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Andhra Pradesh (BIEAP)Class 10 Physical Science★ AP Board (BSEAP) SSC Physical Science · ~6 marks · Board: AP Board

Chemical Reactions and Equations

CHEMICAL REACTIONS — 5 types (combination, decomposition, displacement, double displacement, redox). BALANCING EQUATIONS step by step. The REACTIVITY SERIES. OXIDATION and REDUCTION. CORROSION (rusting) and RANCIDITY — real-world chemical changes. AP Board Class 10 Physical Science.

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By the end of this chapter you'll be able to…

1Balance chemical equations by adjusting coefficients (never changing formulae)
2Identify and write examples of combination, decomposition, displacement, and double displacement reactions
3Identify oxidising and reducing agents in redox reactions
4Describe signs of a chemical reaction: heat change, gas evolution, precipitate formation, colour change

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Why this chapter matters

Chemical Reactions and Equations is the FOUNDATION of Class 10 Chemistry. Balancing chemical equations is tested in every AP SSC Science paper. The five types of reactions (combination, decomposition, displacement, double displacement, redox) are tested as classification and examples questions. Understanding how to balance equations and identify reaction types is essential before studying any other chemistry chapter.

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A 5-minute refresher here will save you 30 minutes of confusion below.

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Class 9 Chemistry: atoms, molecules, chemical formulae, valency

Chemical Reactions and Equations — Class 10 Physical Science

"A chemical reaction is a REARRANGEMENT of atoms. Nothing is created. Nothing is destroyed. Everything is TRANSFORMED."

1. Signs of a Chemical Reaction

Change in COLOUR. Change in TEMPERATURE (exothermic or endothermic). Evolution of GAS (bubbles, fizzing). Formation of PRECIPITATE (solid in liquid). Change in STATE. Emission or absorption of LIGHT.

2. Balancing Chemical Equations

Law of Conservation of Mass: Mass is neither created nor destroyed. Atoms on LEFT = Atoms on RIGHT.

Steps: 1. Write SKELETON equation. 2. Count atoms of each element on BOTH sides. 3. Use COEFFICIENTS (numbers in front) — NEVER change SUBSCRIPTS. 4. Balance metals FIRST. Then non-metals. Then H and O LAST. 5. Verify — count again.

Example — Fe + H₂O → Fe₃O₄ + H₂: Fe: 1 vs 3. H: 2 vs 2. O: 1 vs 4. Multiply Fe ×3 → 3Fe + H₂O → Fe₃O₄ + H₂. Fe: 3 vs 3 (correct). H₂O: need 4 O on left → 3Fe + 4H₂O → Fe₃O₄ + 4H₂. H: 8 vs 8 (correct). O: 4 vs 4 (correct). Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

3. Types of Chemical Reactions

Type	Pattern	Example
Combination	A + B → AB	CaO + H₂O → Ca(OH)₂ (slaking of lime — used in whitewash). 2H₂ + O₂ → 2H₂O
Decomposition	AB → A + B	CaCO₃ →(heat)→ CaO + CO₂ (limestone to quicklime). 2AgCl →(sunlight)→ 2Ag + Cl₂ (photography)
Displacement	A + BC → AC + B	Zn + CuSO₄ → ZnSO₄ + Cu (Cu deposits as reddish-brown). Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement	AB + CD → AD + CB	Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. Precipitation reaction.
Redox	Oxidation + Reduction	Rusting. Burning. Respiration.

4. The Reactivity Series

K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. 'A MORE reactive metal DISPLACES a LESS reactive metal from its salt solution. Fe + CuSO₄ → FeSO₄ + Cu — iron is above copper, so it displaces.'

5. Oxidation and Reduction

Oxidation: GAIN of oxygen. LOSS of hydrogen. LOSS of electrons. Reduction: LOSS of oxygen. GAIN of hydrogen. GAIN of electrons. 'They ALWAYS occur TOGETHER. One substance is oxidised, another is reduced. Hence: REDOX.'

6. Corrosion (Rusting)

Iron + Oxygen + Water → Hydrated Iron (III) Oxide (RUST). 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O. Prevention: Paint. Oil/grease. Galvanisation (zinc coating — sacrificial protection — zinc oxidises BEFORE iron). Alloying (stainless steel — Fe + Cr + Ni — Cr forms protective oxide layer). 'Rusting is a SLOW but COSTLY redox reaction. India loses BILLIONS of rupees annually to corrosion.'

7. Rancidity

Fats and oils OXIDISE when exposed to AIR → unpleasant SMELL and TASTE. Prevention: Add ANTIOXIDANTS (natural: Vitamin C, Vitamin E. synthetic: BHA, BHT). Refrigeration (slows oxidation). Airtight containers (limits oxygen). Nitrogen flushing (packaged chips — nitrogen is INERT, displaces oxygen).

8. Common Mistakes

Changing subscripts to balance — NEVER. Only add COEFFICIENTS. H₂O is water. Changing it to H₂O₂ (hydrogen peroxide) makes a COMPLETELY DIFFERENT substance.
Forgetting diatomic gases: H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂ ALWAYS exist as diatomic molecules when free — never as single atoms.
'All decomposition requires heat' — Photolytic decomposition uses LIGHT (AgCl → Ag + Cl₂). Electrolytic decomposition uses ELECTRICITY (H₂O → H₂ + O₂).

9. AP SSC Exam Focus

Topic	Marks
Balancing equations	4-5
Types of reactions (with examples)	4-5
Reactivity series	2-3
Corrosion and rancidity	3-4

10. Types of Reactions — Detailed Explanations with Examples

Combination Reactions — In Depth

Two or more reactants combine to form ONE product.

Burning of coal: C + O₂ → CO₂ (heat and light).
Formation of water: 2H₂ + O₂ → 2H₂O.
Formation of ammonia: N₂ + 3H₂ → 2NH₃ (Haber's process — high pressure, Fe catalyst).
Quicklime + Water: CaO + H₂O → Ca(OH)₂ (slaked lime). 'This is EXOTHERMIC — the water actually BOILS. That's why the reaction is called "slaking" — it hisses and steams.'

Decomposition Reactions — In Depth

ONE reactant breaks down into TWO OR MORE products.

Thermal Decomposition (heat): CaCO₃ →(1000°C)→ CaO + CO₂↑. 2Pb(NO₃)₂ →(heat)→ 2PbO + 4NO₂↑ + O₂↑ (brown fumes of NO₂).
Photolytic Decomposition (light): 2AgBr →(sunlight)→ 2Ag + Br₂. 'This is the basis of BLACK AND WHITE PHOTOGRAPHY. The silver bromide on photographic film decomposes — silver is deposited as a DARK image.'
Electrolytic Decomposition (electricity): 2H₂O →(electric current)→ 2H₂↑ + O₂↑. 2NaCl →(electricity)→ 2Na + Cl₂↑ (Downs process for sodium metal).

Displacement Reactions — In Depth

A MORE reactive element displaces a LESS reactive element from its compound.

Zn + CuSO₄ → ZnSO₄ + Cu. 'The blue colour of CuSO₄ solution FADES. A reddish-brown coating of copper deposits on the zinc strip.'
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. 'Silver crystals grow on the copper wire — like a metal TREE.'
Pb + CuCl₂ → PbCl₂ + Cu. 'Lead displaces copper — but not all metals displace all others. Consult the REACTIVITY SERIES.'

Double Displacement Reactions — In Depth

Exchange of IONS between two compounds.

Precipitation: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl. 'White insoluble BaSO₄ settles as a PRECIPITATE.'
Neutralisation: HCl + NaOH → NaCl + H₂O (also a double displacement — H⁺ and Na⁺ exchange).
Gas Formation: 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑. 'Brisk effervescence (bubbling) indicates a gas-forming double displacement.'

Redox Reactions — In Depth

OXIDATION and REDUCTION happen SIMULTANEOUSLY.

CuO + H₂ → Cu + H₂O: CuO is REDUCED to Cu (loses oxygen). H₂ is OXIDISED to H₂O (gains oxygen).
ZnO + C → Zn + CO: ZnO → Zn (reduction — loss of oxygen). C → CO (oxidation — gain of oxygen).
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂: MnO₂ is reduced (loses oxygen). HCl is oxidised to Cl₂ (loses hydrogen).

11. Balancing Equations — Step-by-Step Worked Examples

Example 1: NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Na: 1 vs 2 → 2NaOH + H₂SO₄ → Na₂SO₄ + H₂O
H: 2+2=4 vs 2 → 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
O: 2+4=6 vs 4+2=6 (correct)
Balanced: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Example 2: Al + HCl → AlCl₃ + H₂

Al: 1 vs 1 (correct), H: 1 vs 2, Cl: 1 vs 3
Balance Cl first: Al + 3HCl → AlCl₃ + H₂
H: 3 vs 2 → need LCM of 6: 2Al + 6HCl → 2AlCl₃ + 3H₂
Al: 2 vs 2 (correct), H: 6 vs 6 (correct), Cl: 6 vs 6 (correct)
Balanced: 2Al + 6HCl → 2AlCl₃ + 3H₂↑

Example 3: Fe₂O₃ + CO → Fe + CO₂

Fe: 2 vs 1, C: 1 vs 1, O: 3+1=4 vs 2
Balance Fe: Fe₂O₃ + CO → 2Fe + CO₂
O on left = 4, O on right = 2 → need 2CO₂: Fe₂O₃ + 3CO → 2Fe + 3CO₂
C: 3 vs 3 (correct), O: 3+3=6 vs 6 (correct)
Balanced: Fe₂O₃ + 3CO → 2Fe + 3CO₂

12. Corrosion — Detailed Mechanisms

Rusting of Iron: Electrochemical process. Iron atoms at ANODE lose electrons: Fe → Fe²⁺ + 2e⁻. Electrons travel through iron and reduce O₂ at CATHODE: O₂ + 2H₂O + 4e⁻ → 4OH⁻. Fe²⁺ + 2OH⁻ → Fe(OH)₂ (ferrous hydroxide). Further oxidation → Fe₂O₃·xH₂O (reddish-brown rust). 'Rusting REQUIRES both OXYGEN and WATER. Iron in DRY air or BOILED water does NOT rust.'

Corrosion of Other Metals: Silver TARNISHES (reacts with H₂S in air → Ag₂S — black). Copper develops a GREEN PATINA (basic copper carbonate — CuCO₃·Cu(OH)₂). Aluminium forms a PROTECTIVE oxide layer (Al₂O₃) — this is WHY aluminium doesn't rust away.

Prevention Methods: BARRIER protection (paint, oil, plastic coating). SACRIFICIAL protection (zinc/galvanisation — Zn OXIDISES before Fe). ALLOYING (stainless steel). CATHODIC protection (impressed current).

13. Rancidity — In Depth

Chemical Process: Unsaturated fats → OXIDATION at the C=C double bond → formation of ALDEHYDES and KETONES → unpleasant odour. 'Rancidity is AUTOCATALYTIC — once started, it ACCELERATES.'

Types: HYDROLYTIC rancidity (water breaks ester bonds in fats — releases free fatty acids — soapy taste). OXIDATIVE rancidity (oxygen attacks double bonds).

Prevention Methods: ANTIOXIDANTS (BHA, BHT — added to packaged foods. Vitamin E — natural antioxidant in nuts and oils). VACUUM PACKAGING (removes oxygen). NITROGEN FLUSHING (chips packets — N₂ replaces O₂). REFRIGERATION (slows ALL chemical reactions). DARK STORAGE (light accelerates oxidation).

14. Self-Test

Q1: Identify the type of reaction: 2KClO₃ →(heat)→ 2KCl + 3O₂. A1: DECOMPOSITION (thermal decomposition). A single compound (KClO₃) breaks down into TWO simpler products.

Q2: Why does the colour of CuSO₄ solution fade when iron nail is dipped? A2: This is a DISPLACEMENT reaction. Fe + CuSO₄ → FeSO₄ + Cu. Iron displaces copper. BLUE CuSO₄ turns to PALE GREEN FeSO₄. Reddish-brown copper deposits on the nail.

Q3: Balance the equation: C₃H₈ + O₂ → CO₂ + H₂O. A3: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check: C: 3 vs 3 (correct), H: 8 vs 8 (correct), O: 10 vs 6+4=10 (correct).

Q4: Give two differences between corrosion and rancidity. A4: Corrosion affects METALS (iron rusts, silver tarnishes). Rancidity affects FATS and OILS (food spoilage). Corrosion involves OXIDATION of metal atoms. Rancidity involves OXIDATION of C=C bonds in fats.

Q5: In the reaction ZnO + C → Zn + CO, identify the substance being oxidised and reduced. A5: ZnO is REDUCED (loses oxygen → Zn). C is OXIDISED (gains oxygen → CO). This is a REDOX reaction.

Q6: Why is decomposition of AgCl in sunlight a photolytic reaction? A6: Because the reaction is INITIATED by LIGHT (sunlight/photons): 2AgCl →(sunlight)→ 2Ag + Cl₂. The light energy breaks the Ag−Cl bond.

Q7: Why is stainless steel resistant to corrosion? A7: Stainless steel is an ALLOY of iron + chromium + nickel. Chromium forms a THIN, TRANSPARENT, PROTECTIVE oxide layer (Cr₂O₃) on the surface — this layer PREVENTS further oxidation of the iron underneath.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Types of Chemical Reactions

COMBINATION: A + B → AB. Example: 2Mg + O₂ → 2MgO (magnesium burns in oxygen). DECOMPOSITION: AB → A + B. Thermal: 2HgO →(heat)→ 2Hg + O₂. Electrolytic: 2H₂O →(electricity)→ 2H₂ + O₂. DISPLACEMENT: A + BC → AC + B. More reactive displaces less reactive. Zn + CuSO₄ → ZnSO₄ + Cu. DOUBLE DISPLACEMENT: AB + CD → AD + CB (ions exchange). BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl. REDOX: Oxidation (gain O / lose H / lose electrons). Reduction (lose O / gain H / gain electrons). OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). BALANCING: Adjust coefficients only. Atoms of each element must be equal on both sides.

BALANCING STRATEGY: (1) Write the unbalanced equation. (2) Start with the most complex molecule. (3) Balance metals first, then non-metals, then hydrogen and oxygen last. (4) Verify total atoms on both sides. AP SSC frequently asks to balance: Fe₂O₃ + Al → Al₂O₃ + Fe (thermite reaction). Fe₂O₃ + 2Al → Al₂O₃ + 2Fe.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Changing subscripts in formulae to balance equations

✓ NEVER change subscripts (the small numbers inside formulae like the 2 in H₂O). Changing H₂O to H₃O changes the compound entirely. You can ONLY change the COEFFICIENTS (the big numbers in front of formulae). Example: to balance 2H₂ + O₂ → 2H₂O, the coefficient 2 before H₂ means '2 molecules of hydrogen gas', not that you've changed the formula.

Practice problems

Work through this chapter's problems as a readiness check — reveal each solution, mark yourself honestly, and get your gap report at the end.

Readiness check

Are you exam-ready for Chemical Reactions and Equations?

1 problems from this chapter. Try each one, reveal the worked solution, mark yourself honestly — get your gap report at the end.

1 questions~2 min

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Chemical change signs: colour change, gas evolution (bubbles/smell), precipitate formed, temperature change (heat/light produced or absorbed), change cannot be reversed easily.
•Balancing rule: only change COEFFICIENTS (numbers before formulae) — NEVER change subscripts inside a formula. H₂O stays H₂O.
•5 reaction types: COMBINATION (A+B→AB, e.g., 2Mg+O₂→2MgO). DECOMPOSITION (AB→A+B, e.g., 2H₂O→2H₂+O₂). DISPLACEMENT (A+BC→AC+B, e.g., Zn+H₂SO₄→ZnSO₄+H₂). DOUBLE DISPLACEMENT (AB+CD→AD+CB, e.g., BaCl₂+Na₂SO₄→BaSO₄↓+2NaCl). REDOX (oxidation + reduction occur simultaneously).
•OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). Oxidising agent gains electrons (itself gets reduced). Reducing agent loses electrons (itself gets oxidised).
•Endothermic reaction: absorbs heat from surroundings (surroundings cool). Exothermic reaction: releases heat to surroundings (surroundings warm). Combustion is always exothermic.
•Photolysis: decomposition caused by light — e.g., 2AgCl →(sunlight)→ 2Ag + Cl₂. Used in black-and-white photography.
•Thermal decomposition: CaCO₃ →(heat)→ CaO + CO₂. Used in cement manufacture.
•Corrosion: slow oxidation of metals — rust = Fe₂O₃·xH₂O (iron + oxygen + water). Prevention: painting, galvanising (zinc coating), alloying.
•Rancidity: oxidation/hydrolysis of fats and oils → bad smell/taste. Prevention: vacuum packing, antioxidants, refrigeration.
•Precipitation reaction: double displacement forming an insoluble salt (precipitate). e.g., AgNO₃ + NaCl → AgCl↓ (white ppt) + NaNO₃.

Andhra Pradesh (BIEAP) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Industrial chemical manufacturing

The 5 reaction types from this chapter are the foundation of industrial chemistry. The Haber process (N₂ + 3H₂ → 2NH₃) is a combination reaction producing all the world's synthetic fertiliser. The Contact process (SO₂ → SO₃ → H₂SO₄) is a combination/oxidation reaction producing the world's most important industrial acid. Understanding reaction types allows chemists to design industrial processes.

Preservation of food and metals

Rancidity prevention (antioxidant packaging) and corrosion prevention (galvanising, alloying steel) are billion-dollar industries based on slowing down or preventing specific chemical reactions. The galvanised iron roofs common in Indian villages are coated with zinc — which forms ZnO preferentially, protecting the iron underneath. Understanding why these measures work requires knowing the oxidation chemistry from this chapter.

Photography and light-sensitive chemistry

Traditional black-and-white photography uses silver halide crystals (AgBr, AgCl) that decompose on exposure to light — the same photolysis reaction from this chapter. When light hits the film, Ag⁺ is reduced to Ag (black). The photographic developer completes the reduction; fixer washes away unreacted silver halide. Digital photography has replaced this, but photolysis principles still apply in photovoltaic cells and photocatalysis.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Balancing: always verify your answer by counting atoms on both sides before writing it down. Write the balance check explicitly: 'Mg: 2=2 (correct), O: 2=2 (correct)' — this shows the examiner your work and earns full marks even if your equation has a minor error.

For any reaction type question: name the type AND write a balanced example. Naming without example = 1 mark; naming with example = 2 marks.

Redox: state both oxidation and reduction explicitly — which element is oxidised (and its oxidation state change), which is reduced (and its change). Identify the oxidising agent and reducing agent.

Endothermic vs exothermic: common trap — burning (combustion) is exothermic (releases heat). Cooking is often endothermic (requires heat). State 'heat is released/absorbed' in the answer, not just the name.

Corrosion vs rancidity: both are oxidation processes. Corrosion = oxidation of metals. Rancidity = oxidation of fats/oils. AP SSC often asks students to define and distinguish these.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

STRETCH

Research electrochemistry — electrolytic cells (decomposition driven by electricity, e.g., electrolysis of water) vs galvanic/voltaic cells (oxidation-reduction reactions produce electricity, e.g., batteries). The principles of both build directly on the redox concepts from this chapter.

STRETCH

Investigate the Haber-Bosch process in detail — why iron catalyst, why 200 atm pressure, why 450°C? The compromise between equilibrium (favoured by low temperature and high pressure) and reaction rate (favoured by higher temperature) is a fundamental concept in industrial chemistry.

STRETCH

Explore the activity series of metals in detail — why does more reactive metal displace less reactive metal? This connects to electrode potential and Gibbs free energy in Class 12 electrochemistry.

STRETCH

Research the chemistry of cement manufacture — limestone (CaCO₃) is heated to 900°C → CaO (quicklime) + CO₂ (thermal decomposition). CaO + water → Ca(OH)₂ (slaking). Gypsum is added to control setting time. The entire process is a chain of chemical reactions from this chapter.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

AP Board SSC (Class 10)High — balancing and reaction type classification are standard 4–6 mark questions in every AP SSC paper

JEE Main / Advanced (Chemistry)Very High — redox reactions, balancing, and reaction classification are foundational for Class 11–12 chemistry and JEE

AP EAMCET (Engineering stream)High — chemical reactions and equations form the base of Class 11 Physical Chemistry tested in EAMCET

NTSE (Science section)Moderate — reaction types and balancing appear in NTSE science papers

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Subscripts define the actual chemical compound — changing them creates a different substance entirely. H₂O is water; H₂O₂ is hydrogen peroxide. You cannot write H₃O or H₄O to balance — these do not exist in this context. Only coefficients (the numbers in front of the formula) can be changed, because they represent 'how many molecules/formula units' are involved — not what the substance is. The law of conservation of mass is satisfied by adjusting coefficients only.

Rust formation is a CHEMICAL change — iron reacts with oxygen and water to form iron(III) oxide-hydroxide (Fe₂O₃·xH₂O), a completely different substance. The iron cannot be recovered by simple physical means. This distinguishes it from a physical change (like melting), where the original substance can be recovered. AP SSC expects students to justify chemical change by citing evidence: new substance formed (rust), change is not easily reversible.

Step 1: Assign oxidation numbers to all elements in reactants and products. Step 2: Find the element that INCREASED in oxidation number — that element was OXIDISED. The compound containing it is the REDUCING AGENT (it caused the reduction of something else by sacrificing its own electrons). Step 3: Find the element that DECREASED in oxidation number — that element was REDUCED. The compound containing it is the OXIDISING AGENT. Example: In Zn + CuSO₄ → ZnSO₄ + Cu, Zinc goes from 0 to +2 (oxidised, so Zn is the reducing agent); Copper goes from +2 to 0 (reduced, so CuSO₄ is the oxidising agent).

Rancidity occurs when the fatty acids and oils in food undergo oxidation — their carbon-carbon double bonds react with atmospheric oxygen, producing shorter-chain acids with unpleasant odours and tastes. It is a chemical change. Prevention methods: (1) Keep food in sealed, air-tight containers or vacuum packs — no oxygen means no oxidation. (2) Refrigeration — lower temperature slows the oxidation reaction rate. (3) Add antioxidants — vitamin C (ascorbic acid) or vitamin E preferentially react with oxygen, 'scavenging' it before it reaches the fats. (4) Flushing packaged food with nitrogen or inert gases replaces oxygen.

Photosynthesis (6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O) fits the COMBINATION category in the sense that simpler reactants combine to form a more complex molecule (glucose). However, it also involves decomposition (water is split during the light reaction). More accurately, photosynthesis is a REDOX reaction: CO₂ is REDUCED (gains hydrogen → becomes glucose) and H₂O is OXIDISED (loses electrons → releases O₂). In AP SSC classification, it is best described as a combination reaction with redox characteristics.

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