Question Paper

CBSE Board 2020

Section B

3 Marks Questions

Question 15: Complete and balance the following chemical equations:

(a) NaOH(aq) + Zn(s)

Answer: 2NaOH + Zn → Na2ZnO2 + H2

(b) CaCO3(s) + H2O(l) + CO2(g)

Answer: CaCO3 + CO2 + H2O → Ca(HCO3 )2

(c) HCl(aq) + H2O(l)

Answer: HCl(aq) + H2O(l) → H3O+ + Cl-


During electrolysis of brine, a gas ‘G’ is liberated at anode. When this gas ‘G’ is passed through slaked lime a compound ‘C’ is formed, which is used for disinfecting drinking water.

(a) Write formula of ‘G’ and ‘C’

Answer: ‘G’ is Chlorine (Cl) and ‘C’ is bleaching powder (CaOCl2)

(b) State the chemical equation involved

Answer: Ca(OH)2 + Cl2 → CaOCl2 + H2O

(c) What is common name of compound ‘C’? Give is chemical name.

Answer: Common name is bleaching powder and chemical name is calcium oxychloride

Question 16: Mention with reason the colour changes observed when:

(a) Silver chloride is exposed to sunlight.

Answer: Photo decomposition of silver chloride takes place which results in formation of silver and colour changing to grey

(b) Copper powder is strongly heated in the presence of oxygen.

Answer: Copper undergoes oxidation and black coloured copper oxide is formed.

(c) A piece of zinc is dropped in copper sulphate solution.

Answer: Zinc displaces copper and solution changes from blue to colourless

Question 17: Three elements X, Y and Z have atomic numbers 7, 8 and 9 respectively.

(a) Arrange them in the decreasing order of their atomic radii.

Answer: 9 > 8 > 7

(b) Which of the three is most electronegative? Why?

Answer: Z is most electronegative because electronegativity increases when we move from left to right across a period.

(c) Write the formula of compound formed between (i) X and Y and (ii) X and Z

Answer: (i) NO2 (ii) NF3

Question 18: (a) State the role played by the following in the presence of digestion.

(i) Enzyme trypsin
(ii) Enzyme lipase

Answer: Trypsin digests protein, while lipase digests lipid (fat)

(b) List two functions of finger like projections present in the small intestine.

Answer: Finger like projections or villi increase the surface area for absorption and absorb digested food.

Question 19: (a) Define ecosystem.

Answer: The system of interdependency of living beings and non-living components in a given geographical enclosure is called ecosystem.

(b) Autotrophs are at the first level of food chain. Give reason.

Answer: Autotrophs are producers and hence they are at first level of food chain. In other words, food chain begins from autotrophs.

(c) In a food chain of frogs, grass, insects and snakes assign trophic level to frogs. To which category of consumers do they belong to?

Answer: Frog is at second trophic level and is primary consumer.


(a) Explain the role of UV radiation in producing ozone layer.

Answer: Ultraviolet radiation acts on oxygen to produce ozone.

(b) Mention the reaction involved.


chemical equation showing ozone formation

(c) Why is excessive use of CFCs a cause of concern?

Answer: CFCs can convert ozone into oxygen. It results in depletion of ozone layer. So, excessive use of CFCs is a cause of concern.

Question 20: List three factors that could lead to speciation. Which of these cannot be a major factor in the speciation of a self-pollinating plant species and why?

Answer: Three factors that could lead to speciation are: Genetic drift, natural selection and geographical isolation. Since self-pollinating plant does not need another plant to produce offsprings, hence geographical isolation cannot be a major factor in speciation of such plants.

Question 21: A green stemmed rose plant denoted by GG and a brown stemmed rose plant denoted by gg are allowed to undergo a cross with each other.

(a) List your observations regarding

  1. Colour of stem in their F1 progeny.
  2. Percentage of brown stemmed plants in F2 progeny if F1 plants are self pollinated.
  3. Ratio of GG and Gg in the F2 progeny

(b) Based on the finding of this cross, what conclusion can be drawn?

Answer: Following is the Punnette Square for F1 progeny


So, all plants of F1 generation will have green stem

Following the Punnette Square for F2 progeny


25% plants in F2 generation will have brown stem (gg)

Ratio of GG and Gg in F2 generation is 1 : 2

Conclusion from this cross: Every individual possesses a pair of alleles for a particular trait. During gamete formation, a gamete receives only one trait from the alleles. A particular trait can be dominant or recessive in a particular generation.

Question 22: (a) Water has refractive index 1.33 and alcohol has refractive index 1.36. Which of the two media is optically denser? Give reason for your answer.

Answer: The medium with higher refractive index is optically denser. So, alcohol is optically denser than water.

(b) Draw a ray diagram to show the path of a ray of light passing obliquely from water to alcohol.

Answer: When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal. You can make a diagram based on this concept. (Same diagram as in case of light going from air to glass).

(c) State the relationship between angle of incidence and angle of refraction in the above case.

Answer: In all cases following relationship between angle of incidence and angle of refraction holds true:


Question 23: (a) With the help of a labeled ray diagram show the path followed by a narrow beam of monochromatic light when it passes through a glass prism.


monochromatic light passing through prism

(b) What would happen if this beam is replaced by a narrow beam of white light?

Answer: When a narrow beam of white light passes through a prism, it emerges as a spectrum of all components of white light.


(a) A person is suffering from both myopia and hypermetropia.

(i) What kind of lenses can correct this defect?

Answer: Concave lens corrects myopia, while convex lens corrects hypermetropia, so a bifocal lens is required to rectify this defect.

(ii) How are these lenses prepared?

Answer: Lenses are prepared either from glass or transparent plastic. Suitable shape is given with the help of grinders. Bifocal lens is made by using two lenses in one eyepiece. Convex lens is positioned below while concave lens is positioned above.

(b) A person needs a lens of power +3D for correcting his near vision and -3D for correcting his distant vision. Calculate the focal lengths of the lenses required to correct these defects.

Answer: We know that Power = `1/f`

Or, `f = 1/D`

So, focal length of lens for near vision `=1/3=+0.33` m

Focal length of lens for distant vision `=(-1)/3=-0.33` m

Question 24: Give reasons for the following:

(a) There is either a convergence of a divergence of magnetic field lines near the ends of a current carrying straight solenoid.

Answer: If magnetic field lines are near the periphery of solenoid they will converge. If magnetic field lines are near the centre of solenoid they will diverge and will almost appear as straight lines. It happens because magnetic field lines make larger rings as we move towards the centre of solenoid.

(b) The current carrying solenoid when suspended freely rests along a particular direction.

Answer: A current carrying solenoid behaves like a magnet. Hence, it rests along north-south direction when suspended freely.

(c) The burnt out fuse should be replaced by another fuse of identical rating.

Answer: Fuse of lower rating will blow off immediately; requiring frequent replacements. Fuse of higher rating will not break the circuit, even in case of higher load. So, burnt out fuse should be replaced by another fuse of identical rating.

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