Ans: X is Copper (Cu). On heating: 2Cu + O₂ → 2CuO (black). This is an oxidation/combination reaction.

Ans: A reaction in which one compound breaks into two or more simpler substances. Example: CaCO₃ → CaO + CO₂ (on heating limestone).

Ans: Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Type: Redox reaction. Oxidising agent: H₂O (it oxidises Fe).

Ans: Rancidity is the oxidation of fats/oils in food causing bad smell and taste. Prevention: (1) Adding antioxidants like BHA. (2) Storing in airtight containers or refrigerating.

Ans: A double displacement reaction is one in which the ions of two compounds exchange to form two new compounds, often producing a precipitate or water. Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. Here, BaSO₄ is an insoluble white precipitate. Another example: AgNO₃ + NaCl → AgCl↓ (white) + NaNO₃. These are also called metathesis reactions.

Ans: (a) Exothermic: C + O₂ → CO₂ + heat (burning of carbon). Also: CaO + H₂O → Ca(OH)₂ + heat. Respiration: C₆H₁₂O₆ + O₂ → CO₂ + H₂O + energy. (b) Endothermic: 2NH₃ + heat → N₂ + 3H₂. Also: Evaporation of water; decomposition of limestone: CaCO₃ + heat → CaO + CO₂. Photosynthesis: CO₂ + H₂O + light energy → C₆H₁₂O₆ + O₂. (c) Photochemical: 2AgCl + light → 2Ag + Cl₂ (silver chloride decomposes in sunlight). Also: 2AgBr + light → 2Ag + Br₂ (used in photographic film). Photosynthesis is also photochemical.

Ans: Corrosion is the gradual deterioration of metals due to reaction with environmental factors like oxygen, moisture, and CO₂. Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (rust). Corrosion of copper gives green patina (CuCO₃·Cu(OH)₂). Galvanisation: Coating iron with a thin layer of zinc (Zn) to prevent rusting. Zinc is more reactive — it acts as a sacrificial anode (it corrodes preferentially, protecting the iron). Even if the zinc coating is scratched, iron is protected as long as zinc is present. Used in iron pipes, buckets, roofing sheets.

Ans: When lead nitrate, Pb(NO₃)₂, is heated strongly, it decomposes to give lead oxide (PbO — yellow powder), brown fumes of nitrogen dioxide (NO₂), and oxygen gas: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g)↑ + O₂(g)↑. The brown fumes of NO₂ are the characteristic observation. This is a thermal decomposition reaction.

Ans: It is a combination reaction (and it is also exothermic).

Ans: A white precipitate of barium sulphate is formed.

Ans: Paint forms a protective coating that keeps iron away from air (oxygen) and moisture. Without contact with oxygen and water, iron does not rust, so painting prevents corrosion.

Ans: The blue colour of the solution fades, and a reddish-brown deposit of copper forms on the zinc. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Zinc is more reactive than copper and displaces it, so it is a displacement reaction.

Ans: Water decomposes as 2H₂O → 2H₂ + O₂. Water has hydrogen and oxygen in the ratio 2 : 1 by number of atoms, so twice as much hydrogen gas is produced as oxygen. Hydrogen is collected at the cathode (double volume) and oxygen at the anode.

Ans: (a) A hissing sound is heard and a lot of heat is released, so the beaker becomes hot. (b) The product is calcium hydroxide (slaked lime). (c) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. It is an exothermic combination reaction.

Ans: A reaction in which oxidation and reduction occur together is a redox reaction. Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen. In CuO + H₂ → Cu + H₂O, CuO is reduced to Cu (loses oxygen), and H₂ is oxidised to H₂O (gains oxygen); CuO is the oxidising agent and H₂ the reducing agent. In ZnO + C → Zn + CO, ZnO is reduced to Zn and carbon is oxidised to CO.