CBSE class 10 Science Previous Year Question Paper 2017 All India Set-3

The thing with CBSE class 10 science is that: either you are very good at it or you struggle with it a lot. In either way, we can help you score more in CBSE Class 10 Board exams. Have you taken a good look at the syllabus?

CBSE Class 10 Science highlights important concepts which include chemical reactions, principles, equations and a lot more. TopperLearning provides study materials for CBSE Class 10 Science which offers a systematic way to prepare for the final examination. Take our study notes, for example.

The CBSE Class 10 science notes summarize all the key points of a particular chapter in the syllabus so that you can glance through them any time you want. It’s the easiest way to revise the key learnings of every chapter in the class 10 syllabus for CBSE science without even opening your Class 10 science book.  

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Q 1. Write the molecular formula of the 2nd and 3rd member of the homologous series where the first member is ethyne.

Q 2. Why is variation important for a species?

Q 3. In the following food chain, 20,000 J of energy was available to the plants. How much energy would be available to man in this chain?

Q 4. An object is placed at a distance of 15 cm from a concave lens of focal length 30 cm. List four characteristic (nature, position, etc) of the image formed by the lens.

Q 5. You being an environmentalist are interested in contributing towards the conservation of nature resources, List four activities that you can do on your own.

Q 6. Why are coal and petroleum categorised as natural resources? Given a reason as to why they should be used judiciously.

Q 7. Distinguish between esterification and saponification reactions with the help of the chemical equations for each. State one use of each (i) esters, and (ii) saponification process.

Q 8. Write the structural formula of ethanol. What happens when it is heated with excess of conc. H2 SO4 at 443 K? Write the chemical equation for the reaction stating the role of conc. H2 SO4 in this reaction.

Q 9. What is periodicity in properties of elements with reference to the Modern Periodic Table? Why do all the elements of the same group have similar properties? How does the tendency of elements to gain electrons change as we move from left to right in a period? State the reason of this change?

Q 10. Write the electronic configuration two elements X and Y whose atomic numbers are 20 and 17 respectively. Write the Molecular formula of the compound formed when element. X reacts with element Y. Draw electron-dot structure of the product and also state the nature of the bond formed between both the elements.

Q 11. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?

Q 12. What is an organic evolution? It cannot be equated with progress. Explain with the help of a suitable example.

Q 13. List the two types of reproduction. Which one of the two is responsible for bringing in more variations in its progeny and how?

Q 14. What is vegetative propagation? State two advantages and two disadvantages of this method.

Q 15. List three techniques that have been developed to prevent pregnancy. Which one of these techniques is not meant for males? How does the use of these techniques have a direct impact on the health and prosperity of a family?

Q 16. “A lens can form a magnified erect image as well as magnified inverted image of an object placed in front of it”. State the nature of this lens and draw ray diagrams to justify the above statement. Mark the positions of O, F and 2F in the diagram.

Q 17. What is “dispersion of white light”? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why it is essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other?

Q 18. 

(a) Water is an elixir of life a very important natural resource. Your science teacher wants you to prepare a plan for a formative assessment activity, “How to save water, the vital natural resource”. Write any two ways that you will suggest to bring awareness in your neighbourhood, on how to save water’.
(b) Name and explain any one way by which the underground water table does not go down further.

Q 19. With the help of one example for each, distinguish between the acquired traits and the inherited traits. Why are the traits/experiences acquired during the entire lifetime of an individual not inherited in the next generation? Explain the reason of this fact with an example.

Q 20. 

(a) Write the functions of each of the following parts in a human female reproductive system:

  1. Ovary
  2. Uterus
  3. Fallopian tube

(b) Write the structure and functions of placenta in a human female.

Q 21. Why certain compounds are called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur.

Q 22. 

(a)  A student suffering from myopia is not able to see distinctly the object placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams explain.

  1. Why the student is unable to see distinctly the objects placed beyond 5 cm from his eyes.
  2. The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.

(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.

Q 23. Analyse the following observation table showing variation of image distance (v) with object distance (u) in case of a convex lens and answer the questions that follow without doing any calculations:

 

S.No

Object Distance u (cm)

Image Distance v (cm)

1

2

3

4

5

6

-100

-60

-40

-30

-25

-15

+25

+30

+40

+60

+100

+120

 

  1. What is the focal length of the convex lens? Give reason to justify your answer.
  2. Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?
  3. Select an appropriate scale and draw a ray diagram for the observation at S. No 2. Also find the approximate value of magnification. 

Q 24. 

  1. If the image formed by a mirror for all position of the object placed in front of it is always diminished, erect and virtual, state the type of the mirror and also draw a ray diagram to justify your answer. Write one use such mirrors are put to and why.
  2. Define the radius of curvature of spherical mirrors. Find the nature and focal length of a spherical mirror whose radius of curvature is + 24 cm. 

 Q 25. Study the following ray diagram:


 

In this diagram, the angle of incidence, the angle of emergence and the angle of deviation respectively have been represented by

(A)  y, p , z

(B)  x , q , z

(C) p , y , z

(D) P , z , y

Q 26. A student very cautiously traces the path of a ray through a glass slab for different values of the angle of incidence (∠i). He then measures the corresponding values of the angle of refraction (∠r) and the angle of emergence (∠e) for every value of the angle of incidence. On analysing these measurements of angles, his conclusion would be

(A)  ∠ i > ∠ r > ∠ e

(B)  ∠ i = ∠ e > ∠ r

(C)  ∠ i < ∠ r < ∠ e

(D)  ∠ i = ∠ e < ∠ r

Q 27. Study the given ray diagrams and select the correct statement from the following:

 

 

(A) Device X is a concave mirror and device Y is a convex lens, whose focal lengths are 20 cm and 25 cm respectively.

(B) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 10 cm and 25 cm respectively

(C) Device X is a concave lens and device Y is a convex mirror, whose focal lengths are 20 cm and 25 cm respectively.

(D) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

Q 28. 
A student obtains a blurred image of a distant object on a screen using a convex lens. To obtain a distinct image on the screen he should move the lens

(A) away from the screen
(B) towards the screen
(C) to a position very far away from the screen
(D) either towards or away from the screen depending upon the position of the object.

Q 29. While studying the saponification reaction, what do you observe when you mix an equal amount of colourless vegetable oil and 20% aqueous solution of NaOH in a beaker?
(A)  The colour of the mixture has become dark brown
(B)  A brisk effervescence is taking place in the beaker
(C)  The outer surface of the beaker has become hot
(D)  The outer surface of the beaker has become cold

Q 30.  When you add a few drops of acetic acid to a test-tube containing sodium bicarbonate powder, which one of the following is your observation?
(A)  No reaction takes place
(B)  A colourless gas with pungent smell is released with brisk effervescence
(C)  A brown coloured gas is released with brisk effervescence
(D)  Formation of bubbles of a colourless and odourless gas

Q 31. A student requires hard water for an experiment in his laboratory which is not available in the neighbouring area. In the laboratory there are some salts, which when dissolved in distilled water can convert it into hard water. Select from the following groups of salts, a group, each salt of which when dissolved in distilled water will make it hard.
(A)  Sodium chloride, Potassium chloride
(B)  Sodium sulphate, Potassium sulphate
(C)  Sodium sulphate, Calcium sulphate
(D)  Calcium sulphate, Calcium chloride

Q 32. To perform an experiment to identify the different parts of an embryo of a dicot seed, first of all you require a dicot seed. Select dicot seeds from the following group:
Wheat, Gram, Maize, Pea, Barley, Ground-nut
(A) Wheat, Gram and Pea
(B) Gram, Pea and Ground-nut
(C) Maize, Pea and Barley
(D) Gram, Maize and Ground-nut

Q 33. The following vegetables are kept in a basket:
Potato, Tomato, Radish, Brinjal, Carrot, Bottle-gourd
Which two of these vegetables correctly represent the homologous structures?
(A) Carrot and Tomato
(B) Potato and Brinjal
(C) Radish and Carrot
(D) Radish and Bottle-gourd

Q 34. Draw in sequence (showing the four stages), the process of binary fission in Amoeba.

Q 35. A student focuses the image of a candle flame, placed at about 2 m from a convex lens of focal length 10 cm, on a screen. After that he moves gradually the flame towards the lens and each time focuses its image on the screen.
(A) In which direction does he move the lens to focus the flame on the screen?
(B) What happens to the size of the image of the flame formed on the screen?
(C) What difference is seen in the intensity (brightness) of the image of the flame on the screen?
(D) What is seen on the screen when the flame is very close (at about 5 cm) to the lens?

Q 36. Mention the essential material (chemicals) to prepare soap in the laboratory. Describe in brief the test of determining the nature (acidic/alkaline) of the reaction mixture of saponification reaction.

Q 1. Write the molecular formula of the 2nd and 3rd member of the homologous series where the first member is ethyne.

Solution:
The molecular formula of the 2nd and 3rd members of a homologous series where the first member is ethyne (C2H2) is formed by adding –CH2-:

2nd member of alkyne series = propyne (C3H4)  CH- C ≡ CH

3rd member of alkyne series = butyne (C4H6)    CH3 - CH2 - C ≡ CH


Q 2. Why is variation important for a species?
Solution:
Variation increases the chances of survival of a species in a constantly changing Environment.



Q 3. In the following food chain, 20,000 J of energy was available to the plants. How much energy would be available to man in this chain?

Plants → Sheep → Man
Solution:
According to the 10% law, 2 J of energy will be available for the man in this chain.



Q 4. An object is placed at a distance of 15 cm from a concave lens of focal length 30 cm. List four characteristic (nature, position, etc) of the image formed by the lens.

Solution:

  begin mathsize 12px style table attributes columnalign left end attributes row cell Given comma end cell row cell straight u equals negative 15 text  cm (It is to the left of the lens) end text end cell row cell text f = -30 cm (It is a concave lens) end text end cell row cell text Using the lens formula  end text 1 over straight f equals 1 over straight v minus 1 over straight u end cell row cell rightwards arrow 1 over straight v equals 1 over straight f plus 1 over straight u equals fraction numerator 1 over denominator left parenthesis negative 30 right parenthesis end fraction plus fraction numerator 1 over denominator left parenthesis negative 15 right parenthesis end fraction end cell row cell therefore 1 over straight v equals negative 3 over 30 equals negative 1 over 10 end cell row cell therefore text v = -10 cm end text end cell end table end style

The negative sign of the image distance shows that the image is formed on the left side of the concave mirror. Thus, the image formed by a mirror is virtual, erect and on the same side as the object.



Q 5. You being an environmentalist are interested in contributing towards the conservation of nature resources, List four activities that you can do on your own.
Solution:
Four activities which can be done as an environmentalist to conserve natural resources are 

  1. Using public transport for commuting instead of using a personal vehicle.
  2. Avoid using clothes, accessories or articles made of animal skin.
  3. Using energy-efficient electrical appliances to save electricity.
  4. Ensuring no leakage of water taps and pipes at home.



Q 6. Why are coal and petroleum categorised as natural resources? Given a reason as to why they should be used judiciously.
Solution:

  • Coal and petroleum have been formed by natural processes. They have been formed by the degeneration of dead plant and animal biomass buried deep in the earth several million years ago.
  • It has taken millions of years for the formation of these fossil fuels, and the present rate of consumption of these fossil fuels far exceeds the rate at which they are formed.
  • If exhausted, these resources will not be available for use in the near future, and hence, they should be used judiciously.

 



Q 7. Distinguish between esterification and saponification reactions with the help of the chemical equations for each. State one use of each (i) esters, and (ii) saponification process.
Solution:

Esterification

Saponification

1. Carboxylic acid reacts with alcohols in the presence of a little conc. sulphuric acid to form esters.  1. On treating an ester with a base such as NaOH, it is converted back to alcohol and sodium salt of carboxylic acid.
2. Example: Ethanoic acid reacts with ethanol in the presence of a little conc. sulphuric acid to form esters.


begin mathsize 12px style table attributes columnalign left end attributes row cell text C end text subscript text 2 end text end subscript text H end text subscript text 5 end text end subscript text OH+CH end text subscript text 3 end text end subscript text COOH  end text end cell row cell text end text downwards arrow text Conc.H end text subscript text 2 end text end subscript text SO end text subscript text 4 end text end subscript end cell row cell text CH end text subscript text 3 end text end subscript text COOC end text subscript text 2 end text end subscript text H end text subscript text 5 end text end subscript text +H end text subscript text 2 end text end subscript text O end text end cell end table end style
2. Example: Ethyl ethanoate on reaction with sodium hydroxide gives ethanol and sodium ethanoate.

begin mathsize 12px style table attributes columnalign left end attributes row cell CH subscript 3 COOC subscript 2 straight H subscript 5 plus NaOH end cell row cell text end text downwards arrow end cell row cell text C end text subscript text 2 end text end subscript text H end text subscript text 5 end text end subscript text OH   + CH end text subscript text 3 end text end subscript text COONa end text end cell end table end style 

 

Use of esters:
Esters are used in synthetic flavours, perfumes, cosmetics, lacquers, paints and varnishes.


Use of saponification reaction:
It is used in the preparation of soaps on a commercial basis.



Q 8. Write the structural formula of ethanol. What happens when it is heated with excess of conc. H2 SO4 at 443 K? Write the chemical equation for the reaction stating the role of conc. H2 SO4 in this reaction.
Solution:
Structural formula of ethanol:

 begin mathsize 12px style straight H subscript 3 straight C minus straight C with straight H subscript 2 on top minus OH end style
On adding conc. sulphuric acid to ethanol and heating the mixture up to 443 K

(443 K – 273 = 170°C) gives ethene.

 begin mathsize 12px style text C end text subscript text 2 end text end subscript text H end text subscript text 5 end text end subscript text OH end text rightwards arrow from 170 to the power of straight o straight C to Conc. straight H subscript 2 SO subscript 4 of text  CH end text subscript text 2 end text end subscript text =CH end text subscript text 2 end text end subscript text +H end text subscript text 2 end text end subscript end style

The role of conc. H2SO4 in the above reaction is that it is used as a dehydrating agent and causes dehydration of ethanol.



Q 9. What is periodicity in properties of elements with reference to the Modern Periodic Table? Why do all the elements of the same group have similar properties? How does the tendency of elements to gain electrons change as we move from left to right in a period? State the reason of this change?
Solution:
Properties which reappear at regular intervals or in which there is gradual variation at regular intervals are called periodic properties, and the phenomenon is known as the periodicity of elements.
Elements in the same group or column have the same number of electrons in their outermost shell. Hence, elements of the same group have similar properties.
On moving across a period from left to right, the tendency to gain electrons increases. This is due to an increase in the nuclear pull and a decrease in atomic size.



Q 10. Write the electronic configuration two elements X and Y whose atomic numbers are 20 and 17 respectively. Write the Molecular formula of the compound formed when element. X reacts with element Y. Draw electron-dot structure of the product and also state the nature of the bond formed between both the elements.
Solution:
Atomic number of X = 20, electronic configuration = 2, 8, 8, 2

Atomic number of Y = 17, electronic configuration = 2, 8, 7

Molecular formula of the compound = XY2

Electron-dot structure of the compound:

 

An ionic bond is formed between the two elements.


Q 11. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
Solution:
Mendel explained that it is possible that a trait is inherited but not expressed in an organism with the help of a monohybrid cross.

 

 

  • He crossed pure-bred tall plants (TT) with pure-bred dwarf plants (tt).
  • The progeny he received in the first filial generation was tall. The dwarfness did not show up in the F1 generation.
  • He then crossed the tall pea plants of the F1 generation and found that the dwarf plants were obtained in the second generation. He obtained three tall plants and one dwarf plant.

 



Q 12. What is an organic evolution? It cannot be equated with progress. Explain with the help of a suitable example. 
Solution:
Organic evolution can be defined as the slow, progressive, natural and sequential development in primitive organisms to form more complex organisms or a new species.



Q 13. List the two types of reproduction. Which one of the two is responsible for bringing in more variations in its progeny and how?
Solution:
Two types of reproduction:

    • Sexual reproduction
    • Asexual reproduction
  • Sexual reproduction is responsible for bringing in more variations in its progeny.
  • It takes place by the combination of male and female gametes.
  • Gametes are formed from one cell which involves copying of DNA and the cellular apparatus. DNA copying is not absolutely accurate, and errors result in new variations. With every DNA copied, a new variation is introduced, and this DNA copy may already have several variations accumulated from the previous generations.

 



Q 14. What is vegetative propagation? State two advantages and two disadvantages of this method.

Solution:

Vegetative propagation is a type of reproduction in which several plants are capable of producing naturally through their roots, stems and leaves.

Advantages of vegetative propagation:

  • Plants not capable of producing sexually are produced by this method.
  • It is a fast and certain method to obtain plants with desired features.

Disadvantages of vegetative propagation:

  • There is no possibility for variation.
  • The new plant grows in the same area as the parent plant which leads to competition for resources.



Q 15. List three techniques that have been developed to prevent pregnancy. Which one of these techniques is not meant for males? How does the use of these techniques have a direct impact on the health and prosperity of a family?
Solution:

Techniques to prevent pregnancy:

  • Use of intra-uterine devices such as Lippes loop and Copper T
  • Use of condoms
  • Surgical methods (e.g. tubectomy)

Use of intra-uterine devices is not meant for males.

The use of these techniques will keep the mother in good health. With a small family size, parents will be able to provide quality resources to the child such as food, clothes and education. This will improve the overall mental and physical well-being of the family.



Q 16. “A lens can form a magnified erect image as well as magnified inverted image of an object placed in front of it”. State the nature of this lens and draw ray diagrams to justify the above statement. Mark the positions of O, F and 2F in the diagram.
Solution:
Convex lens can form a magnified erect image as well as a magnified inverted image of an object placed in front of it.

 

Position of object

Position of image

Size of image

Nature of image

Between focus Fand optical centre O

On the same side of the lens as the object

Magnified

Virtual and erect

 

Position of object

Position of image

Size of image

Nature of image

Between F1 and 2F1

Beyond 2F2

Magnified

Real and inverted







Q 17. What is “dispersion of white light”? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why it is essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other?
Solution:
The phenomenon of splitting of white light into its constituent seven colours on passing through a glass prism is called dispersion of light.

 

It is essential to place the two identical prisms in an inverted position with respect to each other because the refraction produced by the second prism is equal and opposite to that produced by the first prism.

 



Q. 18

(a) Water is an elixir of life a very important natural resource. Your science teacher wants you to prepare a plan for a formative assessment activity, “How to save water, the vital natural resource”. Write any two ways that you will suggest to bring awareness in your neighbourhood, on how to save water’.
(b) Name and explain any one way by which the underground water table does not go down further.
Solution:
(a)  Two ways by which awareness on how to save water can be created in the neighbourhood:

  • By bringing to notice the current situation of drought in rural areas and its dreadful effects on humans and animals
  • Making people realise the importance of water in life and the shortage of water and its consequences in the near future


(b)   

  • Khadin is one way of recharging groundwater.
  • A khadin consists of a 100–300-m long embankment called bund made of earth. The bund is built across the lower edge of the sloping farmland.
  • Rainwater from the catchment area flows down the slope and collects in front of the bund forming a reservoir.
  • Pathways through the bund allow excess water to flow through and collect in shallow wells dug behind the bund.
  • The water which collects in the reservoir and wells seeps into the land and recharges the groundwater.



Q 19. With the help of one example for each, distinguish between the acquired traits and the inherited traits. Why are the traits/experiences acquired during the entire lifetime of an individual not inherited in the next generation? Explain the reason of this fact with an example.
Solution:

Acquired Trait

Inherited Trait

  • A trait or characteristic which develops in response to the environment and cannot be inherited.
  • A characteristic feature inherited from the previous generation.
  • Example: A person learns to swim.
  • Example: A girl has brown eyes just like her mother.

 

  • Only those traits are inherited which are developed because of changes in genes.
  • An acquired trait or experience is developed as a response to the environment; it is not inherited. These are not developed due to the changes in genes.
  • Example: Human beings experiencing weight loss due to starvation. There will be reduction in weight as a response to starvation. This will result in the reduction in the number of body cells or overall body–mass ratio of the individual. It will not have any effect on the genetic constitution of the individual. Because there is no change in the gene of the individual, it is not an acquired trait.





Q 20. 

(a) Write the functions of each of the following parts in a human female reproductive system:

  1. Ovary
  2. Uterus
  3. Fallopian tube

(b) Write the structure and functions of placenta in a human female.
Solution:
(a)   

  1. Ovary: It produces female gametes. One ovum is released by one ovary every month. It also secretes hormones oestrogen and progesterone.
  2. Uterus: It protects and nourishes the developing embryo.
  3. Fallopian tube: It passes down the ovum towards the uterus released by the ovary.

(b)  Structure of the placenta in human female:

  • The placenta is a disc which is embedded in the uterine wall.
  • It contains villi on the embryo side. The mother’s end of the placenta has blood spaces which surround the villi.

Functions of the placenta in human female:

  • Nutrients and oxygen are received by the foetus from the mother’s blood.
  • The foetus gives away waste products and carbon dioxide to the mother’s blood for excretion.



Q 21. Why certain compounds are called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur.
Solution:
Certain compounds contain only carbon and hydrogen. So, these organic compounds are called hydrocarbons.
General formula for the homologous series of alkanes = CnH2n+2
First member of the alkane family is methane.

 

General formula for the homologous series of alkenes = CnH2n First member of the alkene family is ethene.

 

General formula for the homologous series of alkynes = CnH2n-2 First member of the alkyne family is ethyne.

 

Catalytic hydrogenation is the reaction used to convert alkenes to alkanes.

 begin mathsize 12px style text CH end text subscript text 2 end text end subscript text =CH end text subscript text 2 end text end subscript text +H end text subscript text 2 end text end subscript rightwards arrow from 300 to the power of straight o straight C to Nickel of straight C subscript 2 straight H subscript 6 end style



Q 22. 

(a)  A student suffering from myopia is not able to see distinctly the object placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams explain.

  1. Why the student is unable to see distinctly the objects placed beyond 5 cm from his eyes.
  2. The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.

(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.
Solution:
(a)  This defect may arise due to excessive curvature of the eye lens or elongation of the eyeball. 

  1. A person with this defect has the far point nearer than infinity. Such a person may see clearly up to a distance of a few metres.




    In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.





  2. This defect can be corrected by using a concave lens of suitable power.A concave lens of suitable power will bring the image back onto the retina and thus the defect is corrected.





(b) 
Error converting from MathML to accessible text. 



Q 23. Analyse the following observation table showing variation of image distance (v) with object distance (u) in case of a convex lens and answer the questions that follow without doing any calculations:

 

S.No

Object Distance u (cm)

Image Distance v (cm)

1

2

3

4

5

6

-100

-60

-40

-30

-25

-15

+25

+30

+40

+60

+100

+120

 

  1. What is the focal length of the convex lens? Give reason to justify your answer.
  2. Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?
  3. Select an appropriate scale and draw a ray diagram for the observation at S. No 2. Also find the approximate value of magnification.

Solution:

  1. When the object distance and the image distance are the same, it means that the object is placed at 2f or the image is formed at 2f. From the table, it is clear that 2f = 40 cm. Therefore, the focal length of the convex lens is 20 cm.
  2. Serial number 6 is incorrect. Given that the object is placed at 15 cm which is between the focal length and the lens. Thus, the image should be formed on the same side as the object. The data given in the observation serial number 6 does not satisfy the condition.


begin mathsize 12px style table attributes columnalign left end attributes row cell text Magnification,m= end text straight v over straight u end cell row cell text Let us consider the third observation where end text end cell row cell text u=-40 cm and v = 40 cm end text end cell row cell therefore text m= end text straight v over straight u equals fraction numerator 40 over denominator negative 40 end fraction end cell row cell therefore straight m equals negative 1 end cell end table end style



Q 24. 

  1. If the image formed by a mirror for all position of the object placed in front of it is always diminished, erect and virtual, state the type of the mirror and also draw a ray diagram to justify your answer. Write one use such mirrors are put to and why.
  2. Define the radius of curvature of spherical mirrors. Find the nature and focal length of a spherical mirror whose radius of curvature is + 24 cm. 

Solution:

(a)  A convex mirror always forms a diminished, erect and virtual image of the object placed in front of it.

Position of object

Position of image

Size of image

Nature of image

Between infinity and the pole of the mirror

Between P and F behind the mirror

Diminished

Virtual and erect

 

 

Use of a convex mirror:

  1. Convex mirrors are commonly used as rear view mirrors in vehicles.
  2. They are preferred because they always give an erect image, although diminished. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view a much larger area than would be possible with a plane mirror.


(b)  The radius of curvature of a spherical mirror is the radius of the sphere of which the reflecting surface of the spherical mirror is a part and represented by R.

Radius of curvature R = 24 cm

Radius of curvature = 2 ⨯ focal length

i.e. R = 2 f

24 = 2 ⨯ f

begin mathsize 12px style table attributes columnalign left end attributes row cell straight f equals 24 over 2 equals 12 end cell row cell straight f equals 12 text  cm end text end cell end table end style

 

Q 25. Study the following ray diagram:

 

In this diagram, the angle of incidence, the angle of emergence and the angle of deviation respectively have been represented by

(A)  y, p , z

(B)  x , q , z

(C) p , y , z

(D) P , z , y

Solution:

(A)  y, p, z

The angle between the incident ray and the normal is known as the angle of incidence, and the angle between the emergent ray and the normal is known as the angle of emergence. The emergent ray is bent at an angle with the direction of the incident ray. This angle is called the angle of deviation.



Q 26. A student very cautiously traces the path of a ray through a glass slab for different values of the angle of incidence (∠i). He then measures the corresponding values of the angle of refraction (∠r) and the angle of emergence (∠e) for every value of the angle of incidence. On analysing these measurements of angles, his conclusion would be

(A)  ∠ i > ∠ r > ∠ e

(B)  ∠ i = ∠ e > ∠ r

(C)  ∠ i < ∠ r < ∠ e

(D)  ∠ i = ∠ e < ∠ r
Solution:
(B)  ∠ i = ∠ e > ∠ r
Because the emergent ray is parallel to the incident ray, the angle of incidence is equal to the angle of emergence. The refracted ray travels from a rarer medium to a denser medium (considering the first refraction); it bends towards the normal. Thus, the angle of incidence is greater than the angle of refraction. If we consider the second refraction, then light travels from a denser medium to a rarer medium, due to which it bends away from the normal after refraction. So, in this case, the angle of refraction is again less than the angle of emergence.



Q 27. Study the given ray diagrams and select the correct statement from the following:

 

 

(A) Device X is a concave mirror and device Y is a convex lens, whose focal lengths are 20 cm and 25 cm respectively.

(B) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 10 cm and 25 cm respectively

(C) Device X is a concave lens and device Y is a convex mirror, whose focal lengths are 20 cm and 25 cm respectively.

(D) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

Solution:
(D) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

Device X is a convex lens and device Y is a concave mirror whose focal lengths are 20 cm and 25 cm, respectively. A parallel ray of light incident on a concave mirror gets reflected, and the image is seen on a screen placed before it. A parallel ray of light incident on a convex lens converges to a point.



Q 28. A student obtains a blurred image of a distant object on a screen using a convex lens. To obtain a distinct image on the screen he should move the lens
(A) away from the screen
(B) towards the screen
(C) to a position very far away from the screen
(D) either towards or away from the screen depending upon the position of the object.
Solution:
(B)Inverted and diminished
When the object is at infinity, the distance of the image from the lens will be equal to the focal length of the lens.



Q 29. While studying the saponification reaction, what do you observe when you mix an equal amount of colourless vegetable oil and 20% aqueous solution of NaOH in a beaker?
(A)  The colour of the mixture has become dark brown
(B)  A brisk effervescence is taking place in the beaker
(C)  The outer surface of the beaker has become hot
(D)  The outer surface of the beaker has become cold
Solution:
(A)  The outer surface of the beaker has become hot.
When 20% NaOH solution was added to the beaker containing vegetable oil, it was observed that the beaker’s surface was warm when touched.
A whitish suspension was formed by heating the mixture of vegetable oil and 20% NaOH solution.



Q 30.  When you add a few drops of acetic acid to a test-tube containing sodium bicarbonate powder, which one of the following is your observation?
(A)  No reaction takes place
(B)  A colourless gas with pungent smell is released with brisk effervescence
(C)  A brown coloured gas is released with brisk effervescence
(D)  Formation of bubbles of a colourless and odourless gas
Solution:
(D) Formation of bubbles of a colourless and odourless gas.
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2

There is double displacement in which acetic acid reacts with sodium bicarbonate to form sodium acetate and carbonic acid.
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2CO3

Carbonic acid is unstable and undergoes a decomposition reaction to produce carbon dioxide gas.
H2CO3 → H2O + CO2
Carbon dioxide escapes from the solution as bubbles.



Q 31. A student requires hard water for an experiment in his laboratory which is not available in the neighbouring area. In the laboratory there are some salts, which when dissolved in distilled water can convert it into hard water. Select from the following groups of salts, a group, each salt of which when dissolved in distilled water will make it hard.
(A)  Sodium chloride, Potassium chloride
(B)  Sodium sulphate, Potassium sulphate
(C)  Sodium sulphate, Calcium sulphate
(D)  Calcium sulphate, Calcium chloride
Solution:
(D) Calcium sulphate, calcium chloride
Hard water can be prepared by dissolving sulphates, chlorides or bicarbonate salts of Ca2+ or Mg2+ ions.



Q 32. To perform an experiment to identify the different parts of an embryo of a dicot seed, first of all you require a dicot seed. Select dicot seeds from the following group:
Wheat, Gram, Maize, Pea, Barley, Ground-nut
(A) Wheat, Gram and Pea
(B) Gram, Pea and Ground-nut
(C) Maize, Pea and Barley
(D) Gram, Maize and Ground-nut
Solution:
(B) Gram, Groundnut, Pea
Dicot seeds have two cotyledons.



Q 33. The following vegetables are kept in a basket:
Potato, Tomato, Radish, Brinjal, Carrot, Bottle-gourd
Which two of these vegetables correctly represent the homologous structures?
(A) Carrot and Tomato
(B) Potato and Brinjal
(C) Radish and Carrot
(D) Radish and Bottle-gourd
Solution:
(C) Radish and Carrot
The structures which are same in structure and origin but are modified to perform different functions are called homologous structures.
Although radish and carrot store food and are used as food, the nutrients which each provide are different.



Q 34. Draw in sequence (showing the four stages), the process of binary fission in Amoeba.
Solution:
Binary fission in amoeba:

 



Q 35. A student focuses the image of a candle flame, placed at about 2 m from a convex lens of focal length 10 cm, on a screen. After that he moves gradually the flame towards the lens and each time focuses its image on the screen.
(A) In which direction does he move the lens to focus the flame on the screen?
(B) What happens to the size of the image of the flame formed on the screen?
(C) What difference is seen in the intensity (brightness) of the image of the flame on the screen?
(D) What is seen on the screen when the flame is very close (at about 5 cm) to the lens?
Solution:
(a)  As the candle is moved towards the lens, the image distance increases. Thus, the student moves the lens away from the screen to focus the image.
(b)  The size of the image increases when the object is moved towards the lens.
(c)  Intensity decreases.
(D) When the candle is moved very close to the lens, no image is formed on the screen. A virtual image is formed behind the candle on the same side of the screen.



Q 36. Mention the essential material (chemicals) to prepare soap in the laboratory. Describe in brief the test of determining the nature (acidic/alkaline) of the reaction mixture of saponification reaction.
Solution:
Chemicals required: Vegetable oil, common salt and 20% sodium hydroxide solution When a red litmus paper is dipped in the reaction mixture, the paper changes its colour to blue. Hence, the reaction mixture of the saponification reaction is basic in nature.

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