In-text Questions — Decomposition Reactions — Chemical Reactions and Equations

1. 12 marksNCERT Cl-10 Science, In-text Qs after §1.2.2, Q1

   A solution of a substance 'X' is used for whitewashing. (i) Name the substance 'X' and write its formula. (ii) Write the reaction of the substance 'X' named in (i) above with water.

   Hint. Reread the 'Do You Know?' note right after Activity 1.4 — it names the exact substance used for whitewashing.

   Step 1 — Identify X. The chapter's own note after Activity 1.4 states that a solution of slaked lime is what's used for whitewashing walls. Slaked lime is calcium hydroxide, so X = calcium hydroxide, Ca(OH)₂.

   Step 2 — What part (ii) is really asking. Calcium hydroxide doesn't go on to react further with more water on its own — the 'reaction ... with water' the question means is the one that produces X in the first place, which is exactly Activity 1.4: calcium oxide (quicklime) combining with water, releasing a large amount of heat.

   ✦ Answer: (i) X is calcium hydroxide (slaked lime), Ca(OH)₂. (ii) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat.

   Where students slip. Naming X as calcium oxide (quicklime) itself — quicklime is what whitewashing starts from, but the substance actually dissolved and applied to the walls is calcium hydroxide.

2. 22 marksNCERT Cl-10 Science, In-text Qs after §1.2.2, Q2

   Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

   Hint. Look at how many hydrogen atoms sit in one molecule of water compared to oxygen atoms.

   Step 1 — What Activity 1.7 is doing. Passing electricity through acidified water electrolyses it: 2H₂O(l) → 2H₂(g) + O₂(g).

   Step 2 — Read the ratio off the equation. For every 2 molecules of water broken down, 2 molecules of hydrogen gas form but only 1 molecule of oxygen gas forms — a 2:1 ratio. Since equal numbers of gas molecules occupy equal volumes under the same conditions, the volume of hydrogen collected is twice the volume of oxygen collected.

   ✦ Answer: Because water is H₂O, each molecule contains twice as many hydrogen atoms as oxygen atoms, so electrolysis produces hydrogen and oxygen gas in a 2:1 volume ratio. The gas collected in double the amount is hydrogen.

   Where students slip. Naming oxygen as the gas collected in the larger amount — it's the other way around; oxygen is the smaller-volume gas in this activity.

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.