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What is a chemical reaction… (Class 10 Science 2024, Q34a)

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ScienceCBSE Class 10 · Board exam 2024, set 31/4/1 · 5 marks · Long answer

CBSE Class 10 Science 2024: question 34(a)

Set in the CBSE Class 10 board exam of 2024, paper 31/4/1, section D, question 34(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(a) · 5 marks · Long answerChemical Reactions and Equations

What is a chemical reaction ? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/4/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

A chemical change produces new substances. Think of visible signs such as colour or heat.

Hint 2

Recall the yellow precipitate reaction and the reaction of quicklime with water.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. A chemical reaction is a process in which bonds between atoms in the reactants are broken and new bonds are formed, producing new substances (products) with different properties.

    1 mark

  2. (i) Change of colour, activity: take potassium iodide solution in a test tube and add lead nitrate solution to it. The colourless solutions give a yellow precipitate of lead iodide.

    1 mark

  3. . The yellow precipitate shows that a chemical change has occurred.

    1 mark

  4. (ii) Change in temperature, activity: take a small amount of calcium oxide (quicklime) in a beaker and slowly add water. The reaction is vigorous and the beaker becomes hot.

    1 mark

  5. + heat. The release of heat (rise in temperature) shows a chemical change; slaked lime is formed.

    1 mark

Answer: Chemical reaction: reactants change into new substances by breaking and forming bonds. (i) Pb(NO₃)₂ + KI gives a yellow PbI₂ precipitate; (ii) CaO + H₂O gives Ca(OH)₂ with heat released.

Where the marks are

  • Definition of a chemical reaction (1 mark)
  • Activity showing colour change, e.g. Pb(NO₃)₂ + KI (1 mark)
  • Observation/equation: yellow precipitate of PbI₂ (1 mark)
  • Activity showing temperature change, e.g. CaO + water (1 mark)
  • Observation/equation: heat released, Ca(OH)₂ formed (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Chemical Reactions and Equations, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 31/4/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.