# CBSE Class 9 Science Exam 2020: Important Short Answer Type Questions with Solutions

Here we are providing a set of important questions for class 9 Science which students should practice to prepare for the upcoming annual examination 2020. Class 9 Science important questions provided here are framed to prepare for the 3 marks questions asked in Section-B of class 9 Science exam. In CBSE Class 9 Science paper, there will be a total of 10 short answer type questions (carrying 3 marks each).

All the questions provided here are thoroughly solved. Practicing the important questions given below will certainly help you to learn effectively and improve your performance in examination.

Given below are some very important questions for short answer type questions category in class 9 Science exam:

Question 1. (a) Draw a sketch of Bohr’s model of an atom with three shells.

(b) If K and L shell of an atom are full, then what would be the total number of electrons in the atom?

Answer.

(a) Bohr’s model of an atom with three shells is presented in the following image:

(b) K shell can accommodate maximum 2 electrons and L shell can accommodate 8 electrons. Thus, if both K and L shells are full then the total number of electrons must be 10.

Question 2. What is role of tissues present in multi-cellular organisms? (3 Marks)

Answer.

The formation of tissues is important in multi-cellular organisms as :

(a) Grouping of cells and performing a single function by the group shows division of labour.

(b) Tissues combine to form organ, organ to organ system and organ systems to organisms.

(c) Due to improved organisation and higher efficacy organisms have high survival rate.

CBSE Class 9 Science Notes: All Chapters

Question 3. Give reasons for the following :
(a) Ice though being a solid floats on water.
(b) A gas fills completely the vessel in which it is kept ?
(c) Rate of evaporation of an aqueous solution decreases with increase in humidity

Answer.

(a) Density of ice is less than that of water. This is on account of the reason that the structure of ice is porous. Therefore, for a given mass , the volume of ice is more than that of water.

(b) This happens because the particles of gas have the high kinetic energy due to which they keep moving and hence exert pressure on the walls of the vessel in which it is kept and hence they readily fill the vessel completely, attaining the same volume as that of the vessel.

(c) Humidity is the amount of water vapour present in air. So, the increase in humidity means that air has high water content due to which it becomes difficult for air to take in more water molecues formed as a result of evaporation. Hence, the rate of evaporation of an aqueous solution decreases with the increase of humidity.

Question 4. (a) Name two properties of a substance to check its purity?

(b) Alloys cannot be separated by physical means, though it is considered mixture, Why?

Answer.

(a) A pure substance has a fixed melting point or boiling point at constant pressure. The purity of a substance can be tested by checking its melting point or boiling point. If a substance is impure i.e. it contains traces of another substance, the melting and boiling point of that substance will change.

(b) Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition.

Question 5. When we open the cap of a cola drink (or any carbonated beverage), why does excess of bubbles come out?

Answer.

A cola (or carbonated) beverage is produced by dissolving carbon dioxide in the drink
solution under pressure. Increase in pressure increases the solubility of gases. Therefore, more carbon dioxide dissolves at the higher pressure. When this pressure is suddenly released, e.g. by removing the cap of the bottle, carbon dioxide is less soluble, and it comes out in as bubbles from the solution.

Question 6. Give reasons for the following:
(i) A liquid generally flows easily
(ii) Ice at 0o appears colder to the mouth than water at 0°C. Why?
(iii) Doctors advise to put strips of wet cloth on the forehead of a person having high temperature.

Answer:

(i) A liquid generally flows easily because the intermolecular space between the liquid particles are more as compared to solids. Moreover there particles are loosely packed due to the presence of a little force of attraction between them.
(ii) Ice at 0o C appears colder to the mouth than water at 0 C because the ice absorbs more latent heat as compared to water at the same temperature.
(iii) Doctors advise to put strips of wet cloths on the forehead of a person having high temperature because the water absorbs the heat of the body and then evaporates thereby reducing the temperature of body.

Question 7. (a) Who discovered vaccine for the first time?

(b) What is an antibiotic? Give two examples.

Answer.

(a) Edward Jenner discovered vaccine.

(b) A substance or compound that kills or inhibits the growth of bacteria is called antibiotic.
Examples : Penicillin, Ampicillin.

Question 8. Why air is a mixture not a compound? Give reasons.

Answer.

Air is considered as a mixture for the following reasons:

• Air does not have a fixed composition. Its composition varies from place to place.
• The components of air can be separated by a physical method such as fractional distillation of liquid air.
• Liquid air does not have a definite boiling point.
• We can prepare air artificially by mixing the various components of air in the same proportions in which they occur at a place. This is a physical process because no energy changes occur.

NCERT Solutions for Class 9 Science: All Chapters

Question 9. Calculate the mass of glucose and mass of water required to make 200g of 25% solution of glucose.

Answer.

Given mass of solution(M) = 200g
Concentration of solution = 25%
Since, Mass by Mass percentage of solution = mass of solute(m) × 100 / mass of solution(M).
⇒ 25 = m × 100/200g
⇒ m = 25 × 200/100 = 50g
∴ Mass of solute = 50g
mass of solvent (water) = M - m = 200g - 50g = 150g

Question 10. How crystallization is better than evaporation?

Answer.

The process of evaporation gives residue which may contain impurities. When there is requirement to obtain solid without impurity, crystallization method is preferred.

In general practice, impure solids are obtained by evaporation and are further purified by crystallization. e.g. salt obtained from sea is further purified by the process of crystallisation.

Also in some other cases like in obtaining sugar crystals, simple evaporation cannot be used because sugar gets charred on heating to dryness. In such cases, crystallization method is applied.

Question11. Give three examples of the range of variations that you see in life forms around you.

Answer.

The following range of variations can be seen in different life forms which occurs on Earth.

(i) Variation in size: Some microscopic bacteria are few micrometre in size whereas blue whale
and redwood trees of California are approximately of 30 metres and 100 metres respectively.

(ii) Variation in lifespan : Some pine trees live for thousands of years while insects like mosquitoes die within a few days.

(iii) Variation in body colours: Some life forms such as worms are colourless or even transparent whereas many of the birds, insects, flowers etc., are brightly coloured.

Question12. When a bullet is fired from a rifle its momentum become 20 kg m/s. If the velocity of the bullet is 1000m/s what will be its mass?

Answer.

Given, Momentum (p) = 20 kg m/s

Velocity (v) = 1000m/s

Mass (m) =?

We know that, Momentum (p) = Mass (m) x Velocity (v)

Therefore, 20 kg m/s = m × 1000m/s

⟹          m=20 /1000

⟹          m= 0.02 kg =20 g

Thus, mass of the bullet = 20 g

Question13. Is it possible that an object is in the state of accelerated motion due to external force acting on it, but no work is being done by the force. Explain it with an example.

Answer.

Yes, it is possible, if an object is moving in a circular path because in that case the centripetal force always acts perpendicular to the direction of displacement.

For example: Consider the motion of earth around sun. The earth is constantly moving in a direction perpendicular to the gravitation pull of sun, hence work done by the gravitational force is zero.

CBSE Class 9 Science Solved Practice Paper 2019-2020

Question14. What do you mean by active and passive immunisation?

Answer.

Active immunisation : It promotes the production of circulating antibodies against foreign antigens by injecting a small quantity of modified antigen into the bloodstream.
Passive immunisation: It is the application of antibodies from an immune individual to non- immune patient. This treatment is used when an individual has been or probably will be, exposed to an infectious disease and there is insufficient time for active immunisation.

Question15. (a) Explain the formation of acid rain.
(b) What does the presence of smog in an area indicate?

Answer.

Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) present in atmosphere, react with water to form sulfuric and nitric acids. The Oxides of sulphur and nitrogen are produced by combustion of fossil fuels in industries, automobiles, thermal power plants and domestic appliances etc. Also the sulphur and nitrogen, present in fossil fuels form these oxides by reacting with oxygen of air by the process of combustion.

(b) The presence of smog in an area indicates the high percentage of smoke released in the air by combustion of fossil fuel in industries, thermal power plants or automobiles. It is an indicator of air pollution.

Question16. (a) What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid? When does the buoyant force become maximum?

(b)Define buoyant force. Name two factors on which buoyant force depends.

Answer.

(a) The buoyant or upthrust force increases as more and more volume of a solid object is immersed in a liquid. When object is fully immersed in water the buoyant force becomes maximum.

(b)Fluid applies an upward force on a solid which is partially or fully submerged in liquid. This phenomenon is called buoyancy (Upthrust) and applied upward force is called buoyant force.

Two factors on which buoyant force depends:

• Volume or shape of object.

• Density of fluid in which object is immersed.

Question17. If the distance between two masses is increased by a factor of 5, by what factor would the mass of one of them have to be altered to maintain the same gravitational force?

Answer.

Let Gravitational constant = G

Also, let M and  m be the masses of two objects and d be the distance between them

Then gravitation force,

When the distance between two masses is increased by 5 times, then d’ = 5d

So, new gravitation  force becomes,

Using equations (i) and (ii), we get:

Thus, to maintain  the same gravitational force, the mass should be increased by 25 times.

Question 18. Give reasons for the following:

(a) When a ball is dropped from a height, its speed continuously increases.

(b) When we stop pedaling a bicycle, its speed slows down.

(c) When a tree is shaken vigorously, its fruits and leaves fall down.

Answer.

(a) When a ball is dropped from a height, its speed continuously increases due to force of gravity (as the acceleration due to gravity increase on coming downwards).

(b) When we stop pedaling a bicycle, its speed slows down due to air resistance and force of friction between the tires of bicycle and surface of road. So retardation force is created which causes a decrease in the speed of bicycle.

(c) When tree is shaken, it moves to and fro slightly but its fruits and leave tends to remain at rest due to their inertia and hence get detached from the tree and fall down.

Question 19. Car A of mass 1500 kg, travelling at 25 m/s collides with another car B of mass 1000 kg travelling at 15 m/s in the same direction. After collision, the velocity of car A becomes 20 m/s. Calculate the velocity of car B after collision.

Answer.

mA = 1500kg
vA = 25 m/s
mB = 1000kg
vB = 15 m/s
vA' = 20 m/s
vB' = ?
According to law of conservation of momentum
total momentum before collision = Total momentum after collision
mAvA + mBvB = mAvA' + mBvB'
(1500 × 25) + (1000 × 15) = (1500 × 20) + (1000 × vB')
(37500 + 4000) = 30000 + 1000vB'
52500 – 30000 = 1000vB'

1000vB' = 22500

vB' = 22.5m/s

Question 20. State the meaning of immunisation. Mention the principle on which it is based. List two diseases for which vaccines are provided under public health programme?

Answer.

Process of developing artificial immunity in a person by giving vaccination against a particular disease is known as immunisation. Immunization is used to prevent diseases caused by microorganisms.

Principle of immunisation: The principle of immunization is based on the memory of the immune system on encountering an infectious agent. On subsequent encounters with the same or related microbe, the response of the immune system is multiplied greatly, leading to quick elimination of the infection.

The diseases that can be prevented by immunisation are: Tetanus, diphtheria, whooping cough, measles, and polio.

Question 21. (a) What is amplitude?

(b) Name the characteristics of sound.

Answer.

(a) Amplitude of a wave is the maximum displacement of the particles of the medium from their mean position when a wave passes through the medium. It is denoted by A. the SI unit is metre(m).

(b) The characteristics of sound are:

(i) Loudness

(ii) Pitch or Shrillness

(iii) Quality or timbre

CBSE Class 9 Science Syllabus for Annual Exam 2020

Question 22. Name the phylum to which the organisms showing following features belong and give one example of each:-

(a) Cylindrical worm like animals having bilateral symmetry.

(b) Segmented animals having jointed legs.

(c) Non motile animals having porous body.

Answer.

(a) Nematoda - Ascaris, wuchereria

(b) Arthropod - Cockroach

(c) Porifera- Sycon, spongilla

Question 23. Why are antibiotics successfully able to control bacterial infections but not viral infections? Explain.

Answer.

Antibiotics block biochemical pathways important for bacteria. Many antibiotics work against many species of bacteria.

Viruses do not use these pathways as they are dependent upon host machinery for their life processes.

Question 24. (a) Explain the law of conservation of energy.

(b) Give one example of each energy transformation:-

(i) Heat energy to mechanical energy.

(ii) Mechanical energy to electric energy.

(c) A person is standing from 20 minutes with heavy weight on his head. Does he do any work or not? explain.

Answer.

(a) According to conservation of energy,"Energy can neither be created nor destroyed". It can be transformed from one form to it's another form. Total energy before and after transformation remains the same.

(b) (i) When the fuel in an engine is burnt, heat energy is converted into mechanical energy to run that engine.

(ii) In an electric generator, mechanical energy is converted into electrical energy.

(c) No, the man does not do any work as no displacement occurs when he lifts the load.

Question 25. What is formula unit mass? How does it calculated? Calculate the formula unit mass of a compound Na2S2O3. (Na=23u, S=32u, O=16u)

Answer.

Formula Unit Mass: It is the sum of atomic mass of ions of atoms present in formula for a compound.

Formula unit mass of Na2S2O3 = (2 × Na atomic mass) + (2 × S atomic mass) + (3 × O atomic mass)

= (2 × 23) + (2 × 32) + (3 × 16) = 1584 u.

Question 26. Derive an expression for kinetic energy of a body of mass m, moving with velocity v.

Answer.

Work done = Force (F) × Displacement (d)

But, Force = Momentum (m) × Acceleration (a), so above equation becomes:

27. A wooden cuboid has mass of 8 kg. The length, breadth and height of this wooden cuboid are 100 cm, 50 cm and 20 cm, respectively. Find the pressure on the floors on which this block is kept.

Answer.

The pressure is the ratio of force (F) exerted by a body to the area (A) upon which the body is exerting the force.

Also, weight (W) is the force exerted by a body due to the earth’s gravitational pull,

i.e. F = W.

But W = mg

Where m = mass of the body = 8kg

And g = acceleration due to gravity = 10 m / s2

Thus, the pressure on the floors on which this block is kept = 160N/m2

28. Write the chemical formula of the following using the criss-cross method:

(a) Ammonium sulphate

(b) Magnesium bicarbonate

(c) Barium nitrate

Answer.

(a) Ammonium sulphate

(b) Magnesium bicarbonate

(c) Barium nitrate

29. ‘We can easily move our hand in the air but not through any solid material’. Justify the statement.

Answer.

The particles of air have least amount of attraction between them. Thus, one can easily move the hand air and push the particles of air apart. However, the particles of solid have maximum amount of attraction between them. A greater amount of force is required to move the particles of solid apart. Thus, a much greater force is required to move hand through a solid block of wood.

30. A gas jar containing air is upside down on a gas jar of bromine vapour. It is observed that after some time, the gas jar containing air also becomes completely reddish brown.

(a) Explain why this happens.

(b) Name the process involved.

Answer.

(a) Both air and bromine vapours are in a gaseous state and has a random motion. The particles of a gas have maximum kinetic energy. They move with high speed in all directions. The moving particles of bromine vapour and air collide with each other and bounce about in all directions due to which they get mixed uniformly causing the gas jar containing air become completely reddish brown.

(b) This process is called diffusion.

31 Why does the skin of your fingers shrink when you wash clothes for a long time?

Answer.

Soap solution is a hypertonic solution with high amounts of dissolved salts as solutes. Thus, when hands are kept in this hypertonic solution, water diffuses out of the cells by osmosis (diffusion of water across semi permeable membrane), due to lower concentration of water outside the cells. As a result, the cells shrink. The process is called exosmosis. Therefore, when skin cells come in contact with the detergent solution, they begin to lose more water by exosmosis after some time. As a result, the skin over the fingers shrinks while washing clothes for a long time.

All the CBSE class 9 short answer type questions provided here have been taken from the most important topics of class 9 Science. So, students may expect the similar questions in their class 9 Science annual exam 2019.

Also Check:

CBSE Class 9 Science Exam 2020: Important Very Short Answer Type Questions

CBSE Class 9 Science Exam 2020: Important Long Answer Type Questions

CBSE Class 9 Science Chapter-wise important topics and questions

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