Ans: A chemical equation is a symbolic representation of a chemical reaction using formulas of reactants and products.

Ans: Mass can neither be created nor destroyed in a chemical reaction; the total mass of reactants equals total mass of products.

Ans: Combination: 2H₂ + O₂ → 2H₂O. Decomposition: 2H₂O → 2H₂ + O₂ (electrolysis). Displacement: Fe + CuSO₄ → FeSO₄ + Cu.

Ans: A reaction in which oxidation and reduction occur simultaneously. Example: CuO + H₂ → Cu + H₂O. Here H₂ is oxidised (gains oxygen) and CuO is reduced (loses oxygen).

Ans: Rancidity: oxidation of fats/oils making food smell bad. Prevented by: adding antioxidants, refrigeration, airtight packing, nitrogen flushing. Corrosion: slow destruction of metals by reacting with environment. Example: rusting of iron (Fe → Fe₂O₃). Prevented by painting, galvanisation, alloying.

Ans: A combination (synthesis) reaction is one in which two or more substances combine to form a single product: A + B → AB. Examples: (1) C + O₂ → CO₂ (burning of carbon — exothermic). (2) CaO + H₂O → Ca(OH)₂ (slaked lime) — highly exothermic. (3) SO₃ + H₂O → H₂SO₄ (exothermic). (4) 2Mg + O₂ → 2MgO (burning of magnesium — exothermic, bright white light). Most combination reactions are exothermic because energy is released when new bonds form.

Ans: Decomposition: a single compound breaks into two or more simpler substances. AB → A + B. Types: (1) Thermal decomposition (heat): 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes of NO₂). CaCO₃ → CaO + CO₂ (manufacture of quicklime). 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (ferrous sulphate decomposes to ferric oxide). (2) Electrolytic decomposition (electricity): 2H₂O → 2H₂ + O₂ (electrolysis of water). (3) Photochemical decomposition (light): 2AgCl → 2Ag + Cl₂ (silver chloride decomposes in sunlight — basis of black-and-white photography; AgBr is used in photographic film). AgCl turns grey/black in sunlight.

Ans: A displacement reaction is one where a more reactive element displaces a less reactive element from its compound solution. Fe + CuSO₄ → FeSO₄ + Cu. Explanation: Iron is more reactive than copper (higher in the reactivity series), so it displaces copper from copper sulphate solution. The blue colour of CuSO₄ solution fades and a reddish-brown deposit of copper forms on the iron nail. Another example: Zn + CuSO₄ → ZnSO₄ + Cu (zinc is more reactive than copper).

Ans: Rancidity is the oxidation of fats and oils in food, producing aldehydes and carboxylic acids that have an unpleasant smell and taste. Food becomes "stale" or "rancid." Prevention: (1) Add antioxidants (BHA, BHT, Vitamin E) to delay oxidation. (2) Store in airtight containers (reduces O₂ contact). (3) Refrigerate — lower temperature slows oxidation. (4) Flush packages with nitrogen gas (inert — chips packets). (5) Use vacuum packaging.

Ans: (a) 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Type: Combination (Fe and O combine) / also Redox. (Check: Fe:3=3✓, H:8=8✓, O:4=4✓). (b) HCl + NaOH → NaCl + H₂O. Already balanced. Type: Neutralisation (Double displacement). (c) 2Pb(NO₃)₂ → 2PbO + 4NO₂↑ + O₂↑. Type: Decomposition (thermal). (Check: Pb:2=2✓, N:4=4✓, O:12→4+8+2=12?→2+8+2=12✓). (d) AgNO₃ + NaCl → AgCl↓ + NaNO₃. Already balanced. Type: Double displacement (precipitation — AgCl is insoluble white precipitate).

Ans: A precipitate is an insoluble solid formed when two solutions react. Example: white barium sulphate formed on mixing barium chloride and sodium sulphate solutions.

Ans: During respiration, glucose is oxidised in the cells to carbon dioxide and water with the release of energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Since heat energy is released, it is exothermic.

Ans: Nitrogen gas is used because it is unreactive and keeps oxygen away from the fats and oils in the chips, which prevents oxidation.

Ans: Magnesium is covered with a thin layer of magnesium oxide formed by slow reaction with air. This layer prevents the metal from burning, so the ribbon is cleaned with sandpaper to expose the fresh metal surface.

Ans: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). It is a double displacement (and precipitation) reaction. A white precipitate of barium sulphate is formed.

Ans: In photosynthesis, plants absorb energy from sunlight to combine carbon dioxide and water into glucose: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in sunlight, with chlorophyll). Since energy is absorbed from the surroundings, it is an endothermic reaction.

Ans: A decomposition reaction brought about by light is called photolytic decomposition. Example: 2AgCl → 2Ag + Cl₂ (in sunlight), and 2AgBr → 2Ag + Br₂. White silver chloride turns grey in sunlight. These reactions are used in black and white photography, and silver chloride is stored in dark bottles.

Ans: After some time, the blue colour of the solution fades to pale green and a reddish-brown layer of copper is deposited on the iron nail. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Iron is more reactive than copper, so it displaces copper from copper sulphate solution. It is a displacement reaction.

Ans: Green ferrous sulphate crystals (FeSO₄·7H₂O) first lose their water of crystallisation, and the colour changes to a white or pale solid. On further heating, the solid decomposes to give a brown solid ferric oxide (Fe₂O₃) and gases sulphur dioxide and sulphur trioxide with the smell of burning sulphur. Equations: FeSO₄·7H₂O → FeSO₄ + 7H₂O; 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃. It is a thermal decomposition reaction.

Ans: (a) H₂(g) + Cl₂(g) → 2HCl(g): combination reaction. (b) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq): double displacement (precipitation) reaction. (c) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g): displacement reaction (sodium displaces hydrogen); it is also exothermic.