CBSE Class 10 Science Previous Year Question Paper 2016

SECTION - A

1. Write the name and structure of an aldehyde with four carbon atoms in its molecule.

Ans: Name: Butanal.
Structure: CH₃-CH₂-CH₂-CHO

2. List two functions of ovary of human female reproductive system.

Ans: 1. To produce ova (female gametes).
2. To secrete the female hormones oestrogen and progesterone.

3. In a food chain of frog, grass, insect and snake, assign trophic level to frog.

Ans: Food chain: Grass → Insect → Frog → Snake.
Frog is a secondary consumer (Third Trophic Level).

4. The refractive indices of glass and water with respect to air are 3/2 and 4/3 respectively. If speed of light in glass is 2 × 10⁸ m/s, find the speed of light in water.

Ans: Given: n_g = 3/2, n_w = 4/3, v_g = 2 × 10⁸ m/s.
Speed of light in air, c = n_g × v_g = (3/2) × 2 × 10⁸ = 3 × 10⁸ m/s.
Speed of light in water, v_w = c / n_w = (3 × 10⁸) / (4/3) = (9/4) × 10⁸ = 2.25 × 10⁸ m/s.

5. List four stakeholders which may be helpful in the conservation of forests.

Ans: 1. The Forest Department of the Government.
2. People who live in and around the forest.
3. Industrialists who use forest products.
4. Forest and wildlife activists/Nature enthusiasts.

6. The construction of large dams leads to social and environmental problems. List two problems of each category.

Ans: Social problems: (1) Displacement of large number of tribals and peasants. (2) Lack of sufficient compensation or rehabilitation.
Environmental problems: (1) Deforestation. (2) Loss of biodiversity and ecological imbalance.

7. The position of eight elements in the Modern Periodic Table is given below where atomic numbers of elements are given in the parenthesis.
Period 2: Li(3), Be(4)
Period 3: Na(11), Mg(12)
Period 4: K(19), Ca(20)
Period 5: Rb(37), Sr(38)
(i) Write the electronic configuration of Ca.
(ii) Predict the number of valence electrons in Rb.
(iii) What is the number of shells in Sr?
(iv) Predict whether K is a metal or a non-metal.
(v) Which one of these elements has the largest atom in size?
(vi) Arrange Be, Ca, Mg and Rb in the increasing order of the size of their respective atoms.

Ans: (i) Ca (20): 2, 8, 8, 2.
(ii) Rb belongs to Group 1, so it has 1 valence electron.
(iii) Sr belongs to Period 5, so it has 5 shells.
(iv) K is a metal (Group 1).
(v) Rb has the largest atomic size (size increases down the group).
(vi) Be < Mg < Ca < Rb.

8. Write three different chemical reactions showing the conversion of ethanoic acid to sodium ethanoate. Write balanced chemical equation in each case.

Ans: 1. Reaction with Sodium Carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
2. Reaction with Sodium Bicarbonate: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
3. Reaction with Sodium Hydroxide (Base): CH₃COOH + NaOH → CH₃COONa + H₂O

9. An element `X’ belong to 3rd period and group 13 of the Modern Periodic Table.
(a) Determine the valence electrons and the valency of `X’.
(b) Molecular formula of the compound formed when `X’ reacts with an element `Y’ (atomic number = 8).
(c) Write the name and formula of the compound formed when `X’ combines with chlorine.

Ans: Element X is Aluminium (Al, Z=13).
(a) Valence electrons = 3; Valency = 3.
(b) Y is Oxygen (Z=8, valency 2). Formula: X₂Y₃ (Al₂O₃).
(c) With Chlorine (valency 1), formula is XCl₃ (Aluminium Chloride, AlCl₃).

10. An element `X’ has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of `X’. Also write group number, period number and valency of `X’.

Ans: Atomic Number = Mass Number - Number of Neutrons = 35 - 18 = 17.
Element X is Chlorine (Cl).
Electronic Configuration: 2, 8, 7.
Group Number: 17.
Period Number: 3.
Valency: 1 (It gains 1 electron).

11. (a) List two reasons for the appearance of variations among the progeny formed by sexual reproduction.
(b) (i) Name the part marked `A’ in the diagram (Pistil). (ii) How does `A’ reach part `B’? (iii) State the importance of the part `C’. (iv) What happens to the part marked `D’ after fertilisation is over?

Ans: (a) Reasons for variation: (1) New combinations of genes during gamete formation (meiosis/crossing over). (2) Random fusion of gametes from two different parents.
(b) (i) A is Pollen grain. (ii) A reaches B (Stigma) through pollination agents like wind, insects, etc. (iii) C is Pollen tube; it carries male gametes to the egg cell for fertilization. (iv) D is the Egg cell; after fertilization, it forms the Zygote.

12. Define reproduction. How does it help in providing stability to the population of species?

Ans: Reproduction is the process by which living organisms produce new individuals similar to themselves. It helps in providing stability to the population by ensuring continuity of species generation after generation, replacing dying individuals with new ones, and maintaining the specific chromosome number through mechanisms like meiosis and fertilization.

13. Explain the term “Regeneration” as used in relation to reproduction of organisms. Describe briefly how regeneration is carried out in multicellular organisms like Hydra.

Ans: Regeneration is the ability of an organism to give rise to a new organism from its body parts. In Hydra, if the body is cut into pieces, specialized cells proliferate and make a large number of cells. From this mass, different cells undergo differentiation to form various tissues and organs, eventually reconstructing the complete organism.

14. “Two areas of study namely `evolution’ and `classification' are interlinked”. Justify this statement.

Ans: Classification involves grouping organisms based on similarities in their characteristics. Characteristics that came into existence earlier are more basic than those that appeared later. This hierarchy of characteristics reflects the evolutionary relationships. Organisms with more common characteristics share a more recent common ancestor. Thus, classification is a reflection of their evolutionary history.

15. How do Mendel’s experiment show that traits are inherited independently?

Ans: Mendel performed a dihybrid cross with pea plants differing in two traits, e.g., Round Yellow seeds (RRYY) and Wrinkled Green seeds (rryy). In F1, all plants were Round Yellow (RrYy). In F2 generation, he observed new combinations of traits like Round Green and Wrinkled Yellow, along with the parental types. The appearance of new combinations (recombinants) and the phenotypic ratio of 9:3:3:1 showed that the factors for seed shape and seed color are inherited independently of each other.

16. The activities of man had adverse effects on all forms of living organisms in the biosphere... One of your classmates is an active member of `Eco club’...
(a) Why is it necessary to conserve our environment?
(b) State the importance of green and blue dust-bins in the safe disposal of the household waste.
(c) List two values exhibited by your classmate who is an active member of Eco-club of your school.

Ans: (a) To maintain the ecological balance, protect biodiversity, and ensure resources for future generations.
(b) Green bins are for biodegradable waste (wet waste) which can be composted. Blue bins are for non-biodegradable waste (dry waste) which can be recycled. Segregation helps in efficient waste management.
(c) (1) Environmental consciousness/awareness. (2) Sense of responsibility towards society and nature.

17. The image formed by a spherical mirror is real, inverted and is of magnification –2. If the image is at a distance of 30 cm from the mirror, where is the object placed? Find the focal length of the mirror. List two characteristics of the image formed if the object is moved 10 cm towards the mirror.

Ans: Given: m = -2, v = -30 cm (Real image).
m = -v/u ⇒ -2 = -(-30)/u ⇒ -2 = 30/u ⇒ u = -15 cm. Object is at 15 cm.
Focal length: 1/f = 1/v + 1/u = 1/-30 + 1/-15 = (-1-2)/30 = -3/30 = -1/10. f = -10 cm.
New object position: Moved 10 cm towards mirror ⇒ u' = -15 + 10 = -5 cm.
Since |u'| < |f| (5 < 10), the object is between pole and focus. Image characteristics: (1) Virtual and Erect (2) Magnified.

18. Describe an activity to show that colours of white light splitted by a glass prism can be recombined to get white light by another identical glass prism. Also draw ray diagram to show the recombination of the spectrum of white light.

Ans: Place a glass prism on a table. Allow a narrow beam of white light to fall on it. A spectrum is obtained. Place a second identical prism in an inverted position with respect to the first prism. The spectrum produced by the first prism enters the second prism. The second prism recombines the dispersed rays to form a beam of white light which emerges from the other side. This shows that white light is made of seven colours.

19. It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 cm.
(i) What should be the range of distance of an object placed in front of the mirror?
(ii) Will the image be smaller or larger than the object? Draw ray diagram to show the formation of image in this case.
(iii) Where will the image of this object be, if it is placed 24 cm in front of the mirror? Draw ray diagram for this situation also justify your answer.

Ans: (i) Range: 0 to 12 cm (Between Pole and Focus).
(ii) Image will be larger (magnified). [Ray diagram showing virtual erect image].
(iii) If u = 24 cm and f = 12 cm, object is at Centre of Curvature (C = 2f). Image will be formed at C (24 cm), Real, Inverted, and Same size.

20. Define evolution. How does it occur? Describe how fossils provide us evidences in support of evolution.

Ans: Evolution is the sequence of gradual changes which take place in primitive organisms over millions of years in which new species are produced. It occurs through processes like variation, natural selection, and genetic drift. Fossils are remains of past life. They provide evidence by showing the forms of life that existed in the past. By comparing fossils from different rock layers (older vs younger), we can see the transition from simple to complex forms (e.g., Archaeopteryx showing features of both reptiles and birds).

21. What is placenta? Describe its structure. State its functions in case of a pregnant human female.

Ans: Placenta is a special disc-like tissue embedded in the uterine wall. Structure: It contains villi on the embryo's side and blood spaces on the mother's side surrounding the villi. Functions: (1) Provides nutrients (glucose, oxygen) to the embryo from mother's blood. (2) Removes waste products generated by the embryo into mother's blood.

22. A carbon compound `P’ on heating with excess conc. H<sub>2</sub>SO<sub>4</sub> forms another carbon compound `Q’ which on addition of hydrogen in the presence of nickel catalyst forms a saturated carbon compound `R’. One molecule of `R’ on combustion forms two molecules of carbon dioxide and three molecules of water. Identify P, Q and R and write chemical equations for the reactions involved.

Ans: Since R gives 2 CO₂, R has 2 carbons. R is Ethane (C₂H₆).
Q is Ethene (C₂H₄), formed by hydrogenation of Q to R.
P is Ethanol (C₂H₅OH), which gives Ethene on dehydration.
Reactions:
1. CH₃CH₂OH &xrightarrow{Conc. H₂SO₄, Heat} CH₂=CH₂ + H₂O
2. CH₂=CH₂ + H₂ &xrightarrow{Ni} CH₃-CH₃
3. 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O (Combustion).

23. What is atmospheric refraction? Use this phenomenon to explain the following natural events:
(a) Twinkling of stars
(b) Advanced sun-rise and delayed sun-set.
Draw diagrams to illustrate your answers.

Ans: Atmospheric refraction is the refraction of light caused by the earth's atmosphere due to varying optical densities of air layers.
(a) Twinkling of stars: Starlight passes through changing atmospheric layers. The path of light rays fluctuates, and the amount of light entering the eye flickers, causing the twinkling effect.
(b) Advanced sunrise/Delayed sunset: Light from the sun below the horizon is refracted by the atmosphere, bending it towards the observer. Thus, we see the sun about 2 minutes before actual sunrise and 2 minutes after actual sunset.

24. (a) Define focal length of a divergent lens.
(b) A divergent lens of focal length 30 cm forms the image of an object of size 6 cm on the same side as the object at a distance of 15 cm from its optical centre. Use lens formula to determine the distance of the object from the lens and the size of the image formed.
(c) Draw a ray diagram to show the formation of image in the above situation.

Ans: (a) Distance between optical centre and principal focus of concave lens.
(b) Divergent lens (Concave): f = -30 cm, v = -15 cm (virtual image). h = 6 cm.
Lens formula: 1/v - 1/u = 1/f ⇒ 1/u = 1/v - 1/f = 1/-15 - 1/-30 = (-2+1)/30 = -1/30.
u = -30 cm. Object distance is 30 cm.
Magnification m = v/u = -15/-30 = 0.5.
Image size h' = m × h = 0.5 × 6 = 3 cm.

SECTION - B

25. A student while observing an embryo of a pea seed in the laboratory listed various parts... Select three correct parts from the above list: (d) Radicle, Cotyledon, Plumule.

Ans: (d) Radicle, Cotyledon, Plumule. (These are parts of the embryo).

26. If you are asked to select a group of two vegetables, out of the following, having homologous structures which one would you select?
(a) Carrot and radish (b) Potato and sweet potato...

Ans: (a) Carrot and radish. (Both are modified roots, similar origin).

27. In the following ray diagram (prism) the correctly marked angle are:
(d) ∠r, ∠A and ∠D

Ans: (d) ∠r, ∠A and ∠D.

28. In your laboratory you trace the path of light rays through a glass slab... correct conclusion is:
(a) ∠i is more than ∠r, but nearly equal to ∠e

Ans: (a) ∠i is more than ∠r, but nearly equal to ∠e.

29. To determine the approximate value of the focal length of a given concave mirror, you focus the image of a distant object formed by the mirror on a screen. The image obtained on the serene, as compared to the object is always:
(b) Inverted and diminished

Ans: (b) Inverted and diminished.

30. Suppose you have focused on a screen the image of candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching your laboratory table, on the same screen, what you are expected to do is to move the:
(a) lens slightly towards the screen

Ans: (a) lens slightly towards the screen. (Candle image distance v > f; Sun image distance v = f. So v must decrease).

31. For preparing soap in the laboratory we require an oil and a base... (a) Castor oil and calcium hydroxide (b) Turpentine oil and sodium hydroxide (c) Castor oil and sodium hydroxide...

Ans: (c) Castor oil and sodium hydroxide. (Vegetable oil + strong base).

32. A student puts a drop of reaction mixture of a saponification reaction first a blue litmus paper and then on a red litmus paper. He may observe that:
(c) There is no change in the blue litmus paper and the red litmus paper turns blue.

Ans: (c) There is no change in the blue litmus paper and the red litmus paper turns blue. (Soap is basic).

33. In the neighbourhood of your school, hard water required for an experiment is not available. Select from the following group of salts... to make it hard:
(d) Calcium chloride, magnesium chloride

Ans: (d) Calcium chloride, magnesium chloride. (Calcium and Magnesium salts cause hardness).

34. A student is observing a permanent slide showing sequentially the different stages of asexual reproduction taking place in yeast. Name this process and draw diagrams, of what he observes, in a proper sequence.

Ans: Process is Budding. [Diagram showing parent cell, bud formation, nucleus division, and chain of buds].

35. An object of height 2.5 cm is placed at a distance of 15 cm from the optical centre `O’ of a convex lens of focal length 10 cm. Draw a ray diagram to find the position and size of the image formed.

Ans: u = -15 cm, f = 10 cm.
1/v - 1/u = 1/f ⇒ 1/v = 1/10 - 1/15 = 1/30. v = 30 cm.
m = v/u = 30/-15 = -2. Size h' = -2 × 2.5 = -5 cm.
[Ray diagram showing real, inverted, magnified image beyond 2F].

36. A student adds a spoon full of powdered sodium hydrogen carbonate to a flask containing ethanoic acid. List two main observations... Also write chemical equation.

Ans: Observations: (1) Brisk effervescence (evolution of gas). (2) Gas turns lime water milky (CO₂).
Equation: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂.

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