CBSE Previous Year Question Papers Class 10 Science SA2 Delhi – 2013

CBSE Previous Year Question Papers Class 10 Science SA2 Delhi – 2013

ScienceMathsSanskritEnglishComputer ScienceHindiSocial Science
Time allowed: 3 hours                                                                                           Maximum marks: 90

GENERAL INSTRUCTIONS:

1.  The Question Paper comprises of two Sections, A and B. You are to attempt both the Sections.
2. All questions are compulsory.
3. All questions of Section-A and all questions of Section-B are to be attempted separately.
4. Question numbers 1 to 3 in Section-A are one mark questions. These are to be answered in one word or in one sentence.
5. Question numbers 4 to 6 in Section-A are two marks questions. These are to be answered in about 30 words each.
6. Question numbers 7 to 18 in Section-A are three marks questions. These are to be answered in
   about 50 words each.
7. Question numbers 19 to 24 in Section-A are five marks questions. These are to be answered in about 70 words each.
8.  Question numbers 25 to 36 in Section-B are questions based on practical skills. Question nos. 25 to 33 are MCQs. Each question is a one mark question. You are to select one most appropriate response out of the four provided to you. Question nos. 34 to 36 are short answer questions carrying two marks each.

SET I

SECTION A
Question.1 How many vertical columns are there in the modem periodic table and what are they called?
Answer. There are 18 vertical columns in the modem periodic table and they are called groups.

Question.2 What is speciation?
Answer. The process by which new species develop from the existing species is known as speciation.

Question.3 Why should biodegradable and non-biodegradable wastes be discarded in two separate dustbins?
Answer. Biodegradable and non-biodegradable wastes should be discarded in two separate dust¬bins because biodegradable waste is decomposed by the micro-organisms to form simple harmless substances which can be used as manures for the plants (e.g., in the potted plants in our garden/terrace garden). Non-biodegradable waste cannot be broken down naturally.

Question.4 “The chromosomes number of the sexually reproducing parents and their offspring is the same.” Justify this statement.
Answer. In sexual reproduction, though the genetic material DNA (in the form of chromosomes) from two gametes, male and female gametes, combines together to form a new cell ‘zygote’ but the amount of DNA (the number of chromosomes) in the zygote does not get
doubled because gametes are the special type of cells called reproductive cells which contain only half the amount of DNA (or half the number of chromosomes) as compared to the normal body cells of an organism. So when a male gamete (having half number of chromosomes of the organism) combines with a female gamete (which also contain half the number of chromosomes) during sexual reproduction, then the new cell zygote will have the normal amount of DNA or normal number of chromosomes in it.

Question.5 “A ray of light incident on a rectangular glass slab immersed in any medium emerges parallel to itself.” Draw a labelled ray diagram to justify the statement.
Answer.

Question.6 We often observe domestic waste decomposing in the bylanes of residential colonies. Suggest ways to make people realise that the improper disposal of waste is harmful to the environment.
Answer.

1. When the domestic waste is decomposed by the action of micro-organisms then it becomes the breeding place of flies and mosquitoes. Flies and mosquitoes are the carrier and vector of many dangerous diseases. So some posters related to the spread of such diseases can be pasted on the walls in public places to create awareness.
2.  There should be a strict fine imposed by the sanitary officer from the families which throw their domestic wastes on the roads.

Question.7 Write the name and the structural formula of the compound formed when ethanol is heated at 443 K with excess of cone. H₂SO₄. State the role of cone. H₂SO₄ in this reaction. Write chemical equation for the reaction.
Answer. When ethanol is heated with excess of concentrated sulphuric acid at 443 K, it gets dehydrated to form ethene (an unsaturated hydrocarbon)

In this reaction, concentrated sulphuric acid acts as a dehydrating agent which removes water molecules from the ethanol molecule.

Question.8 Why homologous series of carbon compounds are so called? Write chemical formula of two consecutive members of a homologous series and state the part of these compounds that determines their (i) physical properties, and (ii) chemical properties.
Answer.

1.  All the organic compounds having similar structures show similar properties and
   they are put together in the same groups or series called homologous series.
2. For example, all the alkanes have similar structures with single covalent bonds and show similar chemical properties so they are grouped together in a homologous series having general formula C_nH_2n+2.
3.  The two consecutive members of a homologous series are methane CH₄ and ethane C₂H₆.

Question.9 Given below are some elements of the modem periodic table:

(i) Any two adjacent homologues differ by 1 carbon and 2 hydrogen atoms in their molecular formulae. So the difference in molecular masses of any two adjacent homologues is 14 u. Thus the members of a homologous series show a gradual change in their physical properties with increase in molecular mass.
(ii)All the compounds of a homologous series show similar chemical properties because they have similar structures and similar bonding.
(iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?
Answer.

Question.10 Write the number of periods the modem periodic table has. How do the valency and metallic character of elements vary on moving from left to right in a period? How do the valency and atomic size of elements vary down a group?
Answer.

•  There are seven periods in the modem periodic table.
•  On moving from left to right in a period, the valency of elements increases from 1 to
  4 and then decreases to 0
•  On moving from left to right in a period, the metallic characters of elements decreases as the electropositive character of elements decreases on moving from left to right in a period.
• All the elements in a group have same valency, because the number of valence electrons in a group is same.
• On going down in a group of the periodic table, the size of atoms increases because a
  : new shell of electrons is added to the atoms at every step on moving down in a
  group.

Question.11 (a) Explain the process of regeneration in Planaria.
(b) How is regeneration different from reproduction?
Answer. (a) Planaria, a flatworm, possesses great power of regeneration. If the body of planaria gets cut into a number of pieces, then each body piece can regenerate into a complete planaria by growing all missing parts.
Diagram. See Q. 24, 2011 (I Delhi).
(b) Regeneration is different from reproduction because most simple animals would not
depend on being cut into pieces to be able to reproduce.

Q.12. Write two examples each of sexually transmitted diseases caused by (i) virus, (ii) bacteria. Explain how the transmission of such diseases be prevented?
Answer. Sexually transmitted diseases caused by
(i) Virus:

•  AIDS (Acquired Immuno Deficiency Syndrome)
•  Genital warts

(ii) Bacteria: • Gonorrhoea – • Syphilis
Transmission of such diseases can be prevented by the following ways:
— Screening tests for blood donors
— Mutually faithful monogamous relationships
— Educating people in high risk groups
— Using condoms etc.

Question.13 Tabulate two distinguishing features between acquired traits and inherited traits with one example of each.
Answer. See Q. 17, 2012 (I Outside Delhi).

Question.14 “The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it.” Justify this statement with the help of a flow chart showing determination of sex of a newborn.
Answer. Sex determination flow chart. See Q. 14, 2012 (II Outside Delhi).
Thus in human beings, the sex of the baby is determined by the type of sperm that fuses with ovum. As human male produces two types of sperms in equal proportion, so there are 50% chances of a male baby and 50% chances of a female baby.

Question.15 Mention the types of mirrors used as
(i) rear view mirrors,
(ii) shaving mirrors. List two reasons to justify your answers in each case.
Answer.
(i) Convex mirror is used as rear view mirror in vehicles because

• it always produces an erect image of the objects;
•  the image formed in a convex mirror is highly diminished thus it gives a wide field of view.

(ii)Concave mirrors are used as shaving mirrors because

•  when the face is held within the focus of a concave mirror, then an enlarged image of the face is seen in the concave mirror. This helps in making a smooth shave.

Question.16 An object of height 6 cm is placed perpendicular to the principal axis of a concave lens of focal length 5 cm. Use lens formula to determine the position, size and nature of the image if the distance of the object from the lens is 10 cm.
Answer.


Thus the image is formed at a distance of 3.3 cm from the concave lens. The negative (-) sign for image distance shows that the image is formed on the left side of the concave lens (i.e., virtual). The size of the image is 2 cm and the positive (+) sign for hand image shows that the image is erect.
Thus a virtual, erect, diminished image is formed on the same side of the object (i.e., left side).

Question.17 State the difference in colours of the Sun observed during sunris^sunset and noon.
Give explanation for each.
Answer. The Sim and surrounding sky appear red at sunrise and at sunset because at this time the Sun is near the horizon and sunlight has to travel the greatest distance through the atmosphere to reach us. Thus most of the blue colour present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.
When the Sun is overhead (as at noon) then the light coming from the Sun has to travel a relatively shorter distance through the atmosphere to reach us. Thus only a little of blue colour of the white light is scattered. Since the light coming from the overhead Sun has almost all its component colours in the right proportion, therefore the Sun in the sky overhead appears white.

Question.18 (a) What is an ecosystem? List its two main components.
(b) We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly. Explain.
Answer.
(a) An ecosystem is a self-contained unit of living things (plants, animals and decomposers) and their non-living environment (soil, air and water). An ecosystem needs only the input of sunlight energy for its functioning.
The two main components of an ecosystem are:

• Abiotic component. It includes all non-living components like soil, water, air temperature, light, pressure, etc.
•  Biotic component. It includes all living components like plants, animals, decomposers, etc.

(b) A pond is a self sufficient or independent unit in nature. It contains all the components of the ecosystem. In this ecosystem, producers (hydrophytes) trap the solar energy and then provide the basic food or energy for all other life in the pond. When the producers and consumers die, the decomposers present in the pond act on their dead bodies to return the various elements back to the nutrient pool.
On the other hand, in an aquarium there are not any. producers and nutrient pool to trap solar energy, therefore the fishes living in an aquarium need to be nourished. Moreover due to absence of decomposers the excreta of the fishes cannot be decomposed. Therefore the aquarium needs to be cleaned regularly.

Question.19 (a) Define the term ‘isomers’.
(b) Draw two possible isomers of the compound with molecular formula C₃H₆O and write their names.
(c) Give the electron dot structures of the above two compounds. 
Answer. (a) The organic compounds having same molecular formula but different structures are
known as isomers.

Question.20 (a) List three distinguishing features between sexual and asexual types of reproduction.
(b) Explain why variations are observed in the off springs of sexually reproducing organisms?
Answer.

(b) Reason. There is always a possibility of diversity of characters in the off springs because the offspring is formed as a result of fusion of two gametes produced by two different individuals—the male and the female parents. So there is an opportunity for new combinations of characters.

Question.21 (a) Identify A, 6 and C in the given diagram and write their functions.
(b) Mention the role of gamete and zygote in sexually reproducing organisms

Answer. (a)
A—Stigma. The top part of carpel is called stigma. Stigma is for receiving the pollen grains from the anther of stamen during pollination.
B—Pollen tube. When a pollen grain falls on the stigma, it bursts open and grows a pollen tube downward through the style towards the female gamete in the ovary. A male gamete moves down the pollen tube.
C—Female gamete (ovum). It is a special reproductive female sex cell which combines with male gamete to form zygote.
(b) Sexual reproduction takes place by the combination of special reproductive cells called sex cells. These cells are of two types —male sex cells and female sex cells, which are coming from two different parents—a male and a female. The cells involved in sexual reproduction are called gametes. In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called zygote. This zygote then grows and develops into a new organism in due course of time.

Question.22 (a) State the laws of refraction of light. Give an expression to relate the absolute refractive index of a medium with speed of light in vacuum.
(b) The refractive indices of water and glass with respect to air are 4/3 and 3/2 respectively. If the speed of light in glass is 2×10⁸ ms^-1, find the speed of light in (i) air, (ii) water.
Answer.

Question.23 (a) A person cannot read newspaper placed nearer than 50 cm from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect.
List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
(b) We see advertisements for eye donation on television or in newspapers. Write the importance of such advertisements.
Answer.
(a) If a person cannot read newspaper nearer than 50 cm from his eyes then he is suffering from hypermetropia. It is also called long-sightedness.
Ray diagram, causes and correction of this defect. See Q. 17, 2011 (I Delhi).
(b) We have seen advertisements in media persuading people for eye donation. If cornea of the eyes are removed within 6 hours of the death of a person, it can be transplanted to a person suffering from corneal blindness. There are more than 45 lakh cases of corneal blindness and unfortunately out of these a major portion is that of children aged below 12 years. If a person comes forward for eye donation, he can save two corneal blind persons by donating one eye to each.
How excitingjt feels to think that after one’s death, he/she can make two blind persons see this wondeiful world! Therefore it is a must to promote such advertisements to encourage people and make them aware of this noble cause.

Question.24 The elements of the third period of the Periodic Table are given below:

(a) Which atom is bigger, Na or Mg? Why?
(b) Identify the most
(i) metallic and
(ii) non-metallic element in Period 3.
(c) Which is more non-metallic, S or Cl?
(d) Which has higher atomic mass, A1 or Cl?
Answer.
(a) Na is bigger than Mg because on moving from left to right in a period, the atomic
number of elements increases which means that the number of protons and electrons in the atom increases. (The extra electrons being added to the same shell).
(b) (i) Most metallic element is Na. Most non-metallic element is Cl;
Because on moving from left to right in a period the nuclear charge increases thus the valence electrons are pulled in more strongly by the nucleus and it becomes more and more difficult for the atoms to lose electrons so tendency of atoms to lose electrons (i e., metallic character) decreases on moving from left to right in a period. On the other hand, due to increased nuclear charge, it becomes easier for the atoms to gain electrons. So the tendency to gain electrons (i.e. non-metallic character) increases on moving from left to right in a period.
(c) Cl is more non-metallic from S because on moving from left to right in a period,
nuclear charge increases so the tendency to gain electrons increases (i.e., non-metallic character).
(d) Cl has higher atomic mass because on moving from left to right in a period, atomic number increases. Simultaneously, atomic mass increases.

SECTION B
Question.25 A student takes 2 mL acetic acid in a dry test tube and adds a pinch of sodium hydrogen carbonate to it. He makes the following observations:
I. A colourless and odourless gas evolves with a brisk effervescence.
II. The gas turns lime water milky when it is passed through it.
III.The gas bums with an explosion when a burning splinter is brought near it.
IV. The gas extinguishes the burning splinter that is brought near it.
The correct observations are:
(A) I, II and III (B) II, III and IV (C) III, IV and I (D) IV, I and II
Answer.  (D) IV, I and II

Question.26 A student prepared 20% sodium hydroxide solution in a beaker containing water. The observations noted by him are given below.
I. Sodium hydroxide is in the form of pellets.
II. It dissolves in water readily.
III.The beaker appears cold when touched from outside.
IV. The red litmus paper turns blue when dipped into the solution.
The correct observations are:
(A) I, Hand III (B) II, III and IV (C) III, IV and I (D) I, II and IV
Answer.  (D) I, II and IV

Question.27 Hard water required for an experiment is not available in a school laboratory. However, following salts are available in the laboratory. Select the salts which may be dissolved in water to make it hard for the experiment.
1. Calcium Sulphate
2. Sodium Sulphate .
3. Calcium Chloride
4. Potassium Sulphate
5. Sodium Hydrogen Carbonate
6. Magnesium Chloride
(A) 1, 2 and 4 (B) 1, 3 and 6 (C) 3, 5 and 6 (D) 2, 4 and 5
Answer. (B) 1, 3 and 6

Question.28 A student focussed the image of a distant object using a device ‘X’ on a white screen ‘S’ as shown in the figure. If the distance of the screen from the device is 40 cm, select the correct statement about the device.

(A) The device X is a convex lens of focal length 20 cm.
(B) The device X is a concave mirror of focal length 40 cm.
(C) The device X is a concave mirror of radius of curvature 40 cm.
(D) The device X is a convex lens of focal length 40 cm.
Answer.(D) The device X is a convex lens of focal length 40 cm.

Question.29 Select from the following the best set-up for tracing the path of a ray of light through a rectangular glass slab:

(A) I                                                 (B)II                                             (C)III ,                                     (D) IV
Answer. (A) I

Question.30 In an experiment to trace the path of a ray of light through a glass prism for different values of angle of incidence a student would find that the emergent ray:
(A) is parallel to the incident ray
(B) perpendicular to the incident ray
(C) is parallel to the refracted ray
(D) bends at an angle to the direction of incident ray
Answer.(D) bends at an angle to the direction of incident ray

Question.31 Select the correct statements for the process of budding in yeast:
I. A bud arises from a particular region on a parent body.
II. A parent cell divides into two daughter cells, here the parental identity is lost.
III.Before detaching from the parent body a bud may form another bud.
IV. A bud when detaches from the parent body grows into a new individual.
(A) I, II and III (B) II, III and IV (C) III, IV and I (D) IV, I and II
Answer. (C) III, IV and I

Question.32 Study the different conclusions drawn by students of a class on the basis of observations of preserved/available specimens of plants and animals.
I. Potato and swTeet potato are analogous organs in plants.
II. Wings of insects and wings of birds are homologous organs in animals.
III.Wings of insects and wings of bats are analogous organs in animals.
IV. Thoms of citrus and tendrils of cucurbita are analogous organs in plants.
The correct conclusions are:
(A) I and II (B) II and IV (C) I and III (D) III and IV ‘
Answer.  (C) I and III

Question.33 In the figure, the parts marked A, B and C are sequentially:

(A) Plumule, Radicle and Cotyledon
(B) Radicle, Plumule and Cotyledon
(C) Plumule, Cotyledon and Radicle
(D) Radicle, Cotyledon and Plumule
Answer. (A) Plumule, Radicle and Cotyledon

Question.34 Why does a ray of light while passing through a prism, bend towards its base?

Answer. When a ray of light passes through a prism, it under goes refraction twice. First from rarer to denser medium of glass, it bends towards normal which is towards the base of the prism. Second time from denser to rarer medium, i.e., glass to air, it bends away from the normal i.e., towards the base of the prism.

Question.35 Name two types of fissions. Name two living beings of each type which reproduce by these methods of fission.
Answer.
Two types of fissions are:

1.  Binary Fission
2. Multiple Fission

Two examples of binary fission — Amoeba and Paramoecium
Two types of multiple fission are— Plasmodium and Spirogyra

Question.36 Write two tests you would perform to detect, whether the given colourless liquid is Acetic Acid or not.
Answer.Two tests:

1.  If we put a drop of the given colourless liquid on blue litmus paper, if the blue litmus paper changes to red, then the given acid is Acetic acid.
2.  If we smell the given liquid and the liquid gives a smell like that of vinegar, then the given acid is Acetic acid.

SET II

Except for the following questions, all the remaining questions have been asked in Set-I.
SECTION A
Question.l How many horizontal rows are there in the modern periodic table and what are they called?
Answer. There are seven horizontal rows in the modern periodic table. The horizontal rows in a periodic table are called periods.

Question.2 List any two factors that could lead to speciation.
Answer. Two factors that could lead to speciation:

1.  Geographical isolation of a population caused by various types of barriers {such as mountain range, rivers and sea). This leads to reproductive isolation due to which there is no flow of genes between separated groups of population.
2.  Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.

Question.3 Mention one negative effect of our affluent life, style on the environment.
Answer. Chlorofluorocarbons (CFCs) are the chemicals which are used in refrigerators and air conditioners as coolant when released into the air react with ozone gas present in the ozone layer and destroy it gradually. If the ozone layer in the atmosphere disappears completely, then all the extremely harmful ultraviolet radiations coming from the Sun would reach the earth and cause skin cancer and other ailments in humans, animals and plants.

Question.4 Mention the two functions of human testis.
Answer. Functions of human testis:

1.  Testes make male sex hormones called testosterone.
2.  Testes produce male sex gametes, i.e., sperms.

Question.5 Every one of us can do something to reduce our consumption of various natural resources. List four such activities based on 3-R approach.
Answer.

1. We can reduce the use of LPG by making use of solar cooker for cooking food.
2.  We can reduce the wastage of water by repairing the leakage in taps.
3.  We should collect all the discarded household items like newspapers, broken plastic items, glass bottles etc. and send them to the respective industries for recycling to make fresh paper, plastic and glass objects.
4. We can drink soft cold drinks available in the glass bottles instead of pearipet or metal containers as these glass bottles can be refilled and thus reused again.

Question.8 Name the oxidising agent used for the conversion of ethanol to ethanoic acid. Distinguish between ethanol and ethanoic acid on the basis of (i) litmus test, (ii) reaction with sodium carbonate.
Answer. Acidified K₂Cr₂O₇ can be used as oxidising agent for the conversion of ethanol to ethanoic acid.

(i) Litmus test:

• If an organic compound (to be tested) does not change the colour of either of the litmus solution (blue or red) then the organic compound is neutral in nature. Ethanol is a neutral compound.
•  When some blue litmus solution is added to the organic compound (to be tested), if the blue litmus solution turns red; it shows that the organic compound is acidic and hence it is ethanoic acid.

(ii) Reaction with sodium carbonate:

•  When the organic compound (to be tested) is taken in a test tube and a pinch of sodium carbonate is added to it; if no reaction occurs in the reaction mixture, it is ethanol.

Question.9 (a) Differentiate between alkanes and alkenes. Name and draw the structure of one
member of each.
(b) Alkanes generally burn with clean flame. Why?
Answer.

(b) Alkanes bum in air with a blue and non-sooty flame because the percentage of carbon in the alkane is comparatively low which gets oxidised completely by oxygen . present in air.

Question.11 An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a compound.
(a) Write the position of these elements in the modem periodic table.
(b) Write the formula of the compound formed.
Justify your answer in each case.
Answer. Element ‘X’                     Electronic configuration
Atomic number = 17                        2, 8, 7

Question.16 An object of height 5 cm is placed perpendicular to the principal axis of a concave lens of focal length 10 cm. Use lens formula to determine the position, size and nature of the image if the distance of the object from the lens is 20 cm.
Answer.

Question.20 (а) Write the function of placenta in human females.
(b) List four ways of preventing pregnancy. State, two advantages of using such preventive methods.
Answer.
(a) The exchange of nutrients, oxygen and waste products between the embryo and the mother takes place through placenta.
(b) Methods to prevent pregnancy. See Q. 15, 2012 (I Outside Delhi). [Page P – 47
Advantages of using preventive methods.

1. Contraceptive methods are used to prevent the spreading of sexually transmitted diseases like syphillis, AIDS etc.
2.  Contraceptive methods are also used to control human population by checking the unwanted pregnancies during sexual activities.

Question.23 State Snell’s law of refraction of light. Write an expression to relate refractive index of a medium with speed of light in vacuum.
The refractive index of a medium ‘a’ with respect to medium ‘b’ is 2/3 and the refractive index of medium ‘b’ with respect to medium ‘c’ is 4/3. Find the refractive index of medium ‘c’ with respect to medium ‘a’.
Answer. Snell’s law states that, “the ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media”.

SET III

Except for the following questions, all the remaining questions have been asked in Set-I and
Set-11.
SECTION A
Question.1 Write two reasons responsible for late discovery of noble gases?
Answer. The noble gases were discovered very late because they are very unreactive and present in extremely low concentration in the atmosphere.

Question.2 “Cell division is a type of reproduction in unicellular organisms.” Justify.
Answer. Cell division is a type of reproduction in unicellular organisms because many unicellular • organisms split into two identical halves during cell division leading to the creation of new organisms.

Question.3 List any two measures that you suggest for better management of water resources.
Answer. Rain water harvesting and installation of used water recycling plants are the two measures for better management of water resources.

Question.4 (a) Trace the path of sperms from where they are produced in human body to the
exterior.
(b) Write the functions of secretions of prostate gland and seminal vesicles in humans.
Answer.
(a) Testis —> Epididymis —> Vas deferens —> Urethra —> Penis
(b) Along the path of vas-deferens, the glands called seminal vesicles and prostrate gland, add their secretions to sperms to provide nutrition to the sperms and also make their further transport easier.

Question.5 A ray of light falls normally on the surface of a transparent glass slab. Draw a ray diagram to show its path and also mark angle of incidence and angle of emergence.
Answer.

Question.8 What happen when:
(a) ethanol is burnt in air,
(b) ethanol is heated with excess cone.H₂SO₄at 443 K,
(c) a piece of sodium is dropped into ethanol?
Answer.

Question.10 The elements Li, Na and K, each having one valence electron, are in period 2, 3 and 4 respectively of modern periodic table.
(a) In which group of the periodic table should they be?
(b) Which one of them is least reactive?
(c) Which one of them has the largest atomic radius? Give reason to justify your answer in each case.
Answer.

(a) All these elements belong to 1st group because all elements have one electron in their respective outermost shell.
(b) Li is least reactive as reactivity of metals decreases on going down in a group.
(c) K has the largest atomic radius because Li belongs to 2nd period thus has 2 energy shells, Na belongs to the 3rd period thus has 3 energy shells and K belongs to 4th period thus has 4 energy shells. So as the number of shells increases so does the size of the atom.

Question.13 (a) Name the following:
(i) thread like non-reproductive structures present in Rhizopus.
(ii) ‘blobs’ that develop at the tips of the non-reproductive threads in Rhizopus. (b) Explain the structure and the function of the structures released from the ‘blobs’ in Rhizopus.
Answer. (a)

1.  Thread like non-reproductive structures present in Rhizopus is hyphae.
2. Blob like structure at the tip of hyphae is sporangia.

(b) The thread like non-reproductive parts develop on the substrate (like bread) called hyphae. At the tip of the hyphae tiny blob like structures develop called sporangia which contain spores. These spores can develop into new Rhizopus individuals. The spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow.

Question.15 What are fossils? State their importance in the study of evolution with the help of a suitable example.
Answer. The remains or impressions of extinct animals or plants that lived in the remote past are known as fossils.
Importance of fossils:

1.  Fossils provide evidence for evolution, e.g., a fossil bird called Archaeopteryx looks like a bird but it has many features which are found in reptiles. Archaeopteryx has feathered wings like birds but teeth and tail like those of reptiles. Therefore Archaeopteryx is a connecting link between the reptiles and birds and hence suggests that the Birds have evolved from the reptiles.
2.  Habits and behaviour of extinct species can be inferred. For example, the estimation of age of dinosaur fossils have told that they first appeared on earth about 250 million years ago and became extinct about 65 million years ago.
3.  Fossils provide direct evidence of past life.
4.  With the help of fossils broad historical sequence of biological evolution can be built up.

Question.16 An object of height 4 cm is kept at a distance of 30 cm from a concave lens. Use lens formula to determine the image distance, nature and size of the image formed if focal length of the lens is 15 cm.
Answer.

Thus the image is formed at a distance of 10 cm from the concave lens.
The negative (-) sign for image distance shows the image is formed on the left side of the concave lens, i.e., it is virtual.

Thus a 1.33 cm high image is formed and positive (+) sign of h₂ shows that image is erect. Thus image distance = 10 cm, image height = 1.33 cm.
Nature of the image is virtual and erect.

Question.20 (a) Draw a sectional view of human female reproductive system and label the following parts:
(i) Where the development of egg occurs.
(ii)Where fertilisation takes place.
(b) Describe the changes the uterus undergoes:
(i) to receive the zygote.
(ii) if zygote is not formed.
Answer.
(a) Human female reproductive system.

Parts where:

1.  Development of egg occurs—Ovary
2.  Fertilisation takes place—Fallopian tube

(b)

1.  When the uterus receives the zygote, zygote divides rapidly by mitosis and forms a hollow ball of hundreds of cells called embryo which gets embedded in the thick lining of the uterus. This process is called implantation. After implantation, a disc like special tissue develops between the uterus wall and the embryo which is called placenta. The exchange of nutrients, oxygen and waste products between the embryo and the mother takes place through placenta.
2. If zygote is not received by the uterus, then the thick and soft uterus lining having lot of blood cappillaries in it is not required. The unfertilised ovum dies within a day and the uterus lining also breaks down. The breaking of the uterus lining produces blood along with tissues in the form of bleeding through the vagina. This vaginal bleeding is called menstrual flow or menstruation.

Question.24 Define the term absolute refractive index. The absolute refractive index of diamond is 2.42. What is the meaning of this statement? Refractive indices of media A, B, C and D are given below:

In which of these four media is the speed of light (i) minimum and (ii) maximum? Find the refractive index of medium C with respect to medium B.
Answer. When light is going from vacuum to another medium, then the value of refractive index is called absolute refractive index to the medium.
The absolute refractive index of diamond is 2.42. It means that the speed of light in diamond is 1/2.42 times the speed of light in vacuum.
As the refractive indices increase, speed of the light decreases in the medium.
(i) The refractive index of medium D is maximum (1.65). So the speed of light in medium D is minimum.
(ii) The refractive index of medium A is minimum (1.33). So the speed of light in medium A is maximum.

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