Lesson 7: Chemical Reactions

Chemistry is the study of change: iron rusting on a gate, milk turning to curd, fireworks bursting over Diwali skies. This NCERT lesson covers the signs of chemical reactions, how to write and balance chemical equations, the five types of reactions, oxidation and reduction, and the effects of oxidation in everyday life (corrosion, rancidity). Balanced equations are the grammar of chemistry — master them here.

Signs of a chemical reaction

A chemical reaction rearranges atoms into new substances. Its signs: evolution of a gas (zinc + dilute sulphuric acid fizzing hydrogen; antacid tablets fizzing CO₂), change in colour (iron nail in copper sulphate — blue fades, nail turns pink-brown), change in temperature (quicklime + water heats up; ammonium chloride + water cools), formation of a precipitate (silver nitrate + sodium chloride — white curdy silver chloride), and change in state or smell. One sign suffices to suspect reaction; confirmation needs product identification. Everyday Indian examples: curd setting (Lactobacillus fermenting lactose — sour smell, thick texture), iron gates rusting in monsoon humidity (reddish-brown powder), silver jewellery tarnishing (black silver sulphide), cut apples browning (enzymatic oxidation), biogas plants (anaerobic digestion producing methane), and the pressure cooker's whistle (physical, not chemical — a useful contrast). NCERT wants observation first: what did you see, smell, feel? Distinguish carefully from physical change: melting ghee, dissolving salt, and boiling water make no new substances — reversible, no energy of reaction. Burning the ghee, however, is chemical. The board's favourite confusion: "Is dissolving sugar chemical?" — no, recover it by evaporation. "Is cooking rice chemical?" — yes, starch gelatinises irreversibly. Apply the new-substance test ruthlessly.

Writing chemical equations

A word equation names reactants and products: "magnesium + oxygen → magnesium oxide". A skeletal equation uses formulae: Mg + O₂ → MgO. A balanced equation has equal atoms each side: 2Mg + O₂ → 2MgO. Balancing uses the hit-and-trial method: balance metals first, then non-metals, then hydrogen and oxygen; never change formulae to balance (changing H₂O to H₂O₂ changes the substance); use the smallest whole-number coefficients. State symbols make equations complete: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). 2H₂(g) + O₂(g) → 2H₂O(l). Conditions go above/below the arrow: heat (Δ), catalyst, pressure. For board answers, a fully correct equation — balanced, with state symbols — earns full marks; an unbalanced one earns little. Practise the standard set: combustion (CH₄ + 2O₂ → CO₂ + 2H₂O), photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with sunlight and chlorophyll noted), respiration (the reverse), neutralisation (HCl + NaOH → NaCl + H₂O), metal-acid (Zn + H₂SO₄ → ZnSO₄ + H₂), and thermal decomposition (CaCO₃ → CaO + CO₂ on heating — limestone to quicklime, the chemistry of cement plants). Always check a balanced equation against conservation of mass: CaCO₃ → CaO + CO₂ balances *and* conserves — 100 g of limestone yields exactly 56 g quicklime + 44 g carbon dioxide; the masses on both sides are identical because the atoms are.

Types of chemical reactions

• Combination: two or more reactants form one product — CaO + H₂O → Ca(OH)₂ (slaking lime, exothermic — the heat of whitewashing); 2H₂ + O₂ → 2H₂O.
• Decomposition: one reactant breaks into two or more — CaCO₃ → CaO + CO₂ (Δ); 2H₂O → 2H₂ + O₂ (electrolysis); 2AgBr → 2Ag + Br₂ (sunlight — black-and-white photography's chemistry).
• Displacement: a more reactive element displaces a less reactive one — Fe + CuSO₄ → FeSO₄ + Cu (the nail turns copper-pink); Zn + CuSO₄ → ZnSO₄ + Cu.
• Double displacement: two compounds exchange ions — AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white precipitate); BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl (the sulphate test).
• Oxidation–reduction (redox): oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons — they always occur together. CuO + H₂ → Cu + H₂O: CuO is reduced (loses O), H₂ is oxidised (gains O).

The reactivity series orders metals: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au. A metal displaces any below it — zinc displaces copper and iron but not aluminium. Learn the series with a mnemonic — it predicts displacement, extraction methods, and reactions with acids and water. NCERT examines it constantly.

Oxidation in everyday life — corrosion and rancidity

Corrosion: metals attacked by air and moisture — iron rusts (hydrated iron oxide, Fe₂O₃·xH₂O, reddish-brown, flaky), silver tarnishes black (Ag₂S), copper greens (basic carbonate — the Statue of Liberty's patina, and old temple domes). Rusting needs both oxygen and water — the three-tube nail experiment proves it: Tube 1 (air + water) rusts; Tube 2 (boiled water under oil — no air) does not; Tube 3 (dry air with calcium chloride desiccant) does not. Rust is porous — it flakes off, exposing fresh iron, so rusting continues until the object is consumed. Prevention: painting, oiling, greasing (barriers); galvanisation (zinc coating — zinc corrodes preferentially, protecting even scratched spots); alloying (stainless steel — chromium forms a protective oxide); electroplating (chromium on taps and cycles); sacrificial anodes (magnesium blocks on ships). Match method to situation in board answers: paint for gates, galvanise for buckets and lamp posts, stainless steel for cutlery and surgical instruments. Rancidity: oxidation of fats and oils in food — chips, biscuits and fried snacks develop off smells and tastes. Prevention: antioxidants (BHA, BHT in packaged foods), nitrogen flushing (chip packets — the "air" is inert N₂), refrigeration, airtight opaque packaging, and vacuum packing. The nitrogen-flushed packet is a lovely board example: chemistry preserving crunch. Rancidity is oxidation; corrosion is oxidation — the same electron loss, different victims.

Effects and applications

Exothermic reactions release heat: combustion (LPG burning: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O), respiration, neutralisation, slaking lime, fireworks (Diwali's chemistry — metal salts colouring flames: strontium red, barium green, sodium yellow). Endothermic reactions absorb heat: photosynthesis, thermal decomposition of limestone, dissolving ammonium chloride, the cold packs for sports injuries. Classify by temperature change — the thermometer is the judge. Fireworks deserve their chemistry: oxidiser (potassium nitrate/perchlorate) + fuel (charcoal, sulphur) + colour salts + binder. The colours are atomic emission spectra made celebratory — the same physics as flame tests. But the pollution is real: Delhi's post-Diwali air quality crises show chemistry's double edge, and the Supreme Court's "green cracker" rulings are chemistry meeting policy. Cement and concrete: limestone (CaCO₃) heated to ~1450 °C gives quicklime (CaO) + CO₂; slaked to Ca(OH)₂; mixed with sand and aggregate, it sets by carbonation back to CaCO₃ — chemistry building India's infrastructure, and a major CO₂ source (~8% of global emissions). Fertilisers (Haber process: N₂ + 3H₂ → 2NH₃), soaps and detergents (saponification), and pharmaceuticals are industrial reactions scaled to civilisation.

Balancing practice and board strategy

Balance these (coefficients in brackets): H₂ + O₂ → H₂O [2,1,2]; Fe + O₂ → Fe₂O₃ [4,3,2]; Al + HCl → AlCl₃ + H₂ [2,6,2,3]; C₃H₈ + O₂ → CO₂ + H₂O [1,5,3,4]; NH₃ + O₂ → NO + H₂O [4,5,4,6]. Method: list atoms each side, adjust coefficients, recheck, never touch subscripts. For redox, identify oxidised/reduced species by oxygen gain/loss (Class 10 level) — "OILRIG" (Oxidation Is Loss of electrons, Reduction Is Gain) for the electronic version in higher classes. Board answer structure for "what happens when...": name the reaction type, write the balanced equation with state symbols, state the observation (colour, gas, precipitate, temperature). "Zinc is added to copper sulphate solution": displacement reaction; Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s); blue solution fades, pink-brown copper deposits on zinc. Four elements, full marks.

Common mistakes

• Changing formulae instead of coefficients when balancing (H₂O → H₂O₂ is a different chemical).
• Forgetting diatomic molecules — H₂, O₂, N₂, Cl₂, Br₂, I₂ are written as pairs.
• Omitting state symbols and the ↓ for precipitates and ↑ for gases where boards expect them.
• Calling every gas evolution a chemical reaction — boiling water releases steam physically; look for a new substance.
• Predicting displacement against the reactivity series — copper can never displace zinc; check the series order before writing the equation.
• Mixing up oxidation and reduction — track the oxygen: the species that *gains* oxygen is oxidised; the one that *loses* it is reduced.

Two more reaction families from daily life

Electrolysis of water (electricity as a reactant): water with a few drops of acid, electrodes connected to a battery — hydrogen collects at the cathode, oxygen at the anode, in a 2:1 volume ratio; the hydrogen pops with a flame, the oxygen relights a glowing splinter. The volume ratio is the formula H₂O read aloud by an experiment. Precipitation in daily life: the white scale inside kettles in hard-water districts (calcium carbonate precipitating), the curdling of milk by lemon juice (casein precipitating), and the barium-meal X-ray (barium sulphate — an insoluble precipitate opaque to X-rays) are double-displacement chemistry outside the lab. Corrosion's cost deserves one figure: India loses an estimated 3–4% of GDP to corrosion damage every year — the economic argument for galvanisation, painting and stainless alloys. When a board question asks "why study reactions?", this GDP figure, the rusting of the Delhi Iron Pillar's resistance (its protective phosphide layer — ancient Indian metallurgy), and the electrolysis ratio are three concrete answers.

Exam Experience

MCQ 1. When zinc granules are added to dilute sulphuric acid, the gas evolved is: (a) Oxygen (b) Hydrogen, burning with a pop sound (c) Carbon dioxide (d) Chlorine Key: (b). Zn + H₂SO₄ → ZnSO₄ + H₂; the pop test with a burning splinter confirms hydrogen. MCQ 2 (Assertion–Reason). Assertion (A): The reaction of zinc with copper sulphate solution is a displacement reaction. Reason (R): Zinc is more reactive than copper, so it displaces copper from its salt solution. (a) Both A and R are true and R is the correct explanation of A (b) Both A and R are true but R is not the correct explanation of A (c) A is true but R is false (d) A is false but R is true Key: (a). Position in the reactivity series determines displacement; the fading blue colour and copper coating are the observations. Short answer (3 marks). Balance the following and identify each type: (i) FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (ii) Pb(NO₃)₂ + KI → PbI₂ + KNO₃ *Marking scheme:* (i) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ — decomposition (1½). (ii) Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃ — double displacement (precipitation) (1½). Long answer (5 marks). (a) What is rancidity? Give two methods to prevent it in packaged foods (3). (b) Silver articles turn black in air, while gold does not. Explain using reactivity, and name the black compound (2). *Marking scheme:* (a) Oxidation of fats/oils producing bad smell and taste (1); nitrogen flushing and antioxidants/airtight opaque packing — any two (2). (b) Silver reacts with sulphur compounds (H₂S) in air (1) forming black silver sulphide, Ag₂S; gold is too unreactive (1).