CBSE Class 10 Science Previous Year Question Paper 2020

Series: JBB/1 | Set: 1 | Code no.: 31/1/1

SECTION - A

1. Name acyclic unsaturated carbon compound.

Ans: Acyclic unsaturated carbon compounds are those which contain double or triple bonds between carbon atoms and are acyclic in nature. For example: Butene. [1]

2. The change in magnetic field lines in a coil is the cause of induced electric current in it. Name the underlying phenomenon.

Ans: According to the phenomenon of electromagnetic induction, the change in magnetic field lines in a coil is the cause of induced electric current in it. [2]

Paragraph Questions (3a - 3d): The growing size of the human population is a cause of concern for all people. The rate of birth and death in a given population will determine its size. Reproduction is the process by which organisms increase their population. The process of sexual maturation for reproduction is gradual and takes place while general body growth is still going on. Some degree of sexual maturation does not necessarily mean that the mind or body is ready for sexual acts or for having and bringing up children. Various contraceptive devices are being used by human beings to control the size of the population.

3(a). List two common signs of sexual maturation in boys and girls.

Ans: Two common signs of sexual maturation in boys and girls are:

1. Pubic, underarm and facial hair development.
2. Common signs of acne development. [3]

3(b). What is the result of reckless female foeticide?

Ans: The reckless female foeticide will ultimately result in a decline in the men:women ratio. Since due to foeticide the number of females decreases with respect to the number of males. [4]

3(c). Which contraceptive method changes the hormonal balance of the body?

Ans: The oral contraceptive method changes the hormonal balance of the body so this woman does not ovulate by taking such pills. [4]

3(d). Write two factors that determine the size of a population.

Ans: Two factors which determine the size of population are: 1. Birth rate 2. Death rate [5]

Paragraph Questions (4a - 4d): Human body is made up of five important components, of which water is the main component. Food as well as potable water are essential for every human being. The food is obtained from plants through agriculture. Pesticides are being used extensively for a high yield in the fields. These pesticides are absorbed by the plants from the soil along with water and minerals and from the water bodies these pesticides are taken up by the aquatic animals and plants. As these chemicals are not biodegradable, they get accumulated progressively at each trophic level. The maximum concentration of these chemicals gets accumulated in our bodies and greatly affects the health of our mind and body.

4(a). Why is the maximum concentration of pesticides found in human beings?

Ans: Pesticides are extensively used for good production of crops and these crops will be consumed by us. Hence these pesticides transfer to the human body. Therefore the maximum concentration of pesticides is found in human beings. [6]

4(b). Give one method which could be applied to reduce our intake of pesticides through food to some extent.

Ans: Before eating food we must ensure to wash it properly so that if there is a layer of pesticides on the fruit then it gets washed. [7]

4(c). Various steps in a food chain represent:
(a) Food web
(b) Trophic level
(c) Ecosystem
(d) Biomagnification

Ans: (b) Trophic level. The various steps which are represented in the food chain are known as trophic level. Each trophic level represents a class of organism. [7]

4(d). With regard to various food chains operating in an ecosystem, man is a:
(a) Consumer
(b) Producer
(c) Producer and consumer
(d) Producer and decomposer

Ans: (a) Consumer. According to various food chains operating in the ecosystem, a man is a consumer which does not produce anything of its own. It only consumes. [8]

5. Calcium oxide reacts vigorously with water to produce slaked lime.
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
This reaction can be classified as:
(A) Combination reaction
(B) Exothermic reaction
(C) Endothermic reaction
(D) Oxidation reaction
Which of the following is a correct option?
(a) (A) and (C)
(b) (C) and (D)
(c) (A), (C) and (D)
(d) (A) and (B)

Ans: (d) (A) and (B). The above reaction is an exothermic in nature which releases large amounts of heat and it is an example of a combination reaction. [9]

Or
When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. The reaction is an example of a:
(a) Combination reaction
(b) Displacement reaction
(c) Decomposition reaction
(d) Double displacement reaction

Ans: (d) Double displacement reaction. The reaction can be represented as: H₂S + CuSO₄ → CuS + H₂SO₄. [10, 11]

6. In a double displacement reaction such as the reaction between sodium sulphate solution and barium chloride solution:
(A) exchange of atoms takes place
(B) exchange of ions takes place
(C) a precipitate is produced
(D) an insoluble salt is produced
The correct option is:
(a) (B) and (D)
(b) (A) and (C)
(c) only (B)
(d) (B), (C) and (D)

Ans: (b) (A) and (C).
Note: The source text states "Sodium chloride is not an insoluble salt therefore the correct options are A and C." This appears to contain an error in the source logic regarding statement (A) "exchange of atoms", as double displacement typically involves exchange of ions. However, option (b) is explicitly selected in the provided source. [12]

7. Baking soda is a mixture of:
(a) Sodium carbonate and acetic acid
(b) Sodium carbonate and tartaric acid
(c) Sodium hydrogen carbonate and tartaric acid
(d) Sodium hydrogen carbonate and acetic acid

Ans: (c) Sodium hydrogen carbonate and tartaric acid. Baking soda is sodium bicarbonate (NaHCO₃) which is a mixture of sodium hydrogen carbonate and tartaric acid. [13]

8. The chemical formula for plaster of Paris is:
(a) CaSO₄.2H₂O
(b) CaSO₄.H₂O
(c) CaSO₄.1/2H₂O
(d) 2CaSO₄.H₂O

Ans: (a) CaSO₄.2H₂O.
Note: The source text identifies option (a) CaSO₄.2H₂O as the answer. It should be noted that scientifically, CaSO₄.2H₂O is Gypsum, and Plaster of Paris is CaSO₄.1/2H₂O (hemihydrate), but the source explicitly selects (a). [13, 14]

9. The laws of reflection hold true for:
(a) plane mirrors only
(b) concave mirrors only
(c) convex mirrors only
(d) all reflecting surfaces

Ans: (d) all reflecting surfaces. [14]

Or
When an object is kept within the focus of a concave mirror, an enlarged image is formed behind the mirror. This image is:
(a) real
(b) inverted
(c) virtual and inverted
(d) virtual and erect

Ans: (d) virtual and erect. [15]

10. At the time of short circuit, the electric current in the circuit:
(a) vary continuously
(b) does not change
(c) reduces substantially
(d) increases heavily

Ans: (d) increases heavily. At the time of short circuit the current increases heavily which will burn our devices too. [15, 16]

Or
Two bulbs of 100 W and 40 W are connected in series. The current through the 100 W bulb is 1A. The current through the 40W bulb will be:
(a) 0.4A
(b) 0.6A
(c) 0.8A
(d) 1A

Ans: (d) 1A. The current will always be equal in series connection. Therefore current will be 1A. [16]

12. Incomplete combustion of coal and petroleum:
(A) increases air pollution.
(B) increases the efficiency of machines.
(C) reduces global warming.
(D) produce poisonous gases.
The correct option is:
(a) (A) and (B)
(b) (A) and (D)
(c) (B) and (C)
(d) (C)and (D)

Ans: (b) (A) and (D). Incomplete combustion of petrol will increase the air pollution and produce the poisonous gases consequently. [17]

13. Assertion (A): Esterification is a process in which a sweet-smelling substance is produced.
Reason (R): When esters react with sodium hydroxide an alcohol and sodium salt of carboxylic acid are obtained.

Ans: (b) Both A and R are true, but R is not the correct explanation of the Assertion. Esterification is the process by which ester is formed and ester is a sweet smelling substance. [18, 19]

14. Assertion (A): In the process of nuclear fission, the amount of nuclear energy generated by the fission of an atom of uranium is so tremendous that it produces 10 million times the energy produced by the combustion of an atom of carbon from coal.
Reason (R): The nucleus of a heavy atom such as uranium, when bombarded with low energy neutrons, splits apart into lighter nuclei. The mass difference between the original nucleus and the product nuclei gets converted to tremendous energy.

Ans: (a) Both A and R are true and R is correct explanation of the Assertion. [20, 21]

SECTION - B

15. 1g of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations reactions, the name and the color of the products formed in each case.

Ans: When copper powder is heated, a black colored substance is formed which is known as copper oxide.
Equation: 2Cu + O₂ → 2CuO (Black)
When hydrogen gas is passed over this heated substance (copper oxide), the copper metal is obtained (brown).
Equation: CuO + H₂ → Cu + H₂O [21, 22]

16. List the important products of the Chlor-alkali process. Write one important use of each.

Ans: The important products are sodium hydroxide, chlorine gas, and hydrogen gas.
Reaction: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
Uses:

• Sodium Hydroxide: Manufacturing of soaps and detergents.
• Chlorine gas: Pesticides and manufacturing of PVC pipes.
• Hydrogen gas: Fuels and reduction reactions. [23, 24]

Or
How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?

Ans: Washing soda is prepared from sodium carbonate by adding ten molecules of water of crystallization.
Equation: Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O
It is a basic salt. It removes both temporary hardness and permanent hardness of water. [24, 25]

17. A 3 ml ethanol is taken in a test tube and warmed gently in a water bath. A 5% solution of alkaline potassium permanganate is added first drop by drop to this solution, then in excess.
(i) How is 5% solution of KMnO₄ prepared?
(ii) State the role of alkaline potassium permanganate in this reaction. What happens when you add it in excess?
(iii) Write the chemical equation of this reaction.

Ans: (i) Dissolve 5g of KMnO₄ in 100 ml of water.
(ii) Alkaline potassium permanganate acts as an oxidizing agent. When added in excess, the purple color will persist.
(iii) Chemical equation: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (reaction with KMnO₄ oxidizing ethanol to ethanoic acid). [26-28]

18. A squirrel is in a scary situation. Its body has to prepare for either fighting or running away. State the immediate changes that take place in its body so that the squirrel is able to either fight or run?

Ans: 1. Adrenaline hormone is released, increasing the heart rate and breathing rate.
2. Blood pressure changes and pupils dilate.
3. Blood is diverted to skeletal muscles.
4. Digestive and reproductive activities stop temporarily. [28, 29]

Or
Why is chemical communication better than electrical impulses as a means of communication between cells in a multicellular organism?

Ans: 1. Chemical communication (hormones) reaches all cells of the body, unlike electrical impulses which are limited to regions connected by nerves.
2. Chemical communication does not require specialized nervous tissue.
3. Chemical signals can be maintained steadily, whereas electrical impulses require intervals. [30, 31]

19. Define the term pollination. Differentiate between self pollination and cross pollination. What is the significance of pollination?

Ans: Pollination is the transfer of pollen grains from the anther of a male flower to the stigma of a female flower.
Self-Pollination: Transfer within the same flower. No pollinating agents necessary.
Cross-Pollination: Transfer to a different flower. Pollinating agents necessary.
Significance: It helps in fertilization by bringing the male gamete to the female egg, leading to seed production. [31, 32]

20. What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor? Justify your answer.

Ans: Homologous structures have a common basic structure but perform different functions. Example: Forelimbs of reptiles, amphibians, and mammals (e.g., arms of humans, wings of birds). Yes, they have common ancestors but are modified to perform different functions (like flying vs grasping). [33, 34]

21. Why is the Tyndall effect shown by colloidal particles? State four instances of observing the Tyndall effect.

Ans: The Tyndall effect is shown because colloidal particles scatter light. Instances: 1. Headlights of a car in fog. 2. Sunlight passing through a forest canopy. 3. Light entering a dark room through a hole. 4. Beam of light in a glass of milk. [35, 36]

Or
Differentiate between a glass slab and a glass prism. What happens when a narrow beam of (i) a monochromatic light, and (ii) white light passes through (a) glass slab and (b) glass prism?

Ans: Glass slab has parallel refracting surfaces; prism has angled surfaces.
(i) Monochromatic light: Glass slab - deviates but emergent ray is parallel to incident. Prism - deviates from path, incident and emergent rays not parallel.
(ii) White light: Glass slab - no dispersion (recombines). Prism - dispersion into seven colors (spectrum). [37, 38]

22. Draw a labelled diagram to show (i) reddish appearance of the sun at the sunrise or the sunset and (ii) white appearance of the sun at noon when it is overhead.

Ans: At sunrise/sunset, sunlight travels a longer distance through the atmosphere, scattering blue light, leaving red light to reach the eye. At noon, the distance is shorter, so less scattering occurs, appearing white. [39, 40]

23. A V-I graph for a nichrome wire is given below. What do you infer from this graph? Draw a labelled circuit diagram to obtain such a graph.

Ans: The graph shows a linear relationship between V and I, implying the nichrome wire follows Ohm's law (V = IR) and resistance is constant. The circuit diagram includes the nichrome wire, voltmeter, ammeter, and power source. [41, 42]

24. (a) Write the mathematical expression for Joule’s law of heating.
(b) Compute the heat generated while transferring 96000 coulomb of charge in two hours through a potential difference 40 V.

Ans: (a) H = I²Rt.
(b) Heat H = V × Q = 40V × 96000C = 3,840,000 J = 3.84 × 10⁶ J. [42, 43]

SECTION - C

25. Carbon cannot reduce the oxides of sodium, magnesium and aluminium to their respective metals. Why? Where are these metals placed in the reactivity series? How are these metals obtained from their ores? Take an example to explain the process of extraction along with chemical equations.

Ans: Carbon cannot reduce them because Na, Mg, and Al have a higher affinity for oxygen than carbon does. These metals are placed high in the reactivity series. They are obtained by electrolytic reduction (electrolysis) of their molten salts. Example: Sodium is obtained by electrolysis of molten sodium chloride.
Cathode: Na⁺ + e^- → Na
Anode: 2Cl^- → Cl₂ + 2e^- [44-46]

26. The position of certain elements in the Modern Periodic Table are shown below.

Using the table answer: (i) Which element will form only covalent compounds? (ii) Which element is a non-metal with valency 2? (iii) Which element is a metal with valency 2? (iv) Out of H, C and F which has the largest atomic size? (v) To which family docs H, C and F belong?

Ans: (i) Element E (Group 14) and B (Group 15 - source text discrepancy on table layout, likely refers to Silicon/Carbon group). Source identifies E and B.
(ii) Element E (if Group 16). Source text identifies E as non-metal.
(iii) Element D (Group 2, Period 3) - Magnesium.
(iv) F has the largest atomic size (Source text states F, which is in Period 3, while C is in Period 2. Size increases down a group).
(v) Family of H, C, F. (Source text answer is cut off/vague, likely refers to their respective groups). [46-48]

Or
Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why is it so?

Ans: Atomic size is the distance from the nucleus to the outermost shell. Unit: Angstrom (A). Trend: Decreases across a period (left to right) due to increased effective nuclear charge. Increases down a group due to the addition of new shells. [49-51]

27. (a) Why is there a difference in the rate of breathing between aquatic organisms and terrestrial organisms? Explain.
(b) Draw a diagram of the human respiratory system.

Ans: (a) Aquatic organisms breathe faster because the amount of dissolved oxygen in water is much lower than the amount of oxygen in the air.
(b) [Diagram of respiratory system labeling pharynx, trachea, lungs, diaphragm, alveolar sac required]. [51-53]

Or
(a) Name the organs that form the excretory system in human beings.
(b) Describe in brief how urine is produced in the human body.

Ans: (a) Two kidneys, ureters, urinary bladder, urethra.
(b) Urine is produced by filtration of blood in the nephrons of the kidney. Useful substances are reabsorbed, and waste (urea, uric acid) with water forms urine which is stored in the bladder. [53-56]

28. (a) What is the law of dominance of traits? Explain with an example.
(b) Why are the traits acquired during the lifetime of an individual not inherited? Explain.

Ans: (a) Law of dominance states that in a heterozygous condition, the dominant allele expresses itself over the recessive allele. Example: Tall pea plant (TT) crossed with dwarf (tt) produces all Tall (Tt) offspring.

(b) Acquired traits involve changes in non-reproductive tissues (somatic cells) and do not change the DNA of germ cells, so they are not inherited. [56, 57]

29. Draw a ray diagram in each of the following cases to show the formation of image, when the object is placed:
(i) between the optical centre and principal focus of a convex lens.

(ii) anywhere in front of a concave lens.

(iii) at 2F of a convex lens.

State the signs and values of magnifications in the above-mentioned cases.

Ans: (i) Image is virtual, erect, enlarged. Magnification > 1 (Positive).
(ii) Image is virtual, erect, diminished. Magnification < 1 (Positive).
(iii) Image is real, inverted, same size. Magnification = -1. [58-60]

Or
An object 4.0 cm in size, is placed 25.0 cm in front of a concave mirror of focal length 15.0 cm.
(i) At what distance from the mirror should a screen be placed in order to obtain a sharp image?
(ii) Find the size of the image.
(iii) Draw a ray diagram to show the formation of the image in this case.

Ans: (i) Using mirror formula: 1/v + 1/u = 1/f → 1/v - 1/25 = -1/15. v = -37.5 cm.
(ii) Magnification m = -v/u = -(-37.5)/(-25) = -1.5. Height = m * 4.0 = -6.0 cm (Inverted).
(iii) Ray diagram showing real, inverted, magnified image. [60, 61]

30. (a) What is an electromagnet? List any two uses.
(b) Draw a labelled diagram to show how an electromagnet is made.
(c) State the purpose of the soft iron core used in making an electromagnet.
(d) List two ways of increasing the strength of an electromagnet if the material of the electromagnet is fixed.

Ans: (a) A magnet produced by electric current flowing through a coil. Uses: Lifting heavy loads, transformers.

(b) Diagram showing coil around soft iron core connected to battery.
(c) Soft iron core increases the magnetic field strength significantly due to its high magnetic permeability.
(d) Increase the number of turns in the coil; Increase the current flowing through it. [61-63]

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