CBSE Class 10 Science Previous Year Question Paper 2017

SECTION - A

1. Write the molecular formula of the 2nd and 3rd member of the homologous series where the first member is ethyne.

Ans: The first member is ethyne (C₂H₂). The homologous series differs by -CH₂- unit.
2nd member: Propyne (C₃H₄).
3rd member: Butyne (C₄H₆).

2. Why is variation important for a species?

Ans: Variation is important because it increases the chances of survival of a species in a constantly changing environment. It allows species to adapt to new conditions.

3. In the following food chain, 20,000 J of energy was available to the plants. How much energy would be available to man in this chain?
Plants → Sheep → Man

Ans: According to the 10% law, only 10% of energy is transferred to the next trophic level.
Plants (20,000 J) → Sheep (2,000 J) → Man (200 J).
Note: The source text calculates "2 J" but standard application of 10% law (20000 -> 2000 -> 200) yields 200 J. However, based strictly on the source text provided, it states "2 J".

4. An object is placed at a distance of 15 cm from a concave lens of focal length 30 cm. List four characteristic (nature, position, etc.) of the image formed by the lens.

Ans: Given: u = -15 cm, f = -30 cm.
Lens formula: 1/v - 1/u = 1/f ⇒ 1/v = 1/f + 1/u = 1/-30 - 1/15 = -3/30 = -1/10.
v = -10 cm.
Characteristics: 1. Virtual 2. Erect 3. Diminished (smaller than object) 4. Formed at a distance of 10 cm from the lens on the same side as the object.

5. You being an environmentalist are interested in contributing towards the conservation of nature resources. List four activities that you can do on your own.

Ans: 1. Using public transport instead of personal vehicles.
2. Avoiding the use of clothes/accessories made of animal skin.
3. Using energy-efficient appliances to save electricity.
4. Ensuring no leakage of taps to save water.

6. Why are coal and petroleum categorized as natural resources? Given a reason as to why they should be used judiciously.

Ans: They are formed by natural processes involving the degeneration of dead biomass buried deep in the earth over millions of years. They should be used judiciously because they are non-renewable; their formation takes millions of years while their consumption rate is very high, leading to depletion.

7. Distinguish between esterification and saponification reactions with the help of the chemical equations for each. State one use of each (i) esters, and (ii) saponification process.

Ans: Esterification: Reaction of carboxylic acid with alcohol to form ester.
Equation: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (In presence of conc. H₂SO₄).
Use of Esters: Used in perfumes and flavouring agents.
Saponification: Conversion of ester back to alcohol and sodium salt of acid using a base.
Equation: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa.
Use of Saponification: Preparation of soaps.

8. Write the structural formula of ethanol. What happens when it is heated with excess of conc. H₂SO₄ at 443 K? Write the chemical equation for the reaction stating the role of conc. H₂SO₄ in this reaction.

Ans: Structural formula: CH₃-CH₂-OH.
Reaction: Heating ethanol with excess conc. H₂SO₄ at 443 K results in dehydration to form ethene.
Equation: CH₃CH₂OH &xrightarrow{Conc. H₂SO₄} CH₂=CH₂ + H₂O.
Role of H₂SO₄: It acts as a dehydrating agent.

9. What is periodicity in properties of elements with reference to the Modern Periodic Table? Why do all the elements of the same group have similar properties? How does the tendency of elements to gain electrons change as we move from left to right in a period? State the reason of this change?

Ans: Periodicity refers to the recurrence of similar properties of elements at regular intervals. Elements of the same group have similar properties because they have the same number of valence electrons. The tendency to gain electrons increases from left to right in a period due to increased effective nuclear charge and decreased atomic size, which pulls electrons more strongly.

10. Write the electronic configuration two elements X and Y whose atomic numbers are 20 and 17 respectively. Write the Molecular formula of the compound formed when element X reacts with element Y. Draw electron-dot structure of the product and also state the nature of the bond formed between both the elements.

Ans: X (Z=20): 2, 8, 8, 2 (Calcium). Valency = +2.
Y (Z=17): 2, 8, 7 (Chlorine). Valency = -1.
Molecular Formula: XY₂ (CaCl₂).
Bond Nature: Ionic Bond (Electrovalent bond).
[Electron dot structure would show transfer of 2 electrons from X to two Y atoms].

11. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?

Ans: Mendel explained this through his monohybrid cross. When he crossed pure tall plants (TT) with pure dwarf plants (tt), the F1 generation were all tall (Tt). The dwarf trait was inherited but not expressed in the presence of the dominant tall trait. It only reappeared in the F2 generation when two recessive alleles came together (tt).

12. What is an organic evolution? It cannot be equated with progress. Explain with the help of a suitable example.

Ans: Organic evolution is the gradual change in the characteristics of populations of organisms over successive generations. It cannot be equated with progress because older, simpler designs are not necessarily inefficient or eliminated. For example, bacteria (simple structure) still inhabit diverse and harsh environments like hot springs and ice, surviving alongside complex organisms like humans. Complexity does not always mean 'better' survival.

13. List the two types of reproduction. Which one of the two is responsible for bringing in more variations in its progeny and how?

Ans: The two types are Sexual and Asexual reproduction. Sexual reproduction brings more variations because it involves the fusion of gametes from two different parents. The process of gamete formation (meiosis) and DNA copying introduces errors and new combinations of genes, leading to diverse progeny.

14. What is vegetative propagation? State two advantages and two disadvantages of this method.

Ans: Vegetative propagation is a mode of asexual reproduction where new plants are produced from vegetative parts like roots, stems, or leaves (e.g., Bryophyllum).
Advantages: (1) Plants genetically identical to parent (preserves desirable traits). (2) Faster method of reproduction; plants bear flowers/fruits earlier.
Disadvantages: (1) No genetic variation (susceptible to same diseases). (2) Overcrowding may occur around the parent plant.

15. List three techniques that have been developed to prevent pregnancy. Which one of these techniques is not meant for males? How does the use of these techniques have a direct impact on the health and prosperity of a family?

Ans: Techniques: (1) Barrier methods (Condoms), (2) Chemical methods (Oral pills), (3) Intra-uterine devices (Copper-T), (4) Surgical methods (Vasectomy/Tubectomy).
Technique not for males: Intra-uterine devices (IUDs) and Oral pills (generally).
Impact: Prevents unwanted pregnancies, allowing for gap between children. This improves maternal health and allows parents to provide better resources and education to children, leading to family prosperity.

16. “A lens can form a magnified erect image as well as magnified inverted image of an object placed in front of it”. State the nature of this lens and draw ray diagrams to justify the above statement. Mark the positions of O, F and 2F in the diagram.

Ans: The lens is a Convex Lens.
Case 1 (Magnified Erect): Object placed between Optical Centre (O) and Focus (F). Image is virtual, erect, magnified.
Case 2 (Magnified Inverted): Object placed between F and 2F. Image is real, inverted, magnified (formed beyond 2F).

17. What is “dispersion of white light”? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why it is essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other?

Ans: Dispersion is the splitting of white light into its constituent seven colours. Recombination is shown by placing a second identical prism inverted with respect to the first. The first prism disperses light, and the second inverted prism recombines the dispersed rays back into white light because the refraction produced by the second prism is equal and opposite to the first.

18. (a) Water is an elixir of life a very important natural resource. Your science teacher wants you to prepare a plan for a formative assessment activity, “How to save water, the vital natural resource”. Write any two ways that you will suggest to bring awareness in your neighborhood, on how to save water’.
(b) Name and explain any one way by which the underground water table does not go down further.

Ans: (a) Awareness methods: (1) Campaigning about consequences of drought and water scarcity. (2) Teaching methods to reuse water (e.g., washing water for plants) and fix leaks.
(b) Rainwater Harvesting (or Khadin system): Collecting rainwater and directing it to recharge groundwater levels instead of letting it run off. For example, using embankments to hold water which seeps into the ground.

19. With the help of one example for each, distinguish between the acquired traits and the inherited traits. Why are the traits/experiences acquired during the entire lifetime of an individual not inherited in the next generation? Explain the reason of this fact with an example.

Ans: Acquired Traits: Developed during lifetime due to environment/use. Example: Muscles of a wrestler, swimming skills. These involve somatic cells only.
Inherited Traits: Received from parents via genes. Example: Eye color, skin color. These involve germ cells.
Reason: Acquired traits cause changes in non-reproductive tissues (somatic cells) and do not alter the DNA of germ cells (sperm/egg). Only changes in germ cell DNA are passed to progeny. Example: If a mouse loses its tail, its offspring will still be born with tails.

20. (a) Write the functions of each of the following parts in a human female reproductive system: (i) Ovary (ii) Uterus (iii) Fallopian tube.
(b) Write the structure and functions of placenta in a human female.

Ans: (a) (i) Ovary: Produces eggs (ova) and female hormones (oestrogen, progesterone). (ii) Uterus: Supports and nourishes the developing foetus/embryo. (iii) Fallopian tube: Site of fertilisation; transfers egg to uterus.
(b) Placenta: A disc-like structure embedded in the uterine wall. Contains villi on the embryo side and blood spaces on the mother's side. Function: Facilitates supply of glucose and oxygen to the embryo and removal of waste (CO₂, excreta) from embryo to mother's blood.

21. Why certain compounds are called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur.

Ans: Compounds made of only carbon and hydrogen are called hydrocarbons.
Alkanes: C_nH_2n+2 (Methane, CH₄).
Alkenes: C_nH_2n (Ethene, C₂H₄).
Alkynes: C_nH_2n-2 (Ethyne, C₂H₂).
Reaction: Hydrogenation (Addition reaction).
Equation: CH₂=CH₂ + H₂ &xrightarrow{Ni/Pd catalyst, Heat} CH₃-CH₃.

22. (a) A student suffering from myopia is not able to see distinctly the object placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams explain.
(i) Why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.

Ans: (a) Reasons: Excessive curvature of the eye lens or elongation of the eyeball.
(i) In myopia, the image of distant objects is formed in front of the retina, making them appear blurry.
(ii) Corrective lens: Concave lens. It diverges light rays before they enter the eye so they focus on the retina.
(b) Given Far point = 5 m. For correction, focal length f = -5 m (Concave lens).
Power P = 1/f = 1/-5 = -0.2 D.

23. Analyse the following observation table showing variation of image distance (v) with object distance (u) in case of a convex lens and answer the questions that follow without doing any calculations:
S. No | u (cm) | v (cm)
1 | -100 | +25
2 | -60 | +30
3 | -40 | +40
4 | -30 | +60
5 | -25 | +100
6 | -15 | +120
(a) What is the focal length of the convex lens? Give reason to justify your answer.
(b) Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?
(c) Select an appropriate scale and draw a ray diagram for the observation at S. No. 2. Also find the approximate value of magnification.

Ans: (a) In observation 3, u = -40 and v = +40. When |u| = |v|, the object is at 2F. So 2f = 40 cm, thus focal length f = 20 cm.
(b) S. No. 6 is incorrect. The object distance |u|=15 cm is less than focal length (20 cm). For a convex lens, if u < f, the image must be virtual and formed on the same side (negative v), but the table shows v = +120 (real image).
(c) Magnification for S. No. 2: m = v/u = 30/-60 = -0.5.

24. (a) If the image formed by a mirror for all position of the object placed in front of it is always diminished, erect and virtual, state the type of the mirror and also draw a ray diagram to justify your answer. Write one use such mirrors are put to and why.
(b) Define the radius of curvature of spherical mirrors. Find the nature and focal length of a spherical mirror whose radius of curvature is +24 cm.

Ans: (a) The mirror is a Convex Mirror. Use: Rear-view mirrors in vehicles because they give a wider field of view and always form erect images.
(b) Radius of curvature is the radius of the sphere of which the mirror forms a part. Given R = +24 cm. Focal length f = R/2 = +12 cm. Positive sign indicates it is a Convex Mirror.

SECTION - B

25. Study the following ray diagram:
In this diagram, the angle of incidence, the angle of emergence and the angle of deviation respectively have been represented by:
(A) y, p, z (B) x, q, z (C) p, y, z (D) P, z, y

Ans: (A) y, p, z.
(Note: Based on standard prism diagrams, Angle of incidence is between incident ray and normal, emergence between emergent ray and normal, deviation between extended incident and emergent rays).

26. A student very cautiously traces the path of a ray through a glass slab... On analysing these measurements of angles, his conclusion would be:
(A) i > r > e (B) i = e > r (C) i < r < e (D) i = e < r

Ans: (B) i = e > r. Angle of incidence equals angle of emergence, and both are greater than angle of refraction (as light goes air->glass).

27. Study the given ray diagrams and select the correct statement... (Device X converges parallel rays, Device Y reflects parallel rays to a point in front).

Ans: (D) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

28. A student obtains a blurred image of a distant object on a screen using a convex lens. To obtain a distinct image on the screen he should move the lens...

Ans: (B) towards the screen. (If the image is blurred, and object is distant, image forms at Focus. If screen is farther, lens must move towards it).

29. While studying the saponification reaction, what do you observe when you mix an equal amount of colorless vegetable oil and 20% aqueous solution of NaOH in a beaker?

Ans: The outer surface of the beaker has become hot (Exothermic reaction).

30. When you add a few drops of acetic acid to a test-tube containing sodium bicarbonate powder, which one of the following is your observation?

Ans: (D) Formation of bubbles of a colorless and odorless gas (CO₂).

31. A student requires hard water... Select from the following groups of salts, a group, each salt of which when dissolved in distilled water will make it hard.

Ans: (D) Calcium sulphate, Calcium chloride. (Salts of Ca and Mg cause hardness).

32. To perform an experiment to identify the different parts of an embryo of a dicot seed, first of all you require a dicot seed. Select dicot seeds from the following group:

Ans: (B) Gram, Pea and Ground-nut.

33. Which two of these vegetables correctly represent the homologous structures?

Ans: (C) Radish and Carrot. (Both are modified roots).

34. Draw in sequence (showing the four stages), the process of binary fission in Amoeba.

Ans: [Diagrams showing: Parent Amoeba -> Elongation of Nucleus -> Division of Nucleus -> Division of Cytoplasm -> Two daughter cells].

35. A student focuses the image of a candle flame... (Lens movement directions etc).

Ans: (A) He moves the lens away from the screen (as object moves closer, image moves away).
(B) Size of image increases.
(C) Intensity decreases.
(D) No image on screen (Virtual image formed when u < f).

36. Mention the essential material (chemicals) to prepare soap in the laboratory. Describe in brief the test of determining the nature (acidic/alkaline) of the reaction mixture of saponification reaction.

Ans: Chemicals: Vegetable oil, Sodium Hydroxide (NaOH), Salt (NaCl).
Test: Dip red litmus paper into the mixture. It turns blue, indicating the mixture is alkaline (basic) due to excess NaOH/formation of soap.

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