CBSE Class 10 Science Exam 2021: Check Important Short Answer Type Questions with Solutions

Here we are providing some important short answer type questions for CBSE class 10 Science board exam 2021. All the questions are provided with detailed answers.

Created On: Feb 11, 2021 10:10 IST
CBSE Class 10 Science
CBSE Class 10 Science

Get here a set of important questions for CBSE Class 10 Science Exam 2021. These are the questions that may be asked for 2 and 3 marks in the CBSE Class 10 Science Paper.

In CBSE class 10 Science question paper:

  • Sections B and C will be comprised of questions of 2 and 3 marks respectively. 
  • Section-B will have 6 questions carrying 2 marks each.
  • Section-C will have 7 questions carrying 3 marks each.

All the important questions are provided with detailed and appropriate solutions to help students prepare easily and effectively for the exam.

Note - While practicing with the 1 mark questions provided below, students should take care of the deleted portion of the CBSE Class 10 Science Syllabus 2021. Ignore the questions that are from the chapters/topics that have been excluded from the syllabus.

Given below are some very important short answer type questions to prepare for CBSE Class 10 Science Exam 2021:

1. (a) Why are most carbon compounds poor conductors of electricity?

(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound.


(a) Carbon compounds form Covalent bonds. They do not dissociate into ions and do not have charged particles (ions) to carry electric current.

(b) Cyclohexane

 class 10 important questions

Total number of single bonds present in cyclohexane =18

2. Name the hormones secreted by the following endocrine glands and specify one function of each:

(a) Thyroid

(b) Pituitary

(c) Pancreas


(a) Thyroxine hormone is secreted by thyroid gland. It regulates carbohydrate protein and fat metabolism for balance of body growth.

(b) Pituitary gland secrets growth hormone. It regulates growth and development of body.

(c) Pancreas secrets insulin. It regulates/decreases the blood sugar level.

Pancreas also secrets glucagon that regulates /increases blood sugar level.

NCERT Exemplar Problems and Solutions Class 10 Science: All Chapters

3. Give an explanation for the formation of a rainbow.


Rainbow is formed due to the dispersion of sunlight by raindrops suspended in air. Water drops act like tiny prisms. When sunlight enters into a spherical rain¬drop, it is refracted and dispersed. The different colours of light bent in different angles. When different colours of light fall on the back inner surface of drop, they undergo total internal reflection. The water drop again refracts the different colours, while coming out of the drop. After leaving the drop different colours come out to form a rainbow.

A rainbow is always it formed in the direction opposite to the sun. so, to see a rainbow, the sun must bebehind us.

4. How are we able to see distant and nearby objects clearly? Which part of eye helps in changing curvature of lens? Why no image is formed at blind spot?


We can see distant and near objects clearly because of the property of accommodation of eye. Accommodation is the name given to the ability of the lens of the eye to change its focal length and produce focused images of both distant and near objects on the retina. 

Cilary muscle helps in changing curvature of the lens. They are attached to the lens and contract or release to change the lens shape and curvature.

No image is formed at blind spot because there are no photoreceptors, i.e., cones and rods to detect the light and send it to brain.

5. (a) Why magnification is taken negative for real images and positive for virtual images?

(b) Why convex mirror is used in rear view mirrors and not concave mirror?


(a) If image is real, it will be inverted which means height of image will be taken as negative. Also, the height of object is always positive.

Now as magnification = Height of image / Height of object.

Therefore, in case of real image,

Magnification = Height of image (-) / height of object (+)=Negative.

So, Magnification will be negative.

Similarly, if image is virtual, it will be erect.

Therefore, in case of virtual image,

Magnification = Height of image (+) / height of object (+)= Positive

So, Magnification will be positive.

(b) Convex mirror is commonly used in rear view mirrors in vehicles because it gives an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area which is not possible with a concave mirror.

6. (a) Which property of carbon leads to formation of large number of compounds? Define it.

(b) What is the functional group in the following molecules?


(ii) CH3COOH


(a) The two features of carbon that give rise to a large number of compounds are as follows:

(i) Catenation − It is the ability to form bonds with other atoms of carbon.

(ii) Tetravalency − With the valency of four, carbon is capable of bonding with four other atoms.



Functional group


-OH (Hydroxyl group)

(ii) CH3COOH

-COOH (Carboxyl group)

NCERT Solutions for CBSE Class 10 Science: All Chapters

7. (a) What is translocation? Why is it essential for plants ?

(b) Where in plants are the following synthesized: 

(i) Sugars     

(ii) Hormones


(a) The transport of soluble products of photosynthesis is known as translocation. Translocation in plants occurs by means of the vascular tissue known as phloem. It essential for plants because without it, the food prepared by the leaves cannot reach other parts of the plant.

(b)(i) In plants, the synthesis of sugars takes place in the leaves by the process of photosynthesis.

(ii) Plant hormones unlike animal hormones are not secreted by any gland but are secreted away from their target site. 

8. Draw a diagram of human brain and label on it the following of its parts :
(i) Cerebrum

(ii) Hypothalamus

(iii) Medulla Oblongata

(iv) Cerebellum


Diagram of human brain with required labelling is given below:

9.Draw the structural formula CO2, H2S, S8.


10. (i) How will you convert a given set of resistors so that the equivalent resistance is increased? Give reason for your answer.

(ii) In the given circuit diagram, calculate:

(a) the value of current through each resistor

(b) the total current in the circuit

(c) the total effective resistance of the circuit.


(i) Given resistors will be connected in series as in series the same current passes through each resistor in the chain and the total resistance, RT of the circuit comes out to be equal to the sum of all the individual resistors added together, increasing the equivalent resistance.

11. (a)Explain how does the exchange of gases occur in plants across the surface of stems, roots and leaves.

(b) What are the raw materials used during photosynthesis? Explain the role of each material.

(c) Write chemical equation for photosynthesis.


(a) In plants there are tiny pores called stomata on leaves and lenticels in stem which facilitate the exchange of gases. Carbon dioxide is taken in and oxygen given out {during photosynthesis} and vice versa during respiration.

(b) Following raw materials are used by plants to carry the process of photosynthesis:

(i) Carbon dioxide gas: This gas is released into the atmosphere during respiration which is then utilised by the autotrophic plants which enters the leaf through the stomata present on its surface during the process of photosynthesis.

(ii) Chlorophyll: It is a green pigment in plants, which acts as a catalyst. It is responsible for absorption of the sun’s energy by the plant. The chlorophyll pigments are photoreceptor molecules which play a key role in the photosynthetic process. The different types of chlorophyll molecules are chlorophyll a, b, c, d, e and bacteriochlorophyll; of which chlorophyll a and b are the most common.

(iii) Light: Light energy is absorbed by chlorophyll to convert carbon dioxide and water into carbohydrates. An increased light intensity leads to a high rate of photosynthesis and a low light intensity would mean low rate of photosynthesis.

(iv) Water: Water from under the earth, is transported upward through xylem tissues to the leaves, from where it reaches the photosynthetic cells. This water then splits in the presence of sunlight and chlorophyll.
(c) The chemical equation for photosynthesis is as follows:

12. (a) How does a bar magnet differ from an electromagnet.

(b) What type of core should be put inside a current-carrying solenoid to make an electromagnet?

(c) How can you find out the direction of the magnetic field produced by a current-carrying conductor?



Bar Magnets
The bar magnet is a permanent magnet.
It produces a comparatively weak force of attraction.
The strength of a bar magnet cannot be changed.
The polarity of a bar magnet is fixed and cannot be changed.
An electromagnet is a temporary magnet.
It produces a very strong magnetic force.
The strength of an electromagnet can be changed by changing the number of turns in its coil or by changing the current passing through it.
The polarity of an electromagnet can be changed by changing the direction of current in its coil.

(b) A soft iron core is placed inside a solenoid to make an electromagnet.

(c) Maxwell’s right hand thumb rule can be used to find the direction of lines of force of the magnetic field produced by a straight a current-carrying conductor. According to Maxwell’s right-hand thumb rule, “Imagine that the current carrying wire is in the right hand so that the thumb points in the direction of current, then the direction in which the fingers encircle the wire gives the direction of magnetic lines of force around the wire.

13. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.


Reactions involving the energy supply in the form of heat:

class 10 science questions

14. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid.


  • When 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube, the following reaction takes place:

                                Zn + 2NaOH → Na2ZnO2 + H2

  • When a burning splinter is brought near the gas, it burns with a pop Sound.
  • When the same metal (zinc) reacts with dilute solution of a strong acid, Hydrogen (H2) gas is evolved.

15. "Two areas of study namely 'evolution' and 'classification' are interlinked". Justify this statement.


Evolution is gradual change in inherited characters of organisms over successive generation and thereby generating variations at each level of biological organization. Classification is arrangement of organisms into groups on the basis of their similarities and is preceded by identification and nomenclature. Similarities refer to resemblance of any biological structure or characters and form the basis for taxonomic classification. Therefore, the more the morphological/biochemical/anatomical/physiological similarities between two organisms; the more are the chances for them to share common ancestors which in turn makes them more close from evolutionary prospect.

For example: On the basis of presence or absence of vascular tissues, all the plants are classified as non vascular (algae, fungi, bryophytes) and vascular plants. All vascular plants further evolved under two lineages: non seed plants (Pteridophytes) and seed (Spermatophytes) plants. Further evolution of seed plants lead to emergence of Gymnosperms and Angiosperms. Hence, it can be said that classification of organisms into different taxonomic categories represents their ancestral relationship, lineages and evolutionary history.

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


Reproduction: The process of production of offspring by parent organisms via sexual or asexual methods is termed as reproduction.

Reproduction ensures production of own kind by living organisms i.e. cat gives birth to baby cats only and human to baby human only. Individuals of population of each species face interspecific and intraspecific competition for food, reproduction and shelter. Hence, the process of natural selection and struggle for existence cause loss of many individuals from different population. Further, the process of ageing and death also keeps a check on the population size. All these factors, together, work to reduce the population size continuously. Reproduction is the only process that compensates the loss of individuals caused by above mentioned natural processes and thereby ensures a constant population size of different species.

17. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?


Not all the characteristics those inherited from the parents, are expressed. Some characters remain hidden and are tremed as recessive. Such recessive traits are suppressed by dominant genes. Mendel explained this phenomenon with the help of his experiment on monohybrid inheritence. In this experiment, Mendeltall crossed a tall plant with a dwarf plant which produced all tall plants in Fprogeny. However, when these Ftall plants were crossed with each other, not all the plants produced were tall but some of them were dwarf, indicating both the traits that of tallness and shortness were inherited in the F2 plants.which was not observed in the Fgeneration, reappered in the F2 generation.

With this experiment Mendal proved that the traits of dwarfness which were suppressed in the F1 generation by the tallness traits and reappeared in the F2 generation. So, we can say that a trait that is inherited may not be expressed in an organism.

18. (a) A mixture of ethyne and pure oxygen is used for welding instead of using a mixture of ethyne and air. Why?

(b)Why pure acetic acid is called glacial acetic acid?


(a) Ethyne is an unsaturated carbon compound. It burns in air with a sooty flame. It’s an incomplete combustion since the oxygen available from air is not enough to burn the carbon completely. Therefore, for complete combustion pure oxygen is required, which burns Carbon and Hydrogen completely producing a very hot and clean flame apt for welding. It is not possible to generate such a high temperature without pure oxygen.

(b) The Freezing point of pure acetic acid is very high. It freezes into a solid at a temperature very close to room temperature: 16.5 ºC, or 62 ºF. Hence, during winters it gets frozen due to cold climate and appears like an ice. That is why pure acetic acid is called glacial acetic acid.

19. (a) Why do the faces of persons sitting opposite to us around a camp fire appear to shimmer?

(b) Why is red light used as universal danger signal?


(a) The faces of persons sitting opposite to us around a camp fire appear to shimmer because of the refraction of light. The reflected rays of light from the face of a person, sitting opposite to us, get refracted as they pass through the hot air (produced by the burning of wood). Fast moving hot air molecules cause the optical density to vary continuously. Thus, it causes the path of refracted rays passing through it to change continuously giving rise to a shimmering effect.

(b) The red light is least scattered by the atmosphere since it has the longest wavelength amongst the spectral colours. Hence, red light is able to pass easily through fog or mist, without getting scattered and hence is visible from a long distance. That is why; red light is used as universal danger signal.

20. Mention any four limitations in harnessing wind energy on a large scale.


(i) Continuous blowing of air is required to develop wind energy farms

(ii) Wind speed should be higher than 15 km/h to maintain the speed of the turbine.

(iii) Back-up facilities have is important during a period when there is no wind.

(iv) For preparing 1 MW generator wind farm we need about 2 hectares of land.

21. A coil of insulated wire is connected to a galvanometer. What would be seen if a bar magnet with its north pole towards one face of the coil is 

(i) Moved quickly towards it,

(ii) Moved quickly away from the coil and

(iii) Placed near its one face?

Name the Phenomena involved.


(i) Movement of needle (galvanometer) toward right indicate the flow of current from AB, as the motion in magnets stops deflection becomes zero.

(ii) Withdrawal of North Pole away from coil leads to the deflection of galvanometer to the left, indicating flow of current in opposite direction.

(iii) Magnet kept near the face, the deflection of the galvanometer drops to zero.

This phenomenon is called Electromagnetic Induction

22. (a) Why does micelle formation takes place when soap is added to water? Will a micelle be formed in other solvents like ethanol also?

(b) What is the importance of common salt in the preparation of soap?

(c) Write an equation for the chemical reaction involved in the formation of ester?


(a) Micelle formation takes place when soap is added to water because the hydrocarbon chains of soap molecules are hydrophobic (water fearing) which are insoluble in water, but the ionic ends of soap molecules are hydrophilic (water loving) and hence are soluble in water. The micelle formation will not take place when soap is added to organic solvents like ethanol because the hydrocarbon chains of soap molecules are soluble in organic solvents like ethanol.

(b) Common salt is added to the soap solution to separate out all the soap from the aqueous solution. On adding salt to the solution, the solubility of soap present in it decreases, due to which all the soap separates out from the solution in the form of a solid.

(c) The chemical reaction involved in the formation of ester is:

CBSE Class 10 Science Previous Years' Solved Question Papers

23. (a) An element X belongs to group 2 and another element Y belongs to group 15 of the periodic table:

(i) What is the number of valence electrons in X?

(ii) What is the number of valence electrons in Y?

(iii) What is the valency of X and Y?

(b) Why could no fixed position be given to hydrogen in Mendeleev's periodic table?


(a) (i) The number of valence electrons in X is 2.

(ii) The number of valence electrons in Y is 5.

(iii) The valency of X and Y respectively are 2 and 3.

(b) In Mendeleev’s periodic table, hydrogen (H) has been placed in group I with alkali metals because like other alkali metals; hydrogen also combines with halogens to form compounds having similar formulae. But hydrogen also resembles halogens in some of its properties like forming diatomic molecules. They also form ionic compounds like hydride and also form covalent compounds with non-metals.Thus Mendeleev’s periodic table could not assign a correct position to hydrogen in periodic table.

24. Explain how equal genetic contribution of male and female parents is ensured in the progeny.


In human beings, equal genetic contribution of male and female parents is ensured in the progeny through inheritance of equal number of chromosomes from both parents. There are 23 pairs of chromosomes All human chromosomes are not paired. Out of these 23 pairs, the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosomes represented as X and Y. Females have a perfect pair of two X sex chromosomes and males have a mismatched pair of one X and one Y sex chromosome.
During the course of reproduction, as fertilization process takes place, the male gamete (haploid) fuses with the female gamete(haploid) resulting in formation of the diploid zygote. The zygote in the progeny receives an equal contribution of genetic material from the parents. Out of 23 pairs of chromosomes in progeny, male parent contributes 22 autosomes and one X or Y chromosome and female parent contributes 22 autosomes and one X chromosome.

25. The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses.


  • Sodium Hydrogen Carbonate/ Sodium Bicarbonate (NaHCO3) salt is used to make tasty and crispy pakoras.
  • Chemical equation for the formation of sodium bicarbonate is:

                                NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3

Some of the uses of NaHCO3 are:

  • For making baking powder
  • As ingredient of antacid.
  • In the manufacture of soda-acid fire extinguishers

26. Write one main difference between asexual and sexual mode of reproduction. Which species is likely to have comparatively better chances of survival – the one reproducing asexually or the one reproducing sexually? Give reason to justify your answer.


Any one of the following difference:

(i) In sexual reproduction two opposite sexes are involved where as in asexual reproduction only one individual is involved.

(ii) In sexual reproduction male and female gamete formation takes place where as in asexual no gamete formation occurs.

  • Sexually reproducing organisms have better chances of survival
  • Because more variations are generated.

27. State the laws of refraction of light. Explain the term ‘absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum.


1st law: the incident ray, refracted ray and normal to the interface at the point of incidence lie in the same plane.

2nd law: The sine of angle of incidence bears a constant ratio with sine of angle of refraction for a given pair of media.

science class 10 questions

28. What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of – 20 cm. Write the nature and power of each lens.


Power of lens is the ability of the lens to converge the rays of light falling on it. It is given as the reciprocal of its focal length in meters. It is represented by the letter P. Thus, the power P of a lens of focal length f is given by f = 1/P.

The SI unit of power is dioptre.

science important questions

Nature: Diverging lens or Concave lens.

29. (a) List the factors on which the resistance of a conductor in the shape of a wire depends.

(b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity? Give reason.

(c) Why are alloys commonly used in electrical heating devices? Give reason.


(a) Factors on which resistance of a conductor depends are:

class 10 science 3 marks questions

(b) Metals are good conductor of electricity as they have low resistivity and possess free electrons to carry electric current.  

Gass is a bad conductor of electricity as it has high resistivity and have no free electrons to pass on current.

(c) Alloys are commonly used in electrical heating devices because:

  • They have high resistivity
  • They have high melting point
  • They do not oxidize (Or burn) readily at high temperatures. (anyone)

30. (a) Write the function of following parts in human female reproductive system:

 i. Ovary

ii. Oviduct

iii. Uterus

(b) Describe in brief the structure and function of placenta.


(a) i. Ovary: It produces egg (ovum). It also produces the hormones; oestrogen and progesterone.

ii. Oviduct: It is the site of fertilization. It carries egg (ovum) or fertilized ovum (zygote) to the uterus.

iii. Uterus: It is the site of development of embryo. It protects and nourishes the embryo.

(b) Placenta- It is a disc embedded in uterine wall which contains villi on the embryo side of the tissue and blood space on mother side.

Function of placenta: It provides nourishment to embryo from mother’s blood and also facilitates removal of waste from embryo to mother’s blood.

Also Check:

CBSE Class 10 Science Important Questions of All Formats (with Answers) for Board Exam 2021

CBSE Class 10 Science Important Practical Based Questions

Students must practice these important questions to acquaint with the important topics of class 10 Science and track their preparedness for the upcoming board exam. Moreover, the solutions provided here will give students an idea about how to write perfect solutions in exam to score high marks.

Do not miss* CBSE Class 10 Science Complete Study Material & Guide for Board Exam 2021


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