31/ 3/ 3 2020 Class 10 Science Question Paper Solution
Section – A
Note : For question numbers 1 and 2, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
(a) Both (A) and (R) are true and (R) is correct explanation of the assertion.
(b) Both (A) and (R) are true but (R) is not the correct explanation of the assertion.
(c) (A) is true but (R) is false.
(d) (A) is false but (R) is true.
(1) Assertion (A) : Following are the members of a homologous series: CH3OH, CH3CH2OH, CH3CH2CH2OH
Reason (R) : A series of compounds with same functional group but differing by – CH2 – unit is called a homologous series.
Ans: Both (A) and (R) are true and (R) is the correct explanation of the assertion
(2) Assertion (A) : Alloys are commonly used in electrical heating devices like electric iron and heater.
Reason (R) : Resistivity of an alloy is generally higher than that of its constituent metals but the alloys have low melting points then their constituent metals.
Ans: A is true but R is false.
(3) How are covalent bonds formed?
Ans: Covalent bonds are formed by sharing of electron pair /pairs between two atoms.
(4) Define electropositivity.
Ans: Tendency of an element to lose electrons.
OR
The atomic radii of first group elements are given below :
Group-I element | Atomic Radii (pm) |
Na | 86 |
K | 231 |
Rb | 244 |
Cs | 282 |
State the reason behind the observed trend in the above elements.
Ans: Atomic radii increases from Na to Cs due to addition of new shells.
(5) A cylindrical conductor of length ‘l’ and uniform area of cross section ‘A’ has resistance ‘R’. The area of cross section of another conductor of same material and same resistance but of length ‘2l’ is
(a) A/2
(b) 3A/ 2
(c) 2A
(d) 3A
Ans: 2A
(6) The maximum resistance which can be made using four resistors each of resistance 1/2 W is
(a) 2 W
(b) 1 W
(c) 2.5 W
(d) 8 W
Ans: 2 W
(7) The sky appears dark to passengers flying at very high altitudes mainly because:
(a) Scattering of light is not enough at such heights.
(b) There is no atmosphere at great heights.
(c) The size of molecules is smaller than the wavelength of visible light.
(d) The light gets scattered towards the earth.
Ans: Scattering of light is not enough at such heights
(8) An element ‘X’ is forming an acidic oxide. Its position in modern periodic table will be
(a) Group 1 and Period 3
(b) Group 2 and Period 3
(c) Group 13 and Period 3
(d) Group 16 and Period 3
Ans: Group 16 and Period 3
OR
Consider the following statements about an element ‘X’ with number of protons 13.
(A) It forms amphoteric oxide
(B) Its valency is three
(C) The formula of its chloride is XCl3
The correct statements(s) is/are
(a) only (A)
(b) only (B)
(c) (A) and (C)
(d) (A), (B) and (C)
Ans: (d) / (A), (B) & (C)
(9) An aqueous solution ‘A’ turns phenolphthalein solution pink. On addition of an aqueous solution ‘B’ to ‘A’, the pink colour disappears. The following statement is true for solution ‘A’ and ‘B’.
(a) A is strongly basic and B is a weak base.
(b) A is strongly acidic and B is a weak acid.
(c) A has pH greater than 7 and B has pH less than 7.
(d) A has pH less than 7 and B has pH greater than 7.
Ans: A has pH greater than 7 and B has pH less than 7.
(10) In which of the following, the identity of initial substance remains unchanged ?
(a) Curdling of milk
(b) Formation of crystals by process of crystallisation
(c) Fermentation of grapes
(d) Digestion of food
Ans: Formation of crystals by process of crystallisation.
(11) Several factories were pouring their wastes in rivers A and B. Water samples were collected from these two rivers. It was observed that sample collected from river A was acidic while that of river B was basic. The factories located near A and B are
(a) Soaps and detergents factories near A and alcohol distillery near B.
(b) Soaps and detergents factories near B and alcohol distillery near A.
(c) Lead storage battery manufacturing factories near A and soaps and detergents factories near B.
(d) Lead storage battery manufacturing factories near B and soaps and detergents factories near A.
Ans: Lead storage battery manufacturing factories near A and soaps and detergents factories near B.
(12) A diagram of traditional water harvesting system is given below : The statement which defines the system and its parts is
(a) This is an ideal setting of the Khadin system and A = Catchment area; B = Saline area & C = Shallow dugwell
(b) This is an ideal setting of the Shallow dugwell system and A = Catchment area; B = Saline area and C = Khadin
(c) This is an ideal setting of Catchement area and A = Khadin, B = Saline area and C = Shallow dugwell
(d) This is showing Saline area and A = Catchment area; B = Khadin and C = Shallow dugwell
Ans: This is an ideal setting of the Khadin system and A= catchment area; B= Saline area ; C=Shallow dugwell.
OR
The major ill effect of mono culture practice in forests is on the
(a) biodiversity which faces large destruction
(b) local people whose basic needs can no longer be met from such forests
(c) industries
(d) forest department
Ans: biodiversity which faces large destruction.
(13) Answer question numbers 4(a) to 4(d) on the basis of your understanding of the following paragraph and the related studies concepts.
The Tehri dam is the highest dam in India and one of the highest in the World. The Tehri dam withholds a reservoir of capacity 4.0 km3 and surface area 52 km2 . It is used for irrigation, municipal water supply and the generation of 1000 MW of hydro electricity.
The Tehri Dam has been the object of protests. Environment activist Shri Sunder Lal Bahuguna led the “Anti Tehri Dam Movement” from 1980s to 2014. The protest was against the displacement of town inhabitants and environmental consequences of the weak ecosystem. The relocation of more than 1,00,000 people from the area has led to protracted legal battles over resettlement rights and ultimately resulted in the delayed completion of the project.
(a) How is hydropower harnessed?
Ans: Hydropower is harnessed by converting the potential energy of falling water from a height into electricity.
(b) Define 1 MW.
Ans: It is the power developed when 106 J of work is done per second. / 1MW = 106 watts.
(c) Mention two disadvantages of constructing Tehri Dam.
Ans: Loss of agricultural land / displacement of a large number of peasants and tribals/ destruction of ecosystem.
(d) What happens when water from great heights is made to fall on blades of turbine?
Ans: The blades of turbine move the armature of a generator with high speed to generate electricity.
Questions numbers 3(a) to 3(d) are based on table given below. Study the table in which the levels of Thyroid Stimulating Hormone (TSH) in women are given and answer the questions that follow on the basis of understanding of the following paragraph and the related studied concepts.
Age Range | Normal (mU/L) | Low (mU/L) |
18 – 29 years | 0.4 – 2.34 mU/L | < 0.4 mU/L |
30 – 49 years | 0.4 – 4.0 mU/L | < 0.4 mU/L |
50 – 79 years | 0.46 – 4.68 mU/L | < 0.46 mU/L |
Women are at greater risk for developing abnormal TSH levels during menstruation, while giving birth and after going through menopause. Around 5% of women in the United States have some kind of thyroid problem compared to 3% of men. Despite claims that high TSH increases your risk for heart disease, a 2013 study found no link between high TSH and heart diseases. But a 2017 study showed that older women are especially at risk for developing thyroid cancer if they have high TSH levels along with thyroid nodules.
(a) A 35 year old woman has TSH level 6.03 mU/L. What change should she bring in her diet to control this level?
Ans: She should monitor iodine intake in her diet.
(b) When do women face a greater risk of abnormal TSH level?
Ans: During menstruation / during pregnancy and after going through menopause.
(c) State the consequence of low TSH level.
Ans: Low TSH level leads to swelling of neck region / disease called goiter.
(d) Name the mineral that is responsible for synthesis of hormone secreted by thyroid gland.
Ans: Iodine
Section – B
(15) Identity the type of each of the following reactions. Also write balanced chemical equation for each reaction.
(i) A reaction in which the reaction mixture becomes warm.
Ans: Exothermic
CaO + H2O → Ca (OH)2
(ii) A reaction in which an insoluble substance is formed.
Ans: Double displacement reaction
Na2SO4 + BaCl2 → Ba SO4 + 2NaCl
(16) Study the data of the following three categories A, B and C.
Category | Name of the element | Atomic Mass |
A | Li
Na K |
7
23 39 |
B | N
P As |
14
31 74 |
C | B
Al Ga |
10.8
27 69.7 |
(i) From the given three categories A, B and C, pick the one which forms Dobereiner’s Triads.
Ans: Category A / Li, Na, K
(ii) Why did Mendeleev placed elements of category A, B and C in three different groups?
Ans: Because the physical as well as chemical properties of elements of category A,B and C are different.
(iii) Is Newland law of octaves applicable to all the three categories ? Give reason to justify your answer.
Ans: No
Reason : Because Newlands’ law of octaves was applicable only
upto calcium
(17) Complete and balance the following chemical equations:
(i) Na OH(aq) + Zn(s) →
Ans: 2NaOH (aq) + Zn (s) → Na2 Zn O2(aq) + H2(g)
(ii) CaCO3(s) + H2O (l)+ CO2(g) →
Ans: CaCO3(s)+H2O(l) + CO2(g) → Ca (HC03)2(aq)
(iii) HCL(aq) + H2O(l) →
Ans: HCl (aq) + H2O(l) → H3O + (aq) + Cl– (aq)
OR
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.
(i) Write formula of ‘G’ and ‘C’.
Ans: G = Cl2
C = CaOCl2
(ii) State the chemical equation involved.
Ans: Ca(OH)2 + Cl2 → Ca OCl2 + H2O
(iii) What is common name of compound ‘C’? Give its chemical name.
Ans: Common name – Bleaching Powder
Chemical name – Calcium Oxychloride
(18) (a) State with reason the consequence of decrease in number of carnivores in an ecosystem.
Ans: Population of herbivores will increase as their predator number is less.
(b) In a food chain, state the trophic level at which the concentration of harmful chemicals is maximum. Why is it so ?
Ans:
- Top carnivore/ Tertiary consumer
- They are nonbiodegradable , and while passing from one
trophic level to another they keep on accumulating at each level / Biomagnification.
OR
How is ozone layer formed ? State its importance to all life forms on earth ? Why the amount of ozone in the atmosphere dropped sharply in the 1980s?
Ans: UV radiations split some molecular oxygen ( O2) into free (O) atoms;
These atoms then combine with the molecular oxygen to form ozone.
- It protects all life forms from the harmful effects of ultra violet
radiations. - Because of the release of chemicals like (CFC’s) used in
refrigerators and fire extinguishers .
(19) (a) State the role played by the following in the process of digestion.
(i) Enzyme trypsin
Ans: (i) Enzyme trypsin : Helps in the digestion of proteins.
(ii) Enzyme lipase
Ans: (ii) Enzyme lipase : Helps in the breaking down of emulsified fats.
(b) List two functions of finger like projections present in the small intestine.
Ans: Two functions :
- Increase the surface area,
- Helps in absorption of digested food.
(20) 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
(i) Colour of stem in their F1 progeny
Ans: Green
(ii) Percentage of brown stemmed plants in F2 progeny if F1 plants are self pollinated.
Ans: 25 %
(iii) Ratio of GG and Gg in the F2 progeny
Ans: GG : Gg
1 : 2
(b) Based on the finding of this cross, what conclusion can be drawn?
Ans: The traits which are expressed in F1 progeny are called dominant traits, whereas the traits which are unable to express themselves in F1 progeny but reappear in the F2 progeny are called recessive traits.
(21) (a) Classify the following as homologous or analogous pairs :
(i) Broccoli and Cabbage
Ans: Analogous
(ii) Ginger and Raddish
Ans: Analogous
(iii) Fore limbs of birds and lizard
Ans: Homologous
(iv) Wings of a bat and Wings of a bird
Ans: Analogous
(b) State the main feature that categorises a given pair of organs as homologous or analogous.
Ans: Homologous organs have similar origin and basic structure but perform different functions whereas Analogous organs have different basic structure but perform similar functions.
(22) (a) State Snell’s law of refraction of light.
Ans: Snell’s Law – The ratio of sine of angle of incidence to the sine of angle of refraction is a constant for the light of a given colour and for the given pair of media.
(b) When a ray of light travelling in air enters obliquely into a glass slab, it is observed that the light ray emerges parallel to the incident ray but it is shifted sideways slightly. Draw a labelled ray diagram to illustrate it.
Ans:
Ray diagram labelling
(23) (a) With the help of labelled ray diagram show the path followed by a narrow beam of monochromatic light when it passes through a glass prism.
Ans:
(b) What would happen if this beam is replaced by a narrow beam of white light?
Ans:
(b) Splitting into seven colours / Dispersion / VIBGYOR /
OR
(a) A person is suffering from both myopia and hypermetropia.
(i) What kind of lenses can correct this defect?
Ans: (i) Bifocal Lens
(ii) How are these lenses prepared?
Ans: Upper part of lens is concave and lower part of the lens is convex. /
(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.
Ans: P = +3D
f = 1/ P
= 1/3 m = +100 /3 cm = + 33.3 cm
P = – 3D
f = −100 /3 = −33.3 cm
(24) Give reasons for the following:
(i) There is either a convergence or a divergence of magnetic field lines near the ends of a current carrying straight solenoid.
Ans: The strength of magnetic field is higher near the poles /ends of solenoid.
(ii) The current carrying solenoid when suspended freely rests along a particular direction.
Ans: A current carrying solenoid behaves as a bar magnet.
(iii) The burnt out fuse should be replaced by another fuse of identical rating.
Ans: If a fuse , with a defined rating , is replaced by one with a larger rating then the fuse wire will not burn even when a current greater than safe limit is flowing. As a result the electrical circuit / appliances will be damaged.
Section – C
(25) (a) Compare soaps and detergents on the basis of their composition and cleansing action in hard water.
Ans:
Soaps | Detergents |
· Composition –
Sodium or Potassium salts of long chain fatty acids / carboxylic acids
· Cleansing action in hard water – Forms scum. |
Ammonium or Sulphonate salts of long chain carboxylic acids.
Does not form any scum. |
(b) What happens when ethanol is treated with sodium metal ? State about the behaviour of ethanol in this reaction.
Ans:
- Hydrogen gas is evolved.
- Behaves like an acid.
(c) Draw the structure of cyclohexane.
Ans:
(d) Name the following compound.
Ans:
Ethanal / Acetaldehyde
(26) A metal ‘M’ is stored under kerosene. It vigorously catches fire, if a small piece of this metal is kept open in air. Dissolution of this metal in water releases great amount of energy and the metal catches fire. The solution so formed turns red litmus blue.
(a) Name the metal ‘M’.
Ans: Sodium
(b) Write formula of the compound formed when this metal is exposed to air.
Ans: Na2O
(c) Why is metal ‘M’ stored under kerosene?
Ans: It catches fire, if kept in open
(d) If oxide of this metal is treated with hydrochloric acid, what would be the products?
Ans: NaCl and H2O
(e) Write balanced equations for:
(i) Reaction of ‘M’ with air.
Ans: 4M + O2 → 2M2 O
(ii) Reaction of ‘M’ with water.
Ans: 2M + 2H2 O → 2MOH + H2 + Heat energy.
(iii) Reaction of metal oxide with hydrochloric acid.
Ans: M2 O + 2HCl → 2MCl + H2 O
OR
(a) Write electron dot structures of Ca (At. No. 20) and O(At. No. 8).
Ans: Ca ( atomic number 20) = 2,8,8,2
Ca: O ( atomic number 8) = 2,6
(b) Show the formation of calcium oxide by transfer of electrons.
(c) Name the ions present in this compound.
Ans: Calcium ion , Oxide ion
(d) List four important characteristics of this compound.
Ans: · Solid
- High melting point and boiling point
- Soluble in water
- Conducts electricity in molten state
(27) (a) Write the correct sequence of steps followed during journey of oxygen rich blood from lungs to various organs of human body.
Ans: Oxygenated Blood from Lungs into →
Pulmonary Vein → Left Atrium ( Collects blood on relaxation)
(b) What happens when the system of blood vessels develop a leak?
Ans: Leakage results in loss of blood pressure which would reduce the efficiency of the pumping system.
(28) (a) Suggest any two categories of contraceptive methods to control the size of human population which is essential for the prosperity of a country. Also explain about each method briefly.
Ans: Categories of Contraceptive methods :
(i) Mechanical Barrier – The sperm does not reach the egg.
(ii) Contraceptive Pills – Change the hormonal balance so that the eggs are not released.
(iii) Contraceptive devices – Placed in the uterus to prevent pregnancy.
(iv) Surgical methods- Blocking / cutting of vas deferens/ fallopian tube.
(b) Name two bacterial and two viral infections each that can get sexually transmitted.
Ans: Two bacterial infections:
(i) Gonorrhoea
(ii) Syphillis
Two viral infections:
(i) AIDS
(ii) Warts
(c) List two advantages of using condom during sexual act.
Ans: (i) Avoids unwanted pregnancies.
(ii) Prevents transmission of infections/ STD’s.
OR
(a) Draw a diagram to show spore formation in Rhizopus.
(b) With the help of an example differentiate between the process of Budding and Fragmentation.
Ans:
Budding | Fragmentation |
(i) An outgrowth / bud at a specific site develops into a tiny individual and detaches itself from the parent. e.g. Hydra / Yeast | (i) Organism simply breaks upon maturation into two or more pieces/ fragments that grow into new individuals. e.g. Spirogyra |
(c) Why is vegetative propagation practiced for growing some type of plants?
Ans: Useful for plants that do not produce seeds/ lost the capacity to produce seeds.
(29) (a) A security mirror used in a big showroom has radius of curvature 5 m. If a customer is standing at a distance of 20 m from the cash counter, find the position, nature and size of the image formed in the security mirror.
Ans: It is a convex mirror.So focal length should be positive.
Radius of curvature R= + 5 m
∴ focal length f = 5 /2 = +2.5 m
Object distance u= -20m
Mirror formula 1/ v + 1/ u = 1/ f
1/ v + 1/ −20 = 1/ 2.5
1/ v = 1 /20 + 1/ 2.5
1/ v = 1 /20 + 10 /25
v = 5+ 40 /100 = 45/ 100
v = 100/ 45 = 20 /9 = +2.2m
- Nature of image = virtual and erect image
- Size of image : diminished image
(b) Neha visited a dentist in his clinic. She observed that the dentist was holding an instrument fitted with a mirror. State the nature of this mirror and reason for its use in the instrument used by dentist.
Ans: Concave Mirror Reason : to obtain erect and enlarged image of teeth
OR
Rishi went to a palmist to show his palm. The palmist used a special lens for this purpose.
(i) State the nature of the lens and reason for its use.
Ans: Convex lens to get a magnified image of the lines on the palm.
(ii) Where should the palmist place/hold the lens so as to have a real and magnified image of an object?
Ans: Between F and 2F of the lens / or at F of the lens
(iii) If the focal length of this lens is 10 cm, the lens is held at a distance of 5 cm from the palm, use lens formula to find the position and size of the image.
Ans: focal length f = +10 cm
object distance u = -5 cm
Lens formula
v − 1 /u = 1/ f
1/v − 1 /−5 = 1/ 10
1 /v + 1 /5 = 1/ 10
1 /v = 1/ 10 − 1 /5 = 1−2/ 10
1/v = −1 /10
v = −10 cm
- m = himage /hobject = v/ u
= −10/ −5 = 2
Size of image is 2 times the size of object.
(30) (a) An electric bulb is rated at 200 V; 100 W. What is its resistance?
Ans: P = V 2 /R
100 W = 200 V × 200 V R
R = 400 Ω
(b) Calculate the energy consumed by 3 such bulbs if they glow continuously for 10 hours for complete month of November.
Ans: Energy consumed in 1 day = P × t
= 3 × 100 W × 10 h = 3000 Wh
= 3 kWh = 3 units
Energy consumed in 30 days = 30 × 3 units = 90 units
(c) Calculate the total cost if the rate is ` 6.50 per unit.
Ans: Total Cost = Rs. 6.50 × 90 units
= Rs. 585.00
OR
(a) What is meant by the statement, “The resistance of a conductor is one ohm”?
Ans: If the potential difference across the two ends of a conductor is 1 volt and the current through it is 1 ampere, then the resitance of the conductor is 1 Ω /
1 Ω = 1 volt /1 ampere
(b) Define electric power. Write an expression relating electric power, potential difference and resistance.
Ans:
- Electric Power is defined as the rate at which electric energy is consumed /dissipated.
- P = V 2 /R
(c) How many 132 W resistors in parallel are required to carry 5 A on a 220 V line?
Ans: R = V/ I
= 220 volt/ 5 ampere
R= 44 Ω
Net Resistance = 132 Ω n , where n is the number of bulbs in parallel.
R = 44 = 132/ n
n = 132/ 44 = 3 resistors
CBSE Class 10 Previous Question Paper 2020 Solution
- Science (Set No 31/1/1)
- Science (Set No 31/1/2)
- Science (Set No 31/1/3)
- Science (Set No 31/2/1)
- Science (Set No 31/2/2)
- Science (Set No 31/2/3)
- Science (Set No 31/3/1)
- Science (Set No 31/3/2)
- Science (Set No 31/3/3)
- Science (Set No 31/4/1)
- Science (Set No 31/4/2)
- Science (Set No 31/4/3)
- Science (Set No 31/5/1)
- Science (Set No 31/5/2)
- Science (Set No 31/5/3)