii) H₂S + Cl₂ → 2HCl + S (H₂S loses hydrogen)

Reduction (loss of O / gain of H):
(i) CuO + H₂ → Cu + H₂O (CuO loses oxygen)
(ii) 2FeCl₃ + H₂ → 2FeCl₂ + 2HCl (gains hydrogen)

⭐⭐⭐ Board Level Q17. Shiny brown element 'X' turns black on heating. Name X and the black compound.

✅ X = Copper (Cu) — shiny brown metal.
Black compound = Copper(II) Oxide (CuO)
Reaction: 2Cu(s) + O₂(g) →(Heat) 2CuO(s)

⭐⭐⭐ Board Level Q18. Why do we apply paint on iron articles?

✅ Paint acts as a protective barrier — it prevents iron from coming into contact with oxygen and moisture (water vapour). Without these two, rusting (corrosion) cannot occur. The paint layer essentially starves the iron of the reactants it needs to corrode.

⭐⭐⭐ Board Level Q19. Oil and fat food items are flushed with nitrogen. Why?

✅ Nitrogen is chemically inert — it does not react with oils or fats. Flushing displaces oxygen inside the packet. Without oxygen, oxidation of fats cannot occur, so the food stays fresh and does not become rancid. This increases shelf life.

⭐⭐⭐ Board Level Q20. Explain Corrosion and Rancidity with one example each.

Corrosion: Slow destruction of metals by chemical attack of air, moisture, acids etc.
Example: Rusting of iron — 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (rust)

Rancidity: Oxidation of fats/oils in food causing unpleasant smell and taste.
Example: Butter left open turns rancid due to oxidation by air.
Prevention: Add antioxidants, store in airtight containers, use N₂ gas in packets.

🏆 Final Exam Booster

📌 1-Mark Sure-shots ⭐ Basic

What is a skeletal equation?
Name the gas evolved: Zn + H₂SO₄
Define combination reaction
What is a precipitate?
Give one example of exothermic reaction
Name the phenomenon: iron gets reddish-brown layer

📌 2-Mark Questions ⭐⭐ Moderate

Balance: Fe + H₂O → Fe₃O₄ + H₂
What is rancidity? How prevented?
Difference: Exo vs Endothermic
Why clean Mg ribbon before burning?
What is a redox reaction? Example?
Why does iron nail turn brown in CuSO₄?

📌 3-Mark Questions ⭐⭐ Moderate

Explain electrolysis of water with equation
What are 3 types of decomposition with examples?
Distinguish displacement vs double displacement (with eq)
Why is respiration exothermic? Write equation.
Explain corrosion and 2 ways to prevent it.

📌 5-Mark Board Questions ⭐⭐⭐ Board Level

Define 5 types of reactions with examples each
Balance + identify type: all Q8 parts (a,b,c,d)
Write 3 equations where energy supplied = heat/light/electricity
Explain oxidation & reduction with 2 examples each
What is rancidity + corrosion? Real-life examples + prevention

⭐ Last Day Revision Checklist

☐ I know all 4 indicators of chemical reaction
☐ I can balance any equation using Hit-and-Trial method
☐ I know all 6 types of reactions with 1 example each
☐ I understand Oxidation vs Reduction (OIL RIG)
☐ I can write equations for decomp by heat, light, electricity
☐ I know difference between Displacement and Double Displacement
☐ I understand Corrosion and Rancidity with prevention
☐ I have practised all NCERT Exercise questions

🧠 Final Quick Practice

Write 5 equations from memory — timed (under 3 minutes).
Explain to a friend: what is the difference between Oxidation and Reduction?
Write your own definition of a chemical reaction in 2 sentences.

⚡ Final Boss Test ⭐⭐⭐ Board Level

Q: The reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe releases a lot of heat and is used in welding railways. (a) Name the reaction type. (b) Identify what is oxidised and what is reduced.

✅ (a) Displacement reaction (also Redox / Thermite reaction)
(b) Al is OXIDISED (gains O → forms Al₂O₃) · Fe₂O₃ is REDUCED (loses O → forms Fe)

🚀 Master Summary — Everything in One Box

Indicators: Colour change · Temperature change · Gas evolution · State change
Balanced equation → Law of Conservation of Mass
Combination: A+B→AB | Decomposition: AB→A+B (Opposite!)
Displacement: More reactive displaces less reactive metal
Double Displacement: Ion exchange → often precipitate formed
Exothermic: Heat released (combustion, respiration, CaO+H₂O)
Endothermic: Heat absorbed (decomposition, photosynthesis)
Oxidation: Gain O / Lose H | Reduction: Lose O / Gain H
Corrosion: Slow oxidation of metals (rusting, tarnishing)
Rancidity: Oxidation of food fats → bad smell/taste
States: (s) (l) (g) (aq) · ↓ = precipitate · ↑ = gas evolved

📋 Quick Revision Table Board Must

#	Term / Concept	Definition / Key Point	Example
1	Chemical Reaction	Process where reactants form new products with different properties	Burning of Mg
2	Combination Reaction	A + B → AB (two or more → one product)	CaO + H₂O → Ca(OH)₂
3	Decomposition Reaction	AB → A + B (one → two or more); opposite of combination	2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
4	Displacement Reaction	More reactive element displaces less reactive from compound	Fe + CuSO₄ → FeSO₄ + Cu
5	Double Displacement	Ion exchange between two compounds; precipitate often formed	BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl
6	Exothermic	Energy (heat) released along with products	Combustion, Respiration, CaO+H₂O
7	Endothermic	Energy absorbed from surroundings	Photosynthesis, Electrolysis of water
8	Oxidation	Gain of O / Loss of H / Loss of electrons (OIL)	H₂ → H₂O (gains O)
9	Reduction	Loss of O / Gain of H / Gain of electrons (RIG)	CuO → Cu (loses O)
10	Redox Reaction	Oxidation & Reduction occur simultaneously	CuO + H₂ → Cu + H₂O
11	Corrosion	Slow oxidation of metals by air + moisture	Rusting: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O
12	Rancidity	Oxidation of oils/fats → bad smell & taste	Butter left open turns rancid
13	Precipitate (↓)	Insoluble solid formed when two solutions react	BaSO₄↓ (white), PbI₂↓ (yellow)
14	Photolytic Decomp.	Decomposition caused by light energy	2AgCl →(Light) 2Ag + Cl₂
15	Electrolytic Decomp.	Decomposition caused by electricity	2H₂O →(Elec) 2H₂ + O₂
16	Thermal Decomp.	Decomposition caused by heat	CaCO₃ →(Heat) CaO + CO₂
17	Balanced Equation	Equal atoms of each element on LHS and RHS	3Fe + 4H₂O → Fe₃O₄ + 4H₂
18	State Symbols	(s)=solid, (l)=liquid, (g)=gas, (aq)=aqueous	Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂↑(g)

📅 Previous Year Questions (PYQs)

💡 How to use PYQs

These questions have appeared in CBSE Board exams. Practise them first — then reveal the answer. Stars show difficulty level: ⭐ Basic ⭐⭐ Moderate ⭐⭐⭐ Board Level

2020 · 1 Mark ⭐ Basic

Q: What happens when dilute hydrochloric acid is added to iron filings?

✅ Hydrogen gas is evolved and iron(II) chloride is formed.
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑
Type: Displacement reaction.

2019 · 1 Mark ⭐ Basic

Q: Name the type of reaction: CaO + H₂O → Ca(OH)₂ + Heat

✅ Combination Reaction (also Exothermic). Two substances combine to form one product and heat is released.

2020 · 2 Marks ⭐⭐ Moderate

Q: What is rancidity? Write two ways to prevent it.

✅ Rancidity: Oxidation of fats/oils in food leading to unpleasant smell and taste.
Prevention:
1. Flush with nitrogen (inert gas) — displaces O₂ from packets (e.g., chips).
2. Add antioxidants (e.g., BHA, BHT) to delay oxidation.
3. Store in airtight, refrigerated containers.

2018 · 2 Marks ⭐⭐ Moderate

Q: Balance: Fe + H₂O → Fe₃O₄ + H₂. Add state symbols.

✅ 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
Steps: Balance O (4H₂O), then H (4H₂), then Fe (3Fe). Verify: Fe=3, H=8, O=4 on both sides ✓

2022 · 3 Marks ⭐⭐⭐ Board Level

Q: What is a displacement reaction? With the activity of iron nail in CuSO₄ solution, explain the activity series.

✅ Displacement: A more reactive element replaces a less reactive element from its compound.
Activity: Fe nail dipped in blue CuSO₄ solution → nail turns brownish (Cu deposits), solution turns pale green (FeSO₄).
Reaction: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Reason: Fe is more reactive than Cu in the activity series, so Fe displaces Cu.

2019 · 2 Marks ⭐⭐ Moderate

Q: Why do we apply paint on iron articles? Name the phenomenon being prevented.

✅ Paint forms a protective barrier that prevents iron from coming in contact with oxygen and moisture. Without these two reactants, the reaction of rusting cannot occur.
Phenomenon prevented: Corrosion (rusting)
Reaction prevented: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

2023 · 3 Marks ⭐⭐⭐ Board Level

Q: In the reaction: 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) — (a) name the substance oxidised (b) name the substance reduced (c) identify the type of reaction.

✅ (a) Carbon (C) is oxidised — it gains oxygen to form CO₂.
(b) Lead oxide (PbO) is reduced — it loses oxygen to form Pb.
(c) Redox Reaction (oxidation and reduction occur simultaneously).

2021 · 5 Marks ⭐⭐⭐ Board Level

Q: (a) Define exothermic and endothermic reactions with one example each. (b) Is dissolving of NH₄Cl in water exothermic or endothermic? Why?

✅ (a) Exothermic: Reactions that release heat/energy.
Example: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (Respiration)

Endothermic: Reactions that absorb energy from surroundings.
Example: 2AgCl →(Light) 2Ag + Cl₂ (Photolytic decomposition)

(b) Dissolving NH₄Cl is endothermic — the solution feels cold. Energy is absorbed from surroundings to break the ionic lattice of NH₄Cl.

🎯 Exam Strategy

1-Mark: Definition / Name 2-Mark: Explain + Example 3-Mark: Concept + Equations 5-Mark: Full Answer + Diagrams

📝 How to Answer 1-Mark Questions

Give a direct one-line answer — do not write paragraphs.
For "Name the type" → just write: Displacement Reaction
For "Define" → one sentence with the key term bold.
Always include the chemical symbol if a reaction is given. E.g., "H₂↑" not just "hydrogen".

✍️ How to Answer 2–3 Mark Questions

State the definition → give the balanced equation → explain in 1–2 lines.
Always add state symbols: (s), (l), (g), (aq). Missing symbols = marks deducted.
Write ↓ for precipitate and ↑ for gas evolved — these are expected.
For redox: always identify BOTH the oxidised AND reduced substance.
For decomposition types: mention the energy source (heat / light / electricity).

🏆 5-Mark Board Question Blueprint

Part (a) [2 marks]: Define both terms with clear contrast.
Part (b) [2 marks]: Write balanced equations with state symbols for each.
Part (c) [1 mark]: Real-life application or identify the type of reaction.
For electrolysis: draw the diagram showing cathode (−) / anode (+), H₂ and O₂ collection, 2:1 ratio.
For corrosion/rancidity: write at least 2 prevention methods each.

⚠️ Marks-Saving Tips

Never change a chemical formula while balancing — change coefficients ONLY.
In OIL RIG questions — always say what is oxidised AND what is reduced in the same answer.
Corrosion ≠ Rancidity: Corrosion = metals · Rancidity = food fats. Don't mix them up.
Photosynthesis is endothermic; Respiration is exothermic. This is a common swap mistake.
In displacement: check the activity series mentally before writing the answer.
Write reactions with heat/light conditions ABOVE the arrow: →^Heat

⏱️ Time Management — 3 Hours Exam

Section A (1-mark MCQs): 30 sec each max — do not overthink
Section B (2-mark): 3–4 min each — definition + equation
Section C (3-mark): 5–6 min each — concept + equation + reason
Section D (5-mark): 8–10 min — full structured answer
Keep 15 minutes for revision at the end

🚫 All Common Mistakes — Don't Lose Marks!

🔴 These mistakes cost students 5–10 marks in boards. Read carefully!

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Mistake 7: Writing photosynthesis as exothermic

Photosynthesis = exothermic because plants release oxygen

✅ Photosynthesis ABSORBS sunlight energy → it is ENDOTHERMIC. Respiration RELEASES energy → EXOTHERMIC.

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Mistake 8: Confusing Corrosion and Rancidity

Butter turning rancid is called corrosion

✅ Corrosion = oxidation of METALS (rusting, tarnishing). Rancidity = oxidation of FOOD FATS/OILS. Two completely different phenomena.

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Mistake 9: Wrong ratio for electrolysis

H₂ : O₂ = 1 : 2 collected in electrolysis

✅ H₂ : O₂ = 2 : 1 (by volume). H₂ at cathode (−) is DOUBLE. From equation: 2H₂O → 2H₂ + O₂.

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Mistake 10: Saying "Lead" is oxidised in 2PbO + C → 2Pb + CO₂

Pb is oxidised because it appears as a product

✅ PbO LOSES oxygen → PbO (lead oxide) is REDUCED. C GAINS oxygen → C is OXIDISED. Focus on oxygen gain/loss, not product appearance.

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Mistake 11: Not writing the condition above the arrow

CaCO₃ → CaO + CO₂ (missing heat)

✅ CaCO₃ →^Heat CaO + CO₂ — Always mention energy source above the arrow for decomposition reactions.

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Mistake 12: Treating double displacement as redox

BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl is a redox reaction

✅ No change in oxidation state in double displacement. Ions simply exchange partners. Redox requires actual gain/loss of oxygen, hydrogen, or electrons.

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Mistake 13: Writing (s) for BaBr₂ in displacement

2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)

✅ BaBr₂ is soluble in water → write BaBr₂(aq), not (s). Only insoluble products get (s) and ↓ symbol.