In-text Questions — Oxidation and Reduction — Chemical Reactions and Equations

1. 12 marksNCERT Cl-10 Science, In-text Qs after §1.2.5, Q1

   Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

   Hint. Which ion in solution gives copper sulphate its blue colour, and what replaces it?

   Step 1 — What reaction happens. Iron is more reactive than copper, so it displaces copper from copper sulphate solution: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).

   Step 2 — Why the colour changes. The blue colour of the original solution comes from Cu²⁺ ions. As the reaction proceeds, Cu²⁺ ions are used up (copper metal deposits on the nail) and are replaced in solution by Fe²⁺ ions, which give a pale green colour — so the blue fades as the solution turns pale green.

   ✦ Answer: Iron, being more reactive than copper, displaces copper from the solution (Fe + CuSO₄ → FeSO₄ + Cu). Blue Cu²⁺ ions are replaced by pale green Fe²⁺ ions, so the blue colour fades.

   Where students slip. Saying the solution 'turns colourless' — it turns pale green (from dissolved iron sulphate), not colourless.

2. 22 marksNCERT Cl-10 Science, In-text Qs after §1.2.5, Q2

   Give an example of a double displacement reaction other than the one given in Activity 1.10.

   Hint. You don't have to invent a new reaction — this chapter already showed you one, in Activity 1.2, without ever naming it.

   Step 1 — Recall what a double displacement reaction needs. Two compounds must exchange ions, usually giving an insoluble precipitate — Activity 1.10 used sodium sulphate + barium chloride.

   Step 2 — Pick a different pair from elsewhere in this same chapter. Activity 1.2, described earlier in the chapter, mixed lead nitrate solution with potassium iodide solution. The ions swap partners and a bright yellow precipitate of lead iodide forms, since Pb²⁺ and K⁺ exchange partners just as Na⁺ and Ba²⁺ did in Activity 1.10 — a double displacement (and precipitation) reaction distinct from the barium sulphate example already given.

   ✦ Answer: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) [yellow precipitate] + 2KNO₃(aq).

   Where students slip. Re-describing Activity 1.10 itself (sodium sulphate + barium chloride) instead of giving a genuinely different example, as the question asks.

3. 33 marksNCERT Cl-10 Science, In-text Qs after §1.2.5, Q3

   Identify the substances that are oxidised and the substances that are reduced in the following reactions. (i) 4Na(s) + O₂(g) → 2Na₂O(s) (ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

   Hint. Don't assume both halves of a redox pair are always nameable with just the oxygen/hydrogen definition — check what, if anything, is actually losing oxygen in each equation.

   Step 1 — Apply this chapter's definition: gain of oxygen = oxidised; loss of oxygen = reduced.

   Step 2 — (i) 4Na + O₂ → 2Na₂O. Sodium goes from having no oxygen to being combined with oxygen in Na₂O, so sodium is oxidised. There is no compound on either side of this equation that is losing oxygen — oxygen gas is simply combining with sodium — so, using only this chapter's oxygen-based definition, no substance here is classified as 'reduced.' (The electron-transfer definition, which is beyond this chapter, would call O₂ itself reduced — see the appendix.)

   Step 3 — (ii) CuO + H₂ → Cu + H₂O. Copper oxide loses its oxygen to become copper metal, so CuO is reduced. Hydrogen gains that oxygen to become water, so H₂ is oxidised.

   ✦ Answer: (i) Sodium is oxidised; nothing in this equation is 'reduced' by the oxygen-transfer definition used in this chapter. (ii) CuO is reduced (to Cu); H₂ is oxidised (to H₂O).

   Where students slip. Forcing an answer for 'what is reduced' in part (i) — it's fine, and more accurate, to say the oxygen/hydrogen definition simply doesn't identify one here.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc101.pdf) — three in-text question sets (8 questions total) plus one end-of-chapter Exercise (20 questions); the electron-transfer ('OIL RIG') definition of oxidation/reduction and the terms oxidising/reducing agent do not appear anywhere in the current chapter text. Questions are referenced from the NCERT textbook for identification.