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Class 10 Science Chapter 1: Chemical Reactions and Equations case study question with answers

One case-based question of the kind the board asks in Section E: a passage, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks
At a lime works near Katni, limestone is fed into a kiln and heated strongly for hours; a colourless gas escapes from the chimney and white lumps are left. Masons buy the lumps, drop them into drums of water, where the water hisses and boils, and stir the milky paste. Diluted, it is brushed on walls as whitewash. Two to three days later the walls have a hard, shiny finish. The workers say the calcium 'goes round in a circle'.
Part 1 · 1 mark
The colourless gas escaping from the kiln is
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Answer: (b) carbon dioxide

CaCO₃ → CaO + CO₂ on heating; the gas is carbon dioxide.

Part 2 · 1 mark
The reaction in the masons' drums is
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Answer: (a) combination and exothermic

CaO + H₂O → Ca(OH)₂: two substances form one product, with a large release of heat, so the water hisses.

Part 3 · 2 marks
Explain the workers' remark by writing the formula of the calcium compound at each stage (kiln, drum, wall) and naming the type of reaction at each change.
Answer and marking scheme

Limestone CaCO₃ → (kiln, heat; thermal decomposition) CaO → (drum, water; combination) Ca(OH)₂ → (wall, CO₂ of the air) CaCO₃. The wall ends as calcium carbonate, the same compound as the limestone, so the calcium returns to where it began.

  • CaCO₃, CaO, Ca(OH)₂, CaCO₃ in order (1)
  • decomposition in the kiln and combination in the drum, ending as calcium carbonate (1)
Or
The drums of water boil without any fire, yet the kiln must be fired for hours. Explain this difference in terms of energy.
Answer and marking scheme

Slaking quicklime is exothermic: it releases a large amount of heat, enough to boil the water. Decomposing limestone is endothermic: like all decompositions it needs energy to break the reactant down, so heat must be supplied continuously.

  • slaking quicklime is exothermic, releasing heat (1)
  • decomposition of limestone absorbs energy, so heat must be supplied (1)
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