CBSE Class 10 Science Previous Year Question Paper 2015

SECTION - A

1. Write the number of covalent bonds in the molecule of butane, C₄H₁₀.

Ans: There are thirteen covalent bonds (ten C-H and three C-C bonds) present in a molecule of butane.

2. Name two simple organisms having the ability of regeneration.

Ans: Hydra and Planaria have the ability of regeneration.

3. Which of the following are always at the second tropic level of food chains?

Ans: Primary consumers (herbivores) are always at the second trophic level.

4. Draw a ray diagram to show the path of the reflected ray corresponding to an incident ray of light parallel to the principal axis of a convex mirror and show the angle of incidence and angle of reflection on it.

Ans: [Diagram required: A ray parallel to the principal axis diverges from the convex mirror, appearing to come from the focus behind the mirror. Angle i and r should be marked relative to the normal passing through the center of curvature].

5. Why is sustainable management of natural resources necessary? Out of the two—reuse and recycle—which, in your opinion, is better to practice? Give reason.

Ans: Sustainable management is necessary to preserve natural resources for future generations and control environmental pollution. Reusing is better than recycling because recycling requires a large amount of energy and money, whereas reusing simply involves using the same item again, creating less pollution.

6. What is meant by biodiversity? List two advantages of conserving forests and wild life.

Ans: Biodiversity refers to the existence of many different kinds of plants and animals in an environment. Advantages of conserving forests and wildlife: 1. They maintain the ecological balance and food chains. 2. They provide valuable products like wood, medicines, and other raw materials.

7. Write the name and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen is possible. State the essential condition for an addition reaction. Stating this condition, write a chemical equation giving the name of the reactant and the product of the reaction.

Ans: Addition reaction is possible in Alkenes (General formula: C_nH_2n) and Alkynes (C_nH_2n-2). Essential condition: Presence of unsaturation (double or triple bond) and a catalyst like Nickel (Ni) or Palladium (Pd). Equation: CH₂=CH₂ (Ethene) + H₂ &xrightarrow{Ni} CH₃-CH₃ (Ethane).

8. List two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed.

Ans: 1. Sodium Bicarbonate Test: Add NaHCO₃ to both. Carboxylic acid reacts to produce brisk effervescence of CO₂ gas. Alcohol does not react. 2. Litmus Test: Carboxylic acid turns blue litmus red. Alcohol has no effect on litmus paper.

9. Given below are some elements of the modern periodic table: ₄Be, ₉F, ₁₄Si, ₁₉K, ₂₀Ca. (i) Select the element that has one electron in the outermost shell and write its electronic configuration. (ii) Select two elements that belong to the same group. Give reasons for your answer. (iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?

Ans: (i) K (19) has 1 valence electron. Configuration: 2, 8, 8, 1. (ii) Be (4) [1] and Ca (20) [1, 2] belong to the same group (Group 2) because they both have 2 valence electrons. (iii) K (19) and Ca (20) belong to the same period (Period 4). K has a bigger atomic size than Ca because atomic size decreases across a period.

10. Taking the example of an element of atomic number 16, explain how the electronic configuration of the atom of an element relates to its position in the modern periodic table and how valency of an element is calculated on the basis of its atomic number.

Ans: Atomic number 16 (Sulphur). Configuration: 2, 8, 6. Relation to position: - Number of shells = 3, so Period = 3. - Valence electrons = 6, so Group = 10 + 6 = 16. Valency calculation: Since it has 6 valence electrons, it needs 2 more to complete octet. Valency = 8 - 6 = 2.

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) Crossing over during meiosis (gamete formation). (2) Random fusion of male and female gametes. (b) (i) A is Pollen grain. (ii) A reaches B (Stigma) by pollination. (iii) C is Pollen tube; it carries male gametes to the egg. (iv) D is the Egg cell; after fertilization, it develops into the zygote.

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

Ans: Reproduction is the biological process by which new individuals are produced from parents. It provides stability by ensuring the continuity of species, replacing dying individuals, and maintaining the population size amidst high death rates.

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 regrow lost parts or form a whole new organism from a fragment. In Hydra, if cut into pieces, specialized cells proliferate and form a mass of cells. These cells undergo differentiation to form various tissues and organs, 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. Similarities arise from common ancestry. The more characteristics two species share, the more closely they are related in evolutionary terms. Thus, classification is essentially a reflection of evolutionary relationships.

15. Explain with an example for each, how the following provides evidences in favour of evolution in organisms: (a) Homologous organs (b) Analogous organs (c) Fossils.

Ans: (a) Homologous organs: Organs with same basic structure but different functions (e.g., forelimbs of frog, bird, man). Indicate common ancestry. (b) Analogous organs: Organs with different structure but similar function (e.g., wings of insects and birds). Indicate adaptation to similar environments (convergent evolution). (c) Fossils: Preserved remains of past life (e.g., Archaeopteryx). They show transitions between groups (reptiles to birds) and how simple forms evolved into complex ones.

16. With the help of scattering of light, explain the reason for the difference in colours of the Sun as it appears during sunrise/sunset and noon.

Ans: At sunrise/sunset, sunlight travels a longer distance through the atmosphere. Shorter wavelengths (blue/violet) scatter away, leaving longer wavelengths (red/orange) to reach our eyes, making the Sun appear red. At noon, the Sun is overhead, and light travels a shorter distance, resulting in less scattering, so the Sun appears white.

17. An object of height 5 cm is placed perpendicular to the principal axis of a concave lens of focal length 10 cm. If the distance of the object from the optical centre of the lens is 20 cm, determine the position, nature and size of the image formed using the lens formula.

Ans: Given: h = +5 cm, f = -10 cm, u = -20 cm. Lens formula: 1/v - 1/u = 1/f ⇒ 1/v = 1/f + 1/u = 1/-10 - 1/20 = -3/20. v = -20/3 = -6.67 cm. Position: 6.67 cm in front of the lens. Nature: Virtual and erect (since v is negative). Magnification m = v/u = (-20/3) / -20 = 1/3. Size h' = m × h = 1/3 × 5 = 1.67 cm. (Diminished).

18. State the difference in colours of the Sun observed during sunrise/ sunset and noon. Give explanation for each.

Ans: Same as Question 16. Sunrise/Sunset: Red due to scattering of short wavelengths over long distance. Noon: White due to minimal scattering over short distance.

19. (a) What is an ecosystem? List its two main components. (b) We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly. Explain.

Ans: (a) An ecosystem is a self-sustaining unit consisting of living (biotic) and non-living (abiotic) components interacting with each other. Components: Biotic (plants, animals) and Abiotic (soil, air, water). (b) Ponds/lakes are natural ecosystems with natural decomposers that recycle waste and maintain balance. An aquarium is an artificial ecosystem; it may lack sufficient decomposers, leading to toxic waste accumulation, requiring manual cleaning.

20. (a) Write the function of placenta in females. (b) List four ways of preventing pregnancy. State two advantages of using such preventive methods.

Ans: (a) Placenta facilitates the supply of oxygen and nutrients to the embryo and removal of waste products into the mother's blood. (b) Ways: Barrier methods (condoms), Chemical methods (pills), IUCDs (Copper-T), Surgical methods. Advantages: Prevents unwanted pregnancies, protects against STDs (barrier methods), helps in family planning.

21. (a) List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function. (b) Write the function of retina in human eye. (c) Importance of eye donation.

Ans: (a) Iris and Pupil. The iris adjusts the size of the pupil. In bright light, the iris contracts the pupil to reduce light entry. In dim light, it expands the pupil to allow more light. (b) Retina acts as a screen to form the image. It contains light-sensitive cells (rods and cones) that convert light into electrical signals sent to the brain. (c) Eye donation gives vision to the corneal blind. It is a noble act to help others see the world.

22. (a) Explain terms: optical centre, centres of curvature, principal axis, aperture, principal focus, focal length. (b) A converging lens has focal length of 12 cm. Calculate at what distance the object should be placed from the lens so that it forms an image at 48 cm on the other side of the lens.

Ans: (a) Definitions of lens terms. (b) Converging (Convex) lens: f = +12 cm, v = +48 cm (real image). Formula: 1/v - 1/u = 1/f ⇒ 1/u = 1/v - 1/f = 1/48 - 1/12 = (1-4)/48 = -3/48 = -1/16. u = -16 cm. Object placed at 16 cm from lens.

23. (a) Draw a sectional view of human female reproductive system... (b) Describe, in brief, the changes the uterus undergoes (i) to receive the zygote. (ii) if fertilisation does not take place.

Ans: (a) [Diagram required]. (b) (i) To receive zygote: The uterus wall thickens and becomes richly supplied with blood vessels to nourish the embryo. (ii) If no fertilisation: The thick lining breaks down and comes out as blood and mucus through the vagina (menstruation).

24. Both soap and detergent are some type of salts. What is the difference between them? Describe in brief the cleansing action of soap. Why do soaps not form lather in hard water?

Ans: Difference: Soaps are sodium/potassium salts of fatty acids; Detergents are ammonium/sulphonate salts of long-chain carboxylic acids. Cleansing action: Soap molecules have a hydrophobic tail (attaches to dirt/oil) and hydrophilic head (attaches to water). They form micelles around dirt, lifting it off the fabric into the water. Hard water: Soaps react with Calcium and Magnesium ions in hard water to form insoluble precipitate called scum, preventing lather formation.

SECTION - B

25. A student traces the path of a ray of light through a rectangular glass slab... correct conclusion is: (A) angle i > angle r > angle e (B) angle i = angle e > angle r (C) angle i < angle r < angle e (D) angle i = angle e < angle r

Ans: (B) angle i = angle e > angle r. (Emergent ray is parallel to incident ray, and light bends towards normal in glass).

26. A student traces the path of a ray of light through a triangular glass prism... conclusion is: (B) The emergent ray bends at an angle to the direction of the incident ray.

Ans: (B). This angle is the angle of deviation.

27. A student obtains a sharp image of the distant window (W) on the screen (S) using a concave mirror (M). Which distance gives focal length? (B) MS.

Ans: (B) MS. Distance between mirror and screen (focus).

28. Out of the five incident rays shown in the figure... find three rays obeying laws of refraction for convex lens: (D) 2, 3 and 4.

Ans: (D) 2, 3 and 4. (Parallel to axis -> focus; Through optical center -> undeviated; Through focus -> parallel).

29. Select the best set-up for tracing path through glass slab: (a) I.

Ans: (a) I. (Pins should be sufficiently apart and not too close to edge/normal).

30. Dry raisins were soaked in water... extra water removed by: (d) Filter paper.

Ans: (d) Filter paper.

31. In an experiment to trace the path of a ray of light through a glass prism... emergent ray: (d) bends at an angle to the direction of the incident ray.

Ans: (d).

32. Which diagram correctly shows angle of refraction r? (d) IV.

Ans: (d) IV. (Angle between refracted ray and normal).

33. Correct conclusions: I. Potato and sweet potato are analogous. II. Wings of insects and birds are analogous. (d) I and III (Wait, options in source were I and II analogous/homologous mix). Based on source answer: (a) I and II (Source text logic: Potato/Sweet potato analogous; Wings insect/bird analogous).

Ans: (a) I and II. (Potato is stem, sweet potato is root - Analogous. Wings of insect/bird - Analogous). *Note: Source text says I and II are correct conclusions.*

34. Identify two vegetables representing homologous structures: (b) Radish and carrot.

Ans: (b) Radish and carrot. (Both are modified roots).

35. Parts marked A, B, C in dicot embryo diagram (Plumule, Radicle, Cotyledon). (a) Plumule, Radicle, Cotyledon.

Ans: (a) Plumule, Radicle, Cotyledon.

36. Correct observation for budding in yeast: (d) None of the above (Source text analysis). *Wait, Source text solution says (d) a, d, c, b for sequence Question 29? Let's check specific question text.* *Re-evaluating Question 36 text:* "Select correct statements...". Source solution: (d) None of the above? Or (d) IV, I and II? *Correction based on standard biology:* Budding involves bud arising from parent (I), nucleus dividing (III - sort of), bud detaching (IV). Source text solution is (d).

Ans: (d).

37. Correct sequence of binary fission in Amoeba diagrams: (b) II, III, IV, I (Source says c).

Ans: (c) II, III, IV, I. (Nucleus lengthens -> divides -> cytoplasm constricts -> two daughter cells).

38. Saponification reaction mixture litmus test: (a) Red litmus turns blue and reaction is exothermic (Source says c: Red turns blue, endothermic? No, saponification is exothermic usually, but source text solution says "endothermic and basic" in option c for Q33? Let's stick to source text for Q38/33). *Source Text Q33:* "reaction is endothermic and reaction mixture is basic" (Solution C). *Source Text Q38:* "Statements correct...". Solution (a) I and II. (Red turns blue, exothermic). *Conflict in source text*. Standard science: Saponification is exothermic. Soap is basic. Red turns blue. Blue remains blue. Ans: (a) I and II.

39. Observations for 20% NaOH solution: (a) I, II and IV. (Pellets, dissolves readily, red litmus blue).

Ans: (a) I, II and IV.

40. Salts to make water hard: (b) 1, 3 and 6. (Calcium Sulphate, Calcium Chloride, Magnesium Chloride).

Ans: (b) 1, 3 and 6.

41. Acetic acid + water observation: (a) Acetic acid dissolves readily in water.

Ans: (a) Dissolves readily.

42. Acetic acid + Sodium hydrogen carbonate observations: (d) IV, I and II. (Gas extinguishes splinter, colourless/odourless/brisk effervescence, turns lime water milky).

Ans: (d) IV, I and II.

Go to Link