Answer key for 10th CBSE science 2020

Answer key for 10th CBSE science 2020

SECTION A

1. Name of a cyclic and saturated carbon compound.

Ans- cyclopentane/ cyclohexene.

2. The change in magnetic field lines in a coil is the cause of induced electric current in it. Named underlining phenomenon.

Ans- Electromagnetic induction

Answer question numbers 3(a) to 3(d) and 4(a) to 4(d) on the basis of your understanding of the following paragraphs and the related studied concepts.

3. The growing size of the human population is a cause of concern for all people. The date of birth and death in a given population will determine its size. Reproduction is the process by which organisms increase their population. the process of sexual maturation for reproduction is gradual and takes place while general body growth is a still going on. Some degree of sexual maturation does not necessarily mean that the mind and body is ready for sexual acts for for having and bringing up children. Various contraceptive devices are being used by human beings to control the size of population.

3.(a) list two common signs of sexual maturation in boys and girls.

Ans- Thick hair growth in armpit, genital area /thinning hair on arms, legs and face/ more active oil secretion from glands on skin/ occurence of pimples.

3.(b) what is the result of reckless female foeticide?

Ans- Imbalance in male – female ratio/ decline in child sex ratio.

3.(c) which contraceptive method changes the hormonal balance of the body?

Ans- Oral pills.

4.(d) write two factors that determine the size of a population?

Ans- Rate of birth and death.

4. human body is made up of five important components, of which water is the main component. Food as well as potable water are essential for every human being. The food is obtained from plants through agriculture. Pesticides are being used extensively for high yield in the fields.this pesticides are observed by the plants from the soil along with water and minerals and from the water bodies these pesticides are taken up by the aquatic animals and plants. As these chemicals are not biodegradable, they get accumulated progressively at each trophic level. The maximum concentration of this chemical get accumulated in our body and greatly affects the health of our mind and body.

4.(a) why is the maximum concentration of pesticides found in human beings?

Ans- Human beings are at the top level in any food chain.

4.(b)Give one method which could be applied to reduce our intake of breast sides through food to some extent.

Ans- Washing of vegetables, fruits, grains thoroughly/Organic farming/
Use of bio pesticides.

4.(c) Various steps in a food chain represent:

Ans-(b)  Trophic level.

4.(d) with regard to various food chain operating in an ecosystem man is a:

Ans- (a) Consumer.

5. calcium oxide reacts vigorously with water to produce slaked lime.

CaO(s)+H2O(1) –> Ca(OH)2(aq)

(A) combination reaction

(B) exothermic reaction

(C) endothermic reaction

(D) oxidation reaction

Ans- A and B

Or

When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulfuric acid so formed remains in the solution. The reaction is an example of a:

Ans- (D) double displacement reaction.

6. In a double displacement reaction such as the reaction between sodium sulphate solution and barium chloride solution:

Ans- (D) /b,c,d

7. Baking soda is a mixture of:

Ans- (c) sodium hydrogen carbonate and tataric acid.

8. The chemical formula for plaster of paris is :

Ans- (c) CaSO4, ½H2O

9. The laws of reflection hold true for

Ans- (d) all reflecting surfaces

Or

When an object is kept with the focus of a concave mirror and enlarged image is formed behind the mirror. This image is:

Ans- (d) virtual and erect.

10. at the time of short circuit the electric current in the circuit

Ans- (d) increases heavily

Or

Two bulbs of 100w and 40w are connected in series. The current through the 100w bulb is 1A the current through the 40w bulb will be

Ans- (d) 1A

11. Which one of the following is responsible for the substance of underground water?

Ans- (d) afforestation.

12. Incomplete combustion of coal and petroleum

Ans- (b) a, d

13. Assertion A: esterification is a process in which a sweet smelling substance is produced.

Assertion R: when start react with sodium hydroxide and alcohol and sodium salt of carbonxylic acid are obtained.

Ans- (b) both A and R are true but R is not that correct explanation of the assertion.

14. Assertion A :in the process of nuclear fission the amount of nuclear energy generated by the reason of an atom of uranium is so tremendous that it produces 10 million times the energy produced by the combustion of an atom of carbon from coal.

Assertion R :the nucleus of a heavy atoms such as uranium when bombarded with low energy neutrons is played separate into light a nuclei the mass difference between the original nucleus and the product nuclei gets converted to tremendous energy.

Ans- (a) both A and R true ,but R is the correct explanation of the assertion.

SECTION B

15. Ans-
• A black colour is formed on the surface
Heat
2Cu + O2 2CuO
Brown Copper Oxide; Black Colour

• Original/brown colour is restored.
Heat
CuO + H2 Cu + H2O
Black Copper; Brown

16. Ans-
Products: Hydrogen, Chlorine , Sodium hydroxide
Uses: Hydrogen: In the production of margarine/ ammonia/as a fuel
Chlorine: Water treatment/ swimming pools/ production of PVC/ Disinfectants/CFCs/Pesticides.
Sodium hydroxide: For degreasing metal surfaces/ in making soaps and detergents/ paper making/ artificial fibres.
(any one use of these or any other)

OR

Ans-
• By recrystallisation of sodium carbonate
• Na2CO3 + 10H2O Na2CO3.10H2O
• Basic Salt
• Permanent hardness

17. i) By dissolving 5g of KMnO4 in 100mL of water/
By dissolving 5g of KMnO4 in water to make a final volume of 100ml.
ii) As an oxidizing agent
Purple colour persists
Alkaline KMnO4
iii) CH3CH2OH CH3COOH
heat

18. Ans-• The adrenaline hormone is secreted into the blood.

• The heart beats faster resulting in supply of more oxygen to the
muscles.
• Blood is diverted to skeletal muscles.
• The breathing rate increases.
• The blood supply to digestive systems and skin is reduced.
OR
• Electrical impulses have limited access to only those cells that
are connected by nervous tissue/ neurons, whereas chemical
signals can reach each and every cell of the body .
• Cells need time to reset in order to create repeated/ new
electrical impulses whereas no such time is required for
chemical communication.

19. Ans- • Pollination is the transfer of pollen from anther to stigma.
Self Pollination-
Transfer of pollen in the same
flower.

Cross Pollination-

Transfer of pollen from one flower to another.

• Pollination leads to fertilization resulting in the formation of
zygote.

20. • Homologous structures are those which have similar basic structure and origin but perform different functions.
• Example: forelimbs of reptiles, amphibians, humans, wings of birds
(or any other example)
• Yes
• Similarity in basic design of the structure indicates that their ancestors were common.

21. Because of scattering of light.
Instances:
• When a fine beam of light enters a smoke-filled dark room through a small hole.
• When sunlight passes through a canopy of dense forest in foggy/ misty conditions.
• Blue colour of sky.
• Red colour of the sun during sunrise or sunset.

OR

• Prism has 2 inclined refracting surfaces whereas a glass slab has 2 parallel refracting surfaces.

i) When monochromatic light passes through a glass slab it gets displaced laterally whereas in prism it gets angularly displaced.
ii) When white light passes through a glass slab, it gets laterally displaced whereas in prism, dispersion takes place.

24. i) H = I2Rt
ii) H = V.I.t
= V.Q
Given : V = 40 volts , Q = 96000 C
H = 40 V × 96000 C
= 3.84 × 106J

SECTION C

25. Ans- • These metals have more affinity for oxygen than carbon.
• Towards the top of the reactivity series 
• By electrotytic reduction of their molten ores.
• Example : Extraction of sodium from molten sodium chloride by
electrolysis.
Process :
• Molten NaCl is taken in an electrolytic cell and on passing electricity Na is deposited at cathode and chlorine is liberated atanode .
Reactions –
At cathode – Na+  +  e- → Na
At anode – 2Cl- → Cl2 + 2e-
( or any other example)

26. i) E, it has 4 valence electrons .
ii) B, it needs only 2 electrons to attain stable configuration.
iii) D , it loses two electrons to attain stable configuration .
iv) F, it has the largest size since size increases down the group.
v) Noble gases, outermost shell is complete.

OR

• Atomic size is the distance between the centre of the nucleus and the outermost shell of an isolated atom.
• Picometer /pm
• Trends in Atomic radius
In a group: increases down the group ; due to addition of a new shell.In a period: atomic radius decreases from left to right ;due to increase in pulling power of nucleus /due to addition of electrons in the same shell.

27. Ans- (a) Rate of breathing is faster in aquatic organisms because the amount
of dissolved oxygen in water is lower as compared to the amount of oxygen in air.
b) pic

OR

a) A pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
b) A kidney has a large number of filteration units called nephrons. Each
nephron has cup shaped Bowman’s capsule containing a bunch of
capillaries called glomerulus. Blood gets filtered in the glomerulus.
Filterate gets collected in Bowman’s capsule. Some useful substances
such as glucose, amino acids, salts and water are selectively reabsorbed
as urine flows through nephron tube. The urine formed in each kidney is
eventually stored in the urinary bladder

28 Ans- (a) Law of dominance of traits: -In a cross between a pair of contrasting
characters, only one parental character will be expressed in F1 generation which is called dominant trait and the other is called ressive trait.
For example – in pea plants.

All plants in F1 generation were tall proving that the gene for tallness
is dominant over the gene for dwarfness/ short, which is not able to
express itself in the presence of dominant trait.
(any other example)
b) Traits acquired by an organism during its lifetime are known as
aquired traits.
These traits are not inherited because they occur in somatic cells
only/do not cause any change in the DNA of the germ cell.

30. Ans- (a) A current carrying solenoid is called an electromageet /when soft iron
is placed inside a solenoid carrying current, the soft iron piece behaves
like a magnet so long as electric current passes through it. The magnet
so formed is electromagnet.
Uses: In electric motors, electric bells, ( or any other )
c) Soft iron core is used to increase the strength/power of the electro
magnet.
d) i) By increasing the current
ii) By increasing the number of turns in the coil.