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Class 10 Science Chapter 1: Chemical Reactions and Equations MCQs with answers

10 multiple-choice questions on Chemical Reactions and Equations, each with its answer and the reason, written from the NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
Which of these changes forms a new substance?
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Answer: (c) Slaked lime brushed on a wall turns into a hard, shiny layer over three days.

Bending, boiling and shattering change only shape or state. On the wall, calcium hydroxide reacts with carbon dioxide of the air to form calcium carbonate, a new substance.

Question 2
Which of the following is a word-equation?
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Answer: (b) Zinc + Copper chloride → Zinc chloride + Copper

A word-equation names the reactants and products; the others use chemical formulae, and (d) is not a real reaction at all.

Question 3
Which of these equations is a skeletal equation?
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Answer: (b) Ca + O₂ → CaO

In Ca + O₂ → CaO there are two oxygen atoms on the left and one on the right, so it is unbalanced. The other three have equal atoms of each element on both sides.

Question 4
Asked to balance Al + O₂ → Al₂O₃, a student writes 2Al + O₂ → Al₂O₂. The mistake is that the student has
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Answer: (a) changed the formula of the product instead of changing coefficients

A formula is fixed; only the coefficients in front may be changed. Al₂O₂ is not aluminium oxide. The correct equation is 4Al + 3O₂ → 2Al₂O₃.

Question 5
Propane, a fuel gas, burns in oxygen: C₃H₈ + x O₂ → 3CO₂ + 4H₂O. For the equation to be balanced, x must be
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Answer: (d) 5

Oxygen atoms on the right: 3 × 2 + 4 × 1 = 10, so 10/2 = 5 molecules of O₂. Option (b) gives the number of oxygen atoms, not O₂ molecules.

Question 6
In a closed flask, 2.4 g of magnesium burns completely in 1.6 g of oxygen. The mass of magnesium oxide formed is
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Answer: (d) 4.0 g

Mass is neither created nor destroyed: mass of product = 2.4 g + 1.6 g = 4.0 g.

Question 7
In the equation Zn(s) + CuCl₂(aq) → ZnCl₂(aq) + Cu(s), the symbol (aq) shows that copper chloride is
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Answer: (b) present as a solution in water

(aq) stands for aqueous, a solution in water; (l) would mean liquid, (g) gas and (s) solid.

Question 8
At a building site in Jaipur, a mason pours water on lumps of quicklime in an iron drum. The water hisses and the drum becomes too hot to touch. The product and the nature of the reaction are
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Answer: (a) calcium hydroxide; exothermic

Quicklime (CaO) and water form slaked lime, calcium hydroxide, Ca(OH)₂, with a large release of heat, so the reaction is exothermic.

Question 9
A week after a room is whitewashed, a flake of the shiny wall surface is dropped into dilute hydrochloric acid. It fizzes, and the gas turns lime water milky. The flake is mainly
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Answer: (a) calcium carbonate

Calcium hydroxide on the wall slowly takes up carbon dioxide from the air to form calcium carbonate, the shiny layer. A carbonate with dilute acid gives carbon dioxide, which turns lime water milky.

Question 10
Which of these reactions releases heat but gives out no light?
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Answer: (a) Respiration in the cells of the body

Respiration is exothermic but evolves no light. Burning magnesium and natural gas give both heat and light; photosynthesis absorbs energy.

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