31/ 2/ 3 2020 Class 10 Science Question Paper Solution
Section – A
For question numbers 1 and 2, two statements are given – one labelled as Assertion (A) and the other labelled as 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) : Ethanoic acid is also known as glacial acetic acid.
Reason (R) : The melting point of pure ethanoic acid is 290 K and hence it often freezes during winters in cold climates.
Ans: (a) / Both (A) and (R) are true and (R) is the correct explanation of the assertion.
(2) Assertion (A) : The metals and alloys are good conductors of electricity.
Reason (R) : Bronze is an alloy of copper and tin and it is not a good conductor of electricity.
Ans: (d) / (A) is false, but (R) is true.
(3) Covalent compounds have low melting and boiling point. Why?
Ans: Due to weak intermolecular forces.
(4) How many metals are present in second period of periodic table?
Ans: Two / Lithium and Beryllium
(5) The maximum resistance which can be made using four resistors each of 2 W is
(a) 2 W
(b) 4 W
(c) 8 W
(d) 16 W
Ans: 8Ω
(6) A student plots V-I graphs for three samples of nichrome wire with resistances R1 , R2 and R3 . Choose from the following the statement that holds true for this graph.
(a) R1 = R2 = R3
(b) R1 > R2 > R3
(c) R3 > R2 > R1
(d) R2 > R1 > R3
Ans: R2 > R1 > R3
(7) Consider the following reasons for the reddish appearance of the sun at the sunrise or the sunset:
(a) Light from the sun near the horizon passes through thinner layers of air.
(b) Light from the sun covers larger distance of the earth’s atmosphere before reaching our eyes.
(c) Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles.
(d) Light from the sun near the horizon passes through thicker layers of air.
Ans: (b)
The correct reasons are
(a) A and C only
(b) B, C and D
(c) A and B only
(d) C and D only
Ans: B,C and D
OR
Person suffering from cataract has
(a) elongated eyeball
(b) excessive curvature of eye lens
(c) weakened ciliary muscles
(d) opaque eye lens
Ans: Opaque eye lens
(8) On the basis of electronic configuration of 9 5X, the group number and period of the element ‘X’ is :
(a) Group 15 period 2
(b) Group 13 period 2
(c) Group 9 period 5
(d) Group 13 period 5
Ans: Group 13 period 2
OR
An element ‘X’ with atomic number 11 forms a compound with element ‘Y’ with atomic number 8. The formula of the compound formed is
(a) XY
(b) X2Y
(c) XY2
(d) X2Y3
Ans: X2Y
(9) A visually challenged student, has to perform a lab test to detect the presence of acid in a given solution. The acid-base indicator preferred by him will be:
(a) Blue litmus
(b) Clove oil
(c) Red cabbage extract
(d) Hibiscus extract
Ans: Clove oil
(10) Identify ‘x’, ‘y’ and ‘z’ in the following reaction :
2 KClO3 Photo 2 KCL (x) + O2 (Z)
(a) x = gas ; y = reaction condition; z = gas
(b) x = solid ; y = liquid; z = gas
(c) x = number of moles of KClO3 ; y = reaction condition; z = no. of molecules of oxygen.
(d) (d) x = physical state of KClO3 and KCl; y = reaction condition; z = physical state of O2
Ans: x= Physical state of KClO3 and KCl
y = Reaction condition
z= Physical state of O2
(11) Bandharas and Tals are age old water harvesting concepts / structures found in
(a) Bihar
(b) Maharashtra
(c) Tamil Nadu
(d) Rajasthan
Ans: Maharashtra
(12) Which of the following are water intensive crops?
(a) Wheat and rice
(b) Wheat and sugarcane
(c) Sugarcane and rice
(d) Wheat and gram
Ans: Sugarcane and rice
OR
The most poisonous product formed by incomplete combustion of fossil fuels is
(a) Carbon dioxide
(b) Nitrogen dioxide
(c) Carbon monoxide
(d) Sulphur dioxide
Ans: Carbon monoxide
(13) Answer question numbers 13(a) to 13(d) on the basis of your understanding of the following paragraph and the related studied concepts.
Geothermal energy is the energy produced by the heat of molten rocks formed in the deeper hot regions of the earth’s crust. This energy is harnessed to generate electricity. When water is made to flow deep underground in the rocks it returns as steam (or hot water, which is later converted to steam) to drive a turbine on an electric power generator. In India, exploration and study of geothermal fields started in 1970. The Geological Survey in India has identified 350 geothermal energy locations in the country. The most promising of these is in Puga valley of Ladakh. The estimated potential for geothermal energy
In India is about 10000 MW. There are seven geothermal provinces in India namely the Himalayas, Sohna, West coast, Cambay, Son-Narmada-Tapi; Godavari and Mahanadi. Most power stations in India produce Alternating Current (A.C).
(a) What are geothermal energy hot-spots ?
Ans: Are deeper hot regions of earth’s crust where molten rocks are formed.
(b) Name two countries, other than India, where power plants based on geothermal energy are operational.
Ans: New Zealand / United States of America / China/Indonesia, Philippines / Turkey/ New Mexico.
(c) Name the phenomenon that explains the working of an electric generator.
Ans: Electromagnetic Induction.
(d) State an important advantage of using AC over DC.
Ans: In case of A.C. transmission of power/electricity takes place without much loss of energy.
(14) Answer question numbers 14(a) to 14(d) on the basis of your understanding of the following information and related studied concepts.
Thyroid gland is a bilobed structure situated in our neck region. It secretes a hormone called thyroxine. Iodine is necessary for the thyroid gland to make thyroxine. Thyroxine regulates carbohydrate, protein and fat metabolism in the body. It promotes growth of body tissues also. When there is an excess of thyroxine in the body, a person suffers from hyperthyroidism and if this gland is underactive it results in hypothyroidism. Hyperthyroidism is diagnosed by blood tests that measure the levels of thyroxine and Thyroid Stimulating Hormone (TSH). Hypothyroidism is caused due to the deficiency of iodine in our diet resulting in a disease called goitre. Iodised salt can be included in our diet to control it.
(a) Where is thyroid gland situated in our body ?
Ans: In the neck region
(b) State the function of thyroxine in human body.
Ans: Thyroxine regulates carbohydrate, proteins and fat metabolism in the body./ It promotes growth of body tissue.
(c) What is hyperthyroidism?
Ans: Excess of secretion of throxine in the body /overactivity of the thyroid gland
(d) How can we control hypothyroidism?
Ans: Can be controlled by including iodised salt in our diet.
Section – B
(15) From the elements 39 19 A, 28 14 B, 16 8 C and 40 18 Didentify:
(a) the most electro positive element.
Ans: A
(b) a noble gas.
Ans: D
(c) a metalloid.
Ans: B
(d) an element which will gain 2 electrons to attain nearest noble gas configuration.
Ans: C
(e) formula of compound formed between A and C. (f) elements belonging to same period.
Ans: A2C
(f) elements belonging to same period.
Ans: B & D
(16) A compound ‘A’ is used in the manufacture of cement. When dissolved in water, it evolves a large amount of heat and forms compound ‘B’.
(i) Identify A and B.
Ans: A = CaO / Quick lime/ Calcium oxide B = Ca(OH)2 / Slaked lime / Calcium hydroxide
(ii) Write chemical equation for the reaction of A with water.
Ans: CaO + H2O → Ca(OH)2 + heat or energy
(iii) List two types of reaction in which this reaction may be classified.
Ans: Combination reaction Exothermic reaction
(17) (a) What is ‘Water of crystalisation’?
Ans: Water of crystallization is the fixed number of water molecules present in one formula unit of salt.
(b) With the help of equations, state what happens when
(i) baking soda is heated during cooking ?
Ans: NaHCO3 Na2CO3 + H2O + CO2
(ii) gypsum is heated at 373 K?
Ans:
OR
With the help of labelled diagram, show an experimental setup for the reaction of Magnesium with dilute sulphuric acid.
Diagram
Give equation of the reaction involved.
Ans:
- MgSO4 + H2 → Mg + H2SO4 ↑
(18) (a) State one important function of ozone layer at the higher level in the atmosphere.
Ans: It shields the earth from the harmful ultraviolet (UV) radiations from the sun.
(b) How is ozone formed ?
Ans:
- By the action of UV radiations on oxygen molecule which splits to form free oxygen atoms.
- These free oxygen atoms combine with oxygen molecule to form ozone.
(c) It has been observed that ozone layer is getting depleted. Name the compound responsible for ozone depletion.
Ans: Chloroflurocarbon (CFC)
OR What are decomposers ? List two consequences of their absence in the ecosystem.
Ans:
- Decomposers : Micro-organisms which break up the dead complex organic matter into simpler form. Example Fungi, Bacteria
- Consequences:
(i) No nutrient recycling
(ii) Natural cleaning will not occur (Natural scavenger)
(19) Complete the following flow chart as per the given instruction:
Ans:
Secretions | Functions |
(a) mucus | (d)Protects the inner lining of stomach from the acid / softening of food |
(b) HCl(Hydrochloric acid) | (e)Provides the acidic medium for action of enzyme / Kill the germs. |
(c) Pepsin | (f) Digest proteins |
(20) What are chromosomes ? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.
Ans:
- Chromosomes are thread like structures present in nucleus containing genetic material / DNA
Number of chromosomes are reduced to half during gametes / germ cell formation .
After fertilization of germ cells the number of chromosomes is maintained in progeny.
(21) Explain giving an example how the following provide evidences in favour of evolution in organisms.
(i) Homologous organs
(ii) Fossils
Ans: (i) Homologous organs: Mammals have forelimbs as do birds, reptiles and amphibians .The basic structure of the limbs is similar though it has been modified to perform different functions in various vertebrates. Therefore these are homologus organs.
(ii) Fossils: Study of fossils of Archeopteryx / Dinosaurs show the presence of feathers used for insulation in cold weather but later became useful for flight. So birds have evolved from reptiles.
(22) What happens after refraction, when:
(i) a ray of light parallel to the principal axis passes through a concave lens ?
(ii) a ray of light falls on a convex lens while passing through its principal focus ?
(iii) a ray of light passes through the optical centre of a convex lens?
(23) List three common refractive defects of vision. Suggest the ways of correcting these defects.
Ans:
3 common defects | Corrective Lenses |
Myopia | Concave |
Hypermetropia | Convex |
Presbyopia | Bifocal |
OR
(a) How is a rainbow formed?
Ans: It is formed when light passes through suspended water droplets in air (It could be explained with the help of refraction, dispersion, internal reflection,refraction.)
(b) Why do stars twinkle?
Ans: Refractive indices of atmospher keep on changing due to which stars show twinkling effect.
(c) Why do the sky appear dark instead of blue to an astronaut?
Ans: Scattering of light does not occur due to absence of medium in space.
(24) Two coils of insulated copper wire are wound over a non-conducting cylinder as shown. Coil 1 has comparative large number of turns. State your observations, when
(i) Key K is closed.
Ans: Galvanometer (G) shows deflection (for very short time)
(ii) Key K is opened.
Ans: Galvanometer (G) shows deflection for a very short time in opposite direction to the previous observation.
Give reason for each of your observations.
Ans: Common Reason: Due to variation in current flowing through coil 1, magnetic field associated with coil 2 changes. Due to which an induced current will generate consequently galvanometer shows momentry deflection.
Section – C
(25) Write all the steps in proper sequence of obtaining pure copper from the ore in which it exists in nature.
Ans: Step-I : In nature Cu exists – Cu2S (Copper Sulphide) Cu2S is then heated in air
Step -II : Metal obtained in Step-I is not pure so it needs to be refined electrolytically / Electrolytic Refining
- Anode:Impure Metal
- Cathode : Pure Metal
- Electrolyte – CuSO4
- On passing current through electrolyte, the pure metal from the anode dissolves into the electrolyte and an equivalent amount of pure copper metal from the electrolyte is deposited on Cathode.
OR
(a) Show the formation of aluminium ion and chloride ion by transfer of electrons.
Ans:
(b) State the formula and the nature of compound formed if electron transfer takes place between the two.
Ans:
- AlCl3
- Ionic comound
(c) State with reason any three characteristics of the compounds formed by transfer of electrons.
Ans: (i) Hard and solid- Due to strong force of attraction between the positive and negative ions.
(ii) High melting and boiling points- Because igh aout of energy th break the stron inter-ionic attraction.
(iii) Good conductor of electricity in molten state- It contains ions.
(26) (a) What is a homologous series ? Explain with an example.
Ans: · Homologous series is a group of compounds which have the same functional group , same general formula and where to successive member differ by – CH2 in the molecular formula
Example : CH3-OH , CH3 -CH2-OH
Functional group : -OH , General Formula : CnH2n OH
(b) Define the following terms giving one example of each.
(i) Esterification
Ans: Esterification : The reaction of carboxylic acid with an alcohol in the presence of H2SO4 yields an ester.
CH3COOH+C2H5OH photo CH3COOC2H5 + H20
(ii) Addition reaction
Ans: Addition Reaction : A reaction in which two or more atoms are added across a double or triple bond in presence of catalyst is called addition reactions.
CH2=CH2+H2 photo CH3-CH3
(27) (a) List three different categories of contraception methods.
Ans:
- Chemical Method
- Barrier Method
- Surgical Method
(b) Why has Government of India prohibited prenatal sex determination by law ? State its benefits in the long run.
Ans: Increase in female foeticide / Declining child sex ratio
Benefit : Maintaining male-female sex ratio for a healthy society
(c) Unsafe sexual act can lead to various infections. Name two bacterial and two viral infections caused due to unsafe sex.
Ans: Bacterial → Gonorrhoea Syphilis
Viral → Warts AIDS
OR
(a) In the female reproductive system of human beings, state the funtions of
(i) ovary
Ans: (i) Ovary → Production of female germ cell/egg Production of hormone – estrogen
(ii) oviduct
Ans: (ii ) Oviduct → Site of fertilization
(b) Mention the changes which the uterus undergoes, when
(i) it has to receive a zygote.
Ans: Thickening of the uterus lining
(ii) no fertilization takes place.
Ans: Wall of uterus breaks/Menstruation occurs.
(c) State the function of placenta.
Ans: Providing the nutrition / O2/to the developing embryo /foetus or removal of waste from the fetus.
(28) (a) Draw a diagram of human excretory system and label on it the following parts:
(i) Kidney
(ii) Ureter
(iii) Urinary bladder
(iv) Urethra
Ans:
(b) Write one main function each of the labelled parts.
Ans: Functions:
Kidney → Filtration of blood
Ureter → Transports urine from kidney to bladder
Urinary Bladder → Stores Urine
Urethra → Throws out urine/Excrete Urine
(29) (a) Draw and complete the following ray diagram on you answer sheet to show lateral displacement.
Ans:
(b) Define absolute refractive index. Give its mathematical expression.
Ans: Ratio of speed of light in air/vaccum to speed of light in the given medium.
n = c/v
(c) Study the following diagrams :
Photo
I = Image, O = Object
Identify Mirror 1 and Mirror 2 and state one use of each.
Ans:
- Mirror -1 is Concave
- Uses : By dentists/Shaving Mirror/Torch search light/Vehicle headlight /solar furnace.
- Mirror-2 is convex mirror.
- Uses : Rear view mirror in vehicles/ for security purpose.
OR
(a) Define power of a lens and write its SI unit. Name the type of lens whose power is negative.
Ans: · Ability of a lens to diverge or converge light ray.
- SI unit of Power – diopter (D)
- Concave lens
(b) A convex lens forms a real and inverted image of finite size at a distance of 50 cm from it. Where is the object placed in front of the lens? Give all possible positions of the object stating reason in each case.
Ans: Position of object:
(i) Beyond 2F1
(ii) At 2 F1
(iii) Between 2F1 and F1
(c) Draw labelled ray diagram for any one position of the object mentioned in (b) above.
Ans:
(30) (a) Find the ratio of resistances of two copper rods X and Y of lengths 30 cm and 10 cm respectively and having radii 2 cm and 1 cm respectively.
Ans: R = ρ 1/A
RX = ρ1X/Ax = 30/ π x 2 x 2
Ry = ρ 1y/Ay = ρ 10/ π x 1 x 1
Rx / Ry = ρ .1x/Ax x Ay/ ρ . 1y
= 30/ π x 2 x 2 x π x 1 x 1/ 10
Rx/Ry = 3/4
Rx : Ry = : 3: 4
(b) A current of 500 mA flows in a series circuit containing an electric lamp and a conductor of 10 W when connected to 6 V battery. Find the resistance of the electric lamp.
Ans: Here I = 500 mA ; Resistance of lamp = R1
Resistance of the conductor (R2) = 10 Ω
Pot. Difference (V) = 6 volt
Net resistance of the circuit
R = R1 + R2 = V/1
R1 + 10 Ω = 6/0.5
R1 + 10 Ω = 12 Ω
R1 = (12 – 10) Ω
= 2 Ω
CBSE Class 10 Previous Question Paper 2020 Solution
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