CBSE Class 10 Science Board Paper Set 3 — MCQs with Answers

8 solved questions from this paper

Answer and worked explanation shown for each. The remaining 12 are timed and scored when you sign in free.

1. Q1Easy

   An iron nail dipped in copper sulphate solution slowly acquires a reddish-brown coating, and the blue colour of the solution fades. The reaction Fe + CuSO₄ → FeSO₄ + Cu is an example of a:

   A.combination reaction

   B.decomposition reaction

   C.displacement reaction✓ Correct

   D.double displacement reaction

   Answer: C. displacement reaction

   Explanation: One element, iron, pushes another element, copper, out of its salt solution because iron is the more reactive of the two, and this single exchange is a displacement reaction. In a double displacement reaction the two reactants would swap ions with each other, as sodium sulphate does with barium chloride, and both reactants would have to be compounds, which is not the case here.

2. Q2Easy

   Dilute hydrochloric acid is poured over zinc granules and the gas evolved burns with a pop sound when a burning splinter is brought near it. The gas is:

   A.Hydrogen✓ Correct

   B.Oxygen

   C.Carbon dioxide

   D.Chlorine

   Answer: A. Hydrogen

   Explanation: A dilute acid reacting with a metal gives the corresponding salt and hydrogen, here zinc chloride and H₂. The pop sound is the standard test for hydrogen, produced as the gas burns explosively in a small quantity of air. Carbon dioxide would put the splinter out instead of burning, and it is released by acids acting on carbonates, not on metals.

3. Q3Easy

   Which of the following is a metal that exists in the liquid state at room temperature?

   A.Sodium

   B.Mercury✓ Correct

   C.Bromine

   D.Gallium

   Answer: B. Mercury

   Explanation: Mercury is the one metal that is liquid at ordinary room temperature, which is why it is used in thermometers, and like other metals it conducts electricity and has a lustrous surface. Bromine is also a liquid but it is a non-metal, so it fails the first condition, and gallium, though it melts in the warmth of the hand at about 30 °C, is still a solid at normal room temperature.

4. Q4Medium

   Plaster of Paris is obtained by heating gypsum to about 373 K. Its chemical formula is:

   A.CaSO₄·2H₂O

   B.CaSO₄

   C.CaCO₃·H₂O

   D.CaSO₄·½H₂O✓ Correct

   Answer: D. CaSO₄·½H₂O

   Explanation: Gypsum is CaSO₄·2H₂O, and controlled heating drives off three quarters of that water, leaving half a molecule of water for each formula unit of calcium sulphate. Written for two formula units this is (CaSO₄)₂·H₂O. When water is added back the material reverts to gypsum and sets hard, which is what makes it useful for plaster casts, and heating too strongly would leave the anhydrous salt, which no longer sets.

5. Q5Medium

   Silver ornaments left in the open for a long time slowly turn black. This happens because silver reacts with the ___ of the air to form ___.

   A.oxygen; silver oxide

   B.carbon dioxide; silver carbonate

   C.hydrogen sulphide; silver sulphide✓ Correct

   D.nitrogen; silver nitride

   Answer: C. hydrogen sulphide; silver sulphide

   Explanation: Air contains traces of hydrogen sulphide, and silver reacts with it to form a thin black layer of silver sulphide on the surface, which is called tarnishing. Copper behaves differently, reacting with moist carbon dioxide to acquire a green coating of basic copper carbonate, and confusing the two is the usual error here.

6. Q6Medium

   Ethanol is warmed with ethanoic acid in the presence of a few drops of concentrated sulphuric acid, and a sweet-smelling liquid is obtained. This liquid is:

   A.an ester✓ Correct

   B.a soap

   C.an aldehyde

   D.an alcohol

   Answer: A. an ester

   Explanation: An alcohol heated with a carboxylic acid and a little concentrated sulphuric acid gives an ester and water, a change called esterification in which the acid acts as a dehydrating agent. Esters have pleasant fruity smells and are used in perfumes and flavourings. A soap would be formed only if the ester were afterwards heated with sodium hydroxide, a separate reaction known as saponification.

7. Q7Hard

   A person cannot see objects lying beyond 1 m clearly, although nearby objects are seen without difficulty. The power of the corrective lens required is:

   A.−1.0 D✓ Correct

   B.+1.0 D

   C.−4.0 D

   D.+4.0 D

   Answer: A. −1.0 D

   Explanation: The far point of this myopic eye is 1 m, so the lens must take rays from a very distant object and form their image at that far point. Using 1/f = 1/v − 1/u with v = −1 m and u at infinity gives f = −1 m, and P = 1/f = −1 dioptre, the minus sign showing a diverging lens. Forgetting that the corrective lens must be concave, and quoting +1.0 D instead, is the frequent slip.

8. Q8Hard

   A student connects a 4 Ω resistor and a 12 Ω resistor side by side across a 12 V battery so that each has its own branch. She inserts an ammeter in the branch containing the 12 Ω resistor. The ammeter reading will be:

   A.1 A✓ Correct

   B.3 A

   C.4 A

   D.0.33 A

   Answer: A. 1 A

   Explanation: The two resistors are in parallel, so each branch gets the full battery voltage of 12 V rather than a share of it. For the 12 Ω branch, Ohm's law gives I = V/R = 12/12 = 1 A. Working out the equivalent resistance of 3 Ω and quoting the resulting 4 A is a common trap, but that figure is the total current drawn from the battery, not the current in this one branch.

More Class 10 Science board papers

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• Board Paper Set 1 — 20 questions
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• Board Paper Set 3 — you are here

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