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

Get the CBSE Class 10 Science important long answer type questions to prepare for the Board Exam 2021. These questions will give you an idea of the important topics to be prepared for five marks questions. 

Created On: Feb 12, 2021 18:39 IST
CBSE Class 10 Science Important Questions
CBSE Class 10 Science Important Questions

In CBSE Class 10 Science Exam 2021, Section D will have three (34-36) long answer type questions carrying 5 marks each. Here we are providing a set of important long answer type questions to prepare for CBSE Class 10 Science Exam 2021. All these questions have been provided with appropriate answers in a simple language to help students learn easily and effectively. This set of important questions provided here will help you familiarise the important topics and also assess your preparedness for the exam.

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

Question 1. (a) Write chemical equations for the reactions involved in obtaining pure alumina from the mineral bauxite which has impurities of iron oxide and silica.

(b) Draw a labelled diagram of the electrolytic tank cell used for the extraction of aluminium from alumina.


The reaction involved in obtaining pure alumina from the mineral bauxite having impurities of iron oxide and silica is:

The iron oxide and silica (sand) present in the bauxite ore do not dissolve in sodium hydroxide solution. Therefore, they are separated by filtration.

CBSE Class 10 Science Previous Years' Solved Question Papers

(b) Given below is the labelled diagram of the electrolytic tank cell used for the extraction of aluminium from alumina:

Question 2. (a) What are ‘magnetic field lines’? How is the direction of a magnetic field at a point determined ?

(b) Draw two field lines around a bar magnet along its length on its two sides and mark the field directions on them by arrow marks.


(a) The magnetic field lines are the pictorial representation of the strength and direction of the magnetic field. 

The direction of the magnetic field at a point can be found by placing a small magnetic compass at that point. The north end of the needle of  a compass indicates the direction of magnetic field at a point where it is placed.

(b) Magnetic field lines of a bar magnet emerge from the north pole and terminate at the south pole. Inside the magnet, the field lines emerge from the south pole and terminate at the north pole, as shown in the given figure.

Question 3. (a) Mention the pH range within which our body works. Explain how antacids
give relief from acidity. Write the name of one such antacid.
(b) Fresh milk has a pH of 6. How does the pH will change as it turns to curd? Explain your answer.
(c) A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?
(d) Mention the nature of toothpastes. How do they prevent tooth decay?


(a) Our stomach has pH equal to 2. Antacids neutralizes excess of acid in our body and give relief from hyperacidity. Sodium hydrogen carbonate is one of such antacid.
(b) pH will decrease as it turns to curd because curd is acidic due to the presence of lactic acid.
(c) It takes longer time to set as curd as bacteria do not work well in presence of sodium hydrogen carbonate, i.e. fermentation will take place slowly.
(d) Toothpastes are basic in nature. They neutralize the acid formed in mouth which causes tooth decay.

Question 4. Atoms of eight elements A. B. C. D, E, F. G and H have the same number of electronic shells but different number of electrons in their outermost shell. It was found that elements A and G combine to form an ionic compound. This compound is added in a small amount to almost all vegetable dishes during cooking. Oxides of elements A and B are basic in nature while those of E and F are acidic. The oxide of D is almost neural. Based on the above information answer the following questions:

(i) To which group or period of the periodic table do the listed elements belong?

(ii) What would be the nature of compound formed by a combination of elements B and F?

(iii) Which two of these elements could definitely be metals?

(iv) Which one of the eight elements is most likely to be found in gaseous state at room temperature?

(v) If the number of electrons in the outermost shell of elements C and G are 3 and 7 respectively, write the formula of the compound formed by the combination of C and G.


(i) The listed chemicals belong to third period; it includes Na, Mg, Al, Si, P, S, Cl& Ar.

(ii) Compound formed by the combination of B (Mg) and F (S) will result in formation of salt called MgSO4

(iii) Compound A (Na) and B (Mg) are definitely metals as their oxides are basic in nature.

(iv) Element H (Ar), is likely to be found in gaseous state at room temperature as it is the 8th element of the group so it would have 8 electrons in its outermost shell which is the electronic configuration of an Nobel gas.

(v) They will form AlCl3 (Aluminium chloride)

CBSE Class 10 Science Solved Sample Paper 2019

Question 5. (a) List the three events that occur during the process of photosynthesis. Explain the role of

stomata in this process.
(b) Describe an experiment to show that “sunlight is essential for photosynthesis”.


(a) Three most important steps of photosynthesis are:

  • Absorption of light energy by chlorophyll
  • Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen
  • Reduction of carbon dioxide to carbohydrates

Stomata are present in enormous amount over leafs, this tiny pores helps in the exchange of gaseous by photosynthesis. Since large amount of water can also be lost through these stomata, the plant closes these pores when it does not need carbon dioxide for photosynthesis, opening and closing of pours is regulated by guard cells, it  swell up when water flows inside them, causing the stomata pore to open. Similarly the pore closes if the guard cells shrink.

(b) Experiment to prove that “sunlight is essential for photosynthes” is explained below:

(i) Select two healthy plants.

(ii) Keep them in a dark room for 3 days.

(iii) Now place them in different air tight chambers, one supplied with potassium hydroxide, which is used to absorb carbon dioxide.

(iv) Now keep the plants in sunlight for 2 hours.

(v) Pluck leaf from both plants and check the presence of starch.

(vi) Higher starch content in the jar with potassium hydroxide indicates that in the presence of light the photosynthesis have occurred so higher starch content is found. 

Question 6. (a) How does Mendel’s experiment show that traits may be dominant or recessive?      

(b) How traits get expressed from parents to offsprings? Explain with an example.

(c) Why are traits acquired during the lifetime of an individual not inherited?


(a) Mendel in his experiment on the inheritance of traits selected and crossed tall pea plant with dwarf pea plant. The F1 progeny had Tt genotype but the plant was tall. This shows that ‘T’ is dominant trait whereas ‘t’ is recessive.

(b) Traits are passed through DNA. When an egg is fertilized with sperm, the resulting offspring takes 50 percent of its DNA from each parent. This resulting combination of two halves of DNA determines what specific traits the child will have. The expression of traits in an offspring is determined by dominant and recessive genes. If one or both parents pass on a dominant gene to the child, then the dominant gene will be expressed. However, a recessive gene is expressed only in case when both parents pass on the recessive genes to the offspring.

For example, in case of eye colour, the gene for brown eyes is dominant and that for blue eyes is recessive. So if either parent passes on the brown eye gene, then it will be expressed in the child. On the other hand for the expression of blue eye trait, both the parents should pass blue color traits. Thus people with blue eyes have two recessive genes, but people with brown eyes can have one dominant and one recessive gene or both dominant genes.

(c) Only the traits which are passed through DNA of germ cells are inherited to the offsprings  during sexual reproduction. On the other hand the traits acquired during lifetime do not pass on to the DNA of germ cells that’s why they cannot be inherited.

Question 7. (a) Write the chemical equation for the following reactions:

(i) Methane burns in air

(ii) Ethanoic acid reacts with sodium carbonate

(iii) Ethanoic acid reacts with sodium hydroxide

(b) Why is a mixture of acetylene and oxygen instead of acetylene and air, used for welding purposes.


(a) Chemical equations for given reactions are:

(i) Burning of methane in air:

(b) Acetylene being an unsaturated hydrocarbon, burns incompletely in air due to which a sooty flame is produced with very low temperature. On the other hand when a mixture of acetylene and pure oxygen is burnt, then it gives rise to the complete combustion of acetylene to produce a blue flame with a very high temperature which can be used to weld the metals.

Question 8. A person uses concave lens spectacles. What vision defect does he have? What could be the possible reasons causing this defect?

Draw a diagram
(i) To show the image formed by an eye suffering from vision defect.
(ii) To show the correction of this eye defect using the lens.

A person using concave lens spectacles is suffering from myopia (short-sightedness).

The possible reasons causing myopia are:

  • Convering power of eye lens has been increased due to its short focal length.
  • Eye ball has become longer.

(i) Diagram showing the myopic eye:

(ii) Diagram to show the correction of myopia:

Question 9. (a) What is ovulation?

(b) How is it beneficial for the foetus to have a circulatory system that is not directly attached to the circulatory system of mother?

(c) What changes occur at the time of birth?


(a) When a girl child is born, her ovaries already contain thousands of immature ova (or eggs) which are contained in immature follicles. At the age of puberty, one follicle develops into a mature ovum (or egg). It pinches off from the surface of the ovary and enters the fallopian tube. This process is termed as ovulation.

(b) The circulatory system of the foetus and that of the mother are connected by an umbilical cord that attaches the placenta to the fetus. The separate circulatory system facilitates quick diffusion of nutrients, metabolic waste and respiratory gases between the foetus and the mother. Also placenta acts as a shield for the baby against bacteria and other pathogens.

(b) At the time of birth the strong muscles in the walls of the uterus undergo the rhythmic contractions  that are initiated by decreased progesterone secretion and the secretion of oxytocin hormone by the pituitary gland. These rhythmic contractions followed by labour pains result in the dilation of cervix. When the cervix is fully dilated, contractions help the baby to move from the uterus into the vagina through wbich the baby finally steps into the world. After the child delivery, the placenta is expelled out.

Question 10. (a) What is astigmatism? How is it caused? How can it be corrected?

(b) A person is unable to see both far as well as near objects. What is the defect he is suffering from? What could be the reason for this defect? What type of spectacles should be worn by him to correct his vision.


(a) Astigmatism: The defect by which the person is not able to differentiate horizontal and vertical position, is called astigmatism.

Causes of Astigmatism:
Astigmatism is caused by an unevenly shaped cornea. The uneven shape causes the rays of light to focus on two points on the retina, causing distortion and blurring of images.

Correction of astigmatism:

This defect of vision can be rectified by using cylindrical lenses. 

(b) The defect of eye in which a person is unable to see both far as well as near objects clearly is called presbyopia.

Causes of presbyopia:

This defect of vision commonly occurs to old people because of the following reasons:

  • Weakening of the ciliary muscles
  • Reduction in the flexibility of the eye lens

Correction of presbyopia:

This type of defect can be corrected by using spectacles having bi-focal lenses in which upper part consists of a concave lens to correct myopia (shortsightedness) used for distant visions and the lower part consists of a convex lens to correct hypermetropia (longsightedness) sused for reading purposes.

Question 11. (a) Why does carbon generally forms covalent compounds.

(b) Also state reasons to explain why covalent compounds:
(i) are bad conductors of electricity?
(ii) have low melting and boiling points?

(c) Name an allotrope of carbon that is a good conductor of electricity.


(a) Carbon being tetravalent, is neither capable of losing all of its four valance electrons nor it can easily accept four electrons to complete its octet because both of these conditions are energetically less favourable. Therefore carbon atom prefers to complete its octet by sharing of electrons and hence forms covalently bonded compounds.

(b) (i) Covalent compounds are bad conductors of electricity because they are formed by sharing of electrons. So they don't have any free electron that is required for carrying electricity. Thus they are bad conductors of electricity. 

(ii) CovaLent compounds are formed by the sharing of electrons, hence the intermolecular forces are relatively weaker than the electrostatic forces existing between the ionic compounds that are formed by the transferring of electrons. Due to the weak intermolecular forces, less energy is needed to break the covalent bonds hence they have low melting and boiling points

(c) Structure of graphite accounts for it being a good conductor of electricity. In its structure each carbon atom is directly attached to three carbon atoms through covalent bonds. Therefore, out of the four valence electrons in a carbon atom, only three are used for bonding and the fourth electron remains free and can move from one carbon atom to the other. These free electrons make graphite a good conductor of electricity.

NCERT Exemplar Problems and Solutions Class 10 Science: All Chapters

Question 12. (a) Explain the term “rancidity”. Name and explain the type of chemical reactions responsible for causing rancidity.

(b)Write three methods for preventing rancidity of food.


(a) Rancidity: Rancidity can be defined as the chemical decomposition of fats and oils due to hydrolysis or auto-oxidation or microbial activity, producing aldehydes, hydroxyl acids, keta acids, and other compounds. This causes a considerable damage to the natural structure of fats and oils, producing undesirable odors, flavors and discoloration  in the food containing them; making them unfit for consumption.

For Example: When butter is kept open for a long time, then its smell and taste gets changed (due to the presence of butyric acid (a four-carbon acid)).

Factors responsible for causing rancidity are light, oxygen, trace elements such as iron and zinc, salt, water, bacteria, and molds etc.

Types of chemical reactions responsible for causing rancidity:

(i) Oxidation Reaction:  Oxidation is said to have occurred when a substance reacts with oxygen in the air forming oxides. It causes the formation of peroxide at the double bonds (C=C) of fat molecules with subsequent breakdown of these peroxides to form aldehydes, ketones and acids of lower molecular weight. This results in off-flavor, off-odour and sometimes change in colour.

(ii) Hydrolysis: Hydrolysis can be defined as the breaking down of a chemical compound into two or more simpler compounds by reacting with water. It decomposes a lipid into its component fatty acids and glycerol.

                C-O-CO-R + H2O →  C-O-H + HO-CO-R

(b) Following are the methods for preventing rancidity of food: (write any three)

  • In order to prevent rancidity due to oxygen or water vapours present in a container, vacuum packing should be used or the packing should be filled with inert gases. For Example: Potato Chips are usually packed in thick foil packets filled with Nitrogen.
  • Refrigeration of foods also reduces the rate of most of the reactions that cause rancidity.
  • Reducing the water content in foods by drying or smoking and then storing them in a moisture free environment helps to reduce the chances of rancidity.
  • By adding anti-oxidants (butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) to the food containing fats and oils.
  • Keeping food in air tight containers also helps to reduce oxidation.

Question 13. (a) Define 1 watt.

(b) State the commercial unit of electric energy. Express it in terms of SI unit of energy.

(c) An electric refrigerator rated as 750 W operates 8 hours per day. What is the cost of the energy to operate it for a month of June at Rs. 2.50 per kWh?


(a) Watt: Watt is the SI unit of electric power. It is the power consumed by a device that carries 1 A of electric current when operated at a potential difference of 1 V.

Thus, 1 W = 1 volt × 1 ampere = 1 V A

(b) Commercial Unit of Electric Energy:

The unit ‘watt’ is very small. So, practically a much larger unit called ‘kilowatt’ is used. It is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (Wh).

One watt hour is the energy consumed when 1 watt of power is used for 1 hour.

The commercial unit of electric energy is kilowatt hour (kWh), commonly known as ‘unit’. One kilowatt hour is the energy consumed when 1000 watt of power is used for 1 hour.

1 kW h = 1000 watt × 3600 second

             = 3.6 × 106 watt second

             = 3.6 × 106 joule (J) (where joule ‘J’ is the S.I unit of energy.)

(c)  Given, Power, P = 750 W

        Time, T = 8 hrs per day

We know that, electrical energy is the product of power and time.

Therefore, the energy consumed by the refrigerator in one day

                                     = 750 W × 8.0 hour/day

                                    = 6000 W h = 6 kW h

Now, the total energy consumed by the refrigerator in the month of June (30 Days)

                                    = 30 × Energy/day

                                    = 30 × 6kWh

                                    =180 kWh

Thus, the cost of energy required to operate the refrigerator for the month of June at Rs. 2.50 per kWh = 180 kW h × Rs 2.50 = Rs 450.00

NCERT Solutions for CBSE Class 10 Science: All Chapters

Question 14.  Three incandescent bulbs of 100 W each are connected in series in an electric circuit. In
another circuit another set of three bulbs of the same wattage are connected in parallel to
the same source.
(a) Will the bulb in the two circuits glow with the same brightness? Justify your answer.
(b) Now let one bulb in both the circuits get fused. Will the rest of the bulbs continue to glow in each circuit? Give reason.


Let there be three bulbs B1, B2 and B3, each having resistance R.
Given below are the circuit diagrams representing the two cases, i.e., when bulbs are connected in series and in parallel circuits.

 class 10 science questions

cbse important questions science

Now, in case of parallel circuit,

The resistance of each bulb = R

And voltage across each bulb = V [as a parallel combination has same voltage]

∴ Power consumprion of each bulb in parallel combination is given as:

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Therefore, each bulb in parallel combination will glow 3 times brighter than each bulb connected in series combination.

(ii) When one bulb gets fused in both the circuits, then in series combination, circuit gets broken and current stops flowing, whereas in parallel combination, same voltage continues to apply on the remaining bulbs and hence other bulbs continues to glow with same brightness.

Question 15. Apiece of wire of resistance 20 ohm is drawn out so that its length is increased to twice its original length calculate the resistance of the wire is the new situation?


As, resistance is given as:


CBSE Class 10 Science: Important questions with solutions

Question 16. Find out the following in the electric circuit given in the following figure:
(a) Effective resistance of two 8 ohm resistors in the combination
(b) Current flowing through 4 ohm resistor
(c) Potential difference across 4 ohm resistance
(d) Power dissipated in 4 ohm resistor
(e) Difference in ammeter readings, if any

                        class 10 science questions


(a) Since, two 8 ohm resistors are connected in paralleI then their effective resistance R is given by

class 10 science exam

(b) As the 4 ohm resistor and RP are in series, so the total resistance in the circuit is given as:

Thus, current through 4 ohm resistor is 1 A.

(c) Potential difference across 4 ohm resistor is given as,

            V = IR = 1 × 4 = 4V

(d) Power dissipated in 4 ohm resistor

            P = I2R = 12 × 4 = 4 W

(e) There is no difference in the reading of ammeters A1 and A2 as same current flows through all elements in a series combination.

Question 17. Why is there a need for harnessing non-conventional sources of energy? How can energy be harnessed from the sea in different ways?


There is a need for harnessing non-conventional sources of energy because of the following reasons:

  • The demand for energy is increasing day by day to meet out the basic requirement of our changed life-styles, growing use of machines and industrialisation in order to improve our living standards.
  • The fossil fuels like coal, petroleum, are are non-renewable sources of energy and are present in a limited amount which are likely to finish one day so the need for alternative sources of energy is essential.
    The energy from the sea can be harnessed in the different ways as stated below:

The energy from the sea energy be harnessed from the sea in different ways

(i) Tidal Energy: Due to the gravitational pull of mainly the moon on the spinning earth, the level of water in the sea rises and falls. This phenomenon is called high and low tides and the difference in sea-levels gives us tidal energy. Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity.

(ii) Wave Energy: The kinetic energy possessed by huge waves near the seashore can be trapped to generate electricity. The waves are generated by strong winds blowing across the sea. Wave energy would be a viable proposition only where waves are very strong. A wide variety of devices has been developed to trap wave energy for rotation of turbine and production of electricity.

(iii) Ocean Thermal Energy: The water at the surface of the sea or ocean is heated by the Sun while the water in deeper sections is relatively cold. This difference in temperature is exploited to obtain energy in ocean-thermal-energy conversion plants. These plants can operate if the temperature difference between the water at the surface and water at depths up to 2 km is 20 K (20°C) or more. The warm surface-water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of generator. The cold water from the depth of the ocean is pumped up and condenses vapour again to liquid. The energy potential from the sea (tidal energy, wave energy and ocean thermal energy) is quite large, but efficient commercial exploitation is difficult.

Question 18. Describe an experiment to prove that carbon dioxide is essential for the process of photosynthesis.


Experiment showing that carbon dioxide is essential for photosynthesis is explained below:

Materials required: A potted plant, wide mouthed bottle, a cork, potassium hydroxide solution.


(i) Take a potted plant with elongated leaves which have been destarched by keeping the plant in dark for three to four days.

(ii) Take an empty bottle and put a little amount of potassium hydroxide (KOH) in it.

(iii) Now cut the cork of the bottle into two parts and place it on one of the leaves of the potted plant in between the two parts of the cork.

(iv) Now put the bottle in the presence of sunlight for about 72 – 96 hours.

(v) Now test the leaf for the presence of starch.

You will observe that the part of the leave exposed to the sun light and atmospheric air became bluish- black and the one inside the bottle containing potassium hydroxide which absorbs carbon dioxide in the bottle remained colourless.

Question 19. (a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all the three attempts.

(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.

(c) State Modern periodic law.


(a) Dobereiner Periodic Table

Advantage: To predict the atomic mass of middle element in each triad

Limitation: Dobereiner could identify only three triads

Newland Periodic table

Advantage: Every eighth element had properties similar to that of first element.

Limitation: It was only applicable up to Calcium. It included only 56 elements and no future elements were given place in the table.

Mendeleev’s Periodic Table

Advantage: Elements with similar properties could be grouped.

He also predicted the existence of new elements that had not been discovered at that time. Limitation: No fixed position was assigned to hydrogen.

Position of isotopes was not satisfactory.

Atomic masses increased in an irregular manner.

(b) Henry Moseley was the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.

(c) Properties of elements are a periodic function of their atomic number.

Question 20. (a) Mention any two components of blood.

(b) Trace the movement of oxygenated blood in the body.

(c) Write the function of valves present in between atria and ventricles.

(d) Write one structural difference between the composition of artery and veins.


(a) Components of blood are: Plasma, red blood cells, white blood cells, platelets (write any two)

(b) Lungs → left side of the heart → aorta →body organs

(c) Valves present in between atria and ventricles prevent back flow of blood.

(d) Artery has thick elastic wall and vein is thin walled.

Valves are present in the veins and not in arteries

Question 21. (a) Define excretion.

(b) Name the basic filtration unit present in the kidney.

(c) Draw excretory system in human beings and label the following organs of excretory system which perform following functions :

i. form urine.

ii. is a long tube which collects urine from kidney.

iii. stores urine until it is passed out


(a) Process involved in removal of nitrogenous or harmful metabolic waste from the body is termed as excretion.

(b) Nephron is the basic filtration unit present in the kidney.

(c) Diagram of Human Excretory System with labelling of the parts; i. kidney ii. ureter iii. urinary bladder, is shown below:

class 10 science questions

Question 22. (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.

(a) State Fleming’s left hand rule.

(b) Write the principle of working of an electric motor.

(c) Explain the function of the following parts of an electric motor.

i. Armature

ii. Brushes

iii. Split ring


(a) Fleming’s left-hand rule: Stretch the forefinger, middle finger and thumb of left hand in such a way that they are mutually perpendicular to each other. If the forefinger point in the direction of magnetic field, middle finger point in the direction of current then the thumb show the direction of force or motion on the current carrying conductor.

(b) Principle of working of electric motor: A coil carrying electric current placed in an external magnetic field experiences a force.

(c) i. Function of Armature: It enhances the power of the motor and induces motion.

ii. Function of brushes: They help in easy transfer of charge between the coil and the external circuit.

iii. Function of split rings: They reverse the direction of current after each half rotation of the coil so that the coil can keep rotating continuously.

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 CBSE Class 10 Science Important Questions to excel their preparations and increase their chances to score high in the exam.