31/1/2 2015 Class 10 Science Question Paper Solution
(1) Write the name and formula of the 2nd member of homologous series having general formula Cn H2n + 2 .
Ans:
- Name : Ethane ½
- Formula : C2H6
(2) What is the magnification of the images formed by plane mirrors and why?
Ans: 1; size of image = size of object
(3) What is meant by power of a lens?
Ans: Power of a lens – the degree of convergence or divergence of light rays achieved by a lens/ Reciprocal of focal length of a lens
(4) Write two differences between binary fission and multiple fission in a tabular form.
Ans:
Binary fission | Multiple fission |
(i) Parent nucleus divides into two and moves to two daughter cells. | (i) Parent nucleus divides into many daughter nuclei, each surrounded by cytoplasm. |
(ii) No protective covering is formed. | (ii) A protective cyst is formed. |
(5) (a) Why do we need to manage our resources carefully?
Ans: These are not unlimited and with a tremendous increase in human population, the demand for resources is increasing at an exponential rate.
(b) Why management of natural resources requires a long term perspective?
Ans: Long term perspective required to meet the needs of the present as well as for the generations to come.
(6) List four measures that can be taken to conserve forests.
Ans:
Four measures:
(i) Reforestation of the deforested areas as soon as possible.
(ii) Ban on the indiscriminate cutting of trees.
(iii) Felling of trees for fuel wood should be avoided.
(iv) Overgrazing in forests should be discouraged.
(7) Na, Mg and Al are the elements of the same period of Modern Periodic Table having one, two and three valence electrons respectively. Which of these elements
(i) has the largest atomic radius,
(ii) is least reactive? Justify your answer stating reason for each case.
Ans: (i) Na / Sodium.
Reason – The atomic size decreases from left to right due to the increase in the nuclear charge.
(ii) Al / Aluminium.
Reason – The tendency to lose electrons decreases from left to right.
(8) From the following elements:
4Be; 9F; 19K; 20C
(i) Select the element having one electron in the outermost shell.
(ii) two elements of the same group.
Write the formula of and mention the nature of the compound formed by the union of 19K and element X (2,8,7).
Ans: (i) K / Potassium.
(ii) Be and Ca.
KX or KCl
Ionic / Electrovalent.
(9) What is meant by isomers? Draw the structures of two isomers of butane, C4 H10. Explain why we cannot have isomers of first three members of alkane series.
Ans: Isomers are compounds having the same molecular formula but different structures.
- Since branching is not possible, isomers (two different structures) are not possible for the first three members of alkane series.
(10) What is the difference between the molecules of soaps and detergents, chemically? Explain the cleansing action of soaps.
Ans: Soaps are sodium or potassium salts of long chain carboxylic acids.
Detergents are ammonium or sulphonate salts.
Cleansing action of soap – One part of soap molecule is ionic / hydrophilic and dissolves in water. The other part is non-ionic / carbon chain / hydrophobic part which dissolves in oil.
Thus soap molecules arrange themselves in the form of a micelle / diagram of a micelle.
On rinsing with water, soap is washed off, lifting the oily dirt particles with it.
(11) What are sexually transmitted diseases. List two example of each diseases caused due to (i) bacterial infection and (ii) viral infection. Which device or devices may be used to prevent the spread of such diseases.
Ans: · Diseases which are transmitted from an infected person to a healthy person due to unsafe sex.
- Two examples –
Bacterial disease : Gonorrhoea and syphilis
Viral disease : Warts and AIDS
(12) What is DNA copying? State its importance.
Ans: · A process where a DNA molecule produces two similar copies of itself in a reproducing cell.
- Importance –
(i) It makes possible the transmission of characters from parents to the next generation.
(ii) It causes variation in the population.
(13) Explain budding in hydra with the help of labelled diagrams only.
Drawing
Two labeling – Bud, Tentacles
(14) What is speciation? List four factors responsible for speciation.
Ans: Speciation : formation of new species from the pre-existing population.
Four factors :
(i) Genetic Drift
(ii) Natural Selection
(iii) Geographical Isolation
(iv) Change in gene / mutation
(15) (i) Planaria, insects, octopus and vertebrates all have eyes. Can we group eyes of these animals together to establish a common evolutionary origin? Justify your answer.
Ans: (i) No, the structure of the eye in each of the organisms is different.
(ii) “Birds have evolved from reptiles”. State evidence to prove the statement.
Ans:
- Fossils of certain dinosaurs / reptiles show imprints of feathers along
with their bones but they could not fly presumably using the feathers for insulation;
- Later they developed / evolved and adapted feathers for flight, thus becoming the ancestors of present day birds. (OR any other suitable evidence/example)
(16) After the examinations Rakesh with his friends went on a picnic to a nearby park. All friends carried cooked food packed in plastic bags or plastic bags etc and planned to dispose them off by burning. Rakesh immediately checked them and suggested to segregate the leftover foods and peels of fruits from the plastic materials and respectively dispose them off separately in the green and red dustbins placed in the corner of the park.
(a) In your opinion, is burning plastic an eco-friendly method of waste disposal? Why? State the advantage of method suggested by Rakesh.
Ans: (a) No, it pollutes air.
Advantage: Segregation of wastes into biodegradable and non biodegradable wastes at the initial stage of disposal saves time and energy.
(b) How can we contribute in maintaining the parks and roads neat and clean?
Ans: By putting wastes in proper dustbins
(17) To construct a ray diagram we use two rays of light which are so chosen that it is easy to determine their directions after reflection from the mirror. Choose these two rays and state the path of these rays after reflection from a concave mirror. Use these two rays to find the nature and position of the image of an object placed at a distance of 15 cm from a concave mirror of focal length 10 cm.
Ans: The candidate may choose any two of the following rays:
(i) A ray parallel to the principal axis, after reflection, will pass through the principal focus of a concave mirror.
(ii) A ray passing through the principal focus of a concave mirror after reflection will emerge parallel to the principal axis.
(iii) A ray passing through the centre of curvature of a concave mirror after reflection is reflected back along the same path.
(iv) A ray incident obliquely to the principal axis towards the pole of a concave mirror is reflected obliquely, making equal angles with the principal axis.
Note: The candidate must draw the ray diagram as per the two rays chosen by him/her. In the diagram shown above first two rays have been chosen/used.
(18) With the help of a labelled diagram, explain why the sun appears reddish at the sun-rise and the sun-set.
Ans:
- Light from the Sun near the horizon passes through thicker layers of air and longer distance
- Most of the blue light and shorter wavelengths of sunlight are scattered away by the particles. Light of larger wavelength reaches us giving the reddish appearance
(19) The image of a candle flame placed at a distance of 30 cm from a spherical lens is formed on a screen placed on the other side of the lens at a distance of 60 cm from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 3 cm, find the height of its image.
Ans: h1 = +3cm u = 30cm v = +60cm
1/f = 1/v – 1/u
1/f = 1/60 – 1/- 30
1/f = 1/60 + 1/30 = +3/60
\ f = +20 cm
Lens – Convex lens
H’ = v/u x h = +60cm/-30cm x 3cm = -6cm
(20) (a) State the laws of refraction of light. Explain the term absolute refractive index of a medium and write an expression to relate it with the speed of light in vacuum.
Ans: Statement of laws of Refraction of light (two laws)
When a ray of light travels from vacuum or air into a given medium then ratio of sin i to sin r is called absolute refractive index of the medium.
Absolute refractive index = Speed of light in vacuum/ Speed of light in the medium
(b) The absolute refractive indices of two media ‘A’ and ‘B’ are 2.0 and 1.5 respectively. If the speed of light in medium ‘B’ is 2×108 m/s, calculate the speed of light in:
(i) Vacuum,
(ii) medium ‘A’
Ans: nA = 2.0 na = 1.5 v8 = 2 x 108 m/s
(i) n8 = c/v8
\ c = n8 v8 =1.5 x 2.108 m/s = 3 x 108 m/s
(ii) nA = c/vA
\ vA = c/nA = 3 x 108 m/s/2 = 1.5x 108 m/s
(21) A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect. Draw ray diagram for the:
(i) defect of vision and also
(ii) for its correction.
Ans: Defect – Myopia / Nearsightedness
Correction – By using a concave lens of suitable power
(i)
(ii)
(22) Explain why carbon forms compounds mainly by covalent bond. Explain in brief two main reasons for carbon forming a large number of compounds. Why does carbon form strong bonds with most other elements?
Ans: Carbon has 4 electrons in its outermost shell, and needs to gain or lose 4 electrons to attain noble gas configuration.
Losing or gaining 4 electrons is not possible due to energy considerations; hence it shares electrons to form covalent bonds.
Two reasons for large number of carbon compounds:
Catenation: The unique ability of carbon to form bonds with other atoms of carbon giving rise to long chains of different types of compounds.
Tetravalency : Since carbon has a valency of 4, it is capable of bonding with four other atoms of carbon or atoms of elements like oxygen, hydrogen, nitrogen, sulphur, chlorine, etc.
The reason for the formation of strong bonds by carbon is its small size which enables the nucleus to hold on to the shared pairs of electrons strongly.
(23) How many pairs of chromosomes are present in human beings? Out of these how many are sex chromosomes? How many types of sex chromosomes are found in human beings?
“The sex of a newborn child a matter of chance and none of the parents may be considered responsible for it”. Draw a flow chart showing determination of sex of a newborn to justify this statement.
Ans: 23 pairs of chromosomes
One pair, two types
Flow chart
- Justification: Women produce only one type of ovum / (carrying X chromosome) and males produce two types of sperms (carrying either X or Y chromosome) in equal proportions. So the sex of a child is a matter of chance depending upon the type of sperm fertilizing the ovum.
(24) Write the functions of the following in human female reproductive system:
Ovary, oviduct, uterus
How does the embryo get nourishment inside the mother’s body? Explain in brief.
Ans: Functions: –
Ovary: (i) Production of female hormone / oestrogen and progesterone.
(ii) Production of female gamete / egg /germ cell.
Oviduct : (i) Transfer of female gamete from the ovary.
(ii) Site of fertilization.
Uterus: (i) Implantation of Zygote / embryo.
(ii) Nourishment of developing embryo.
(25) A student has obtained a point image of a distant object using the given convex lens. To find the focal length of the lens he should measure the distance between the:
(A) lens and the object only
(B) lens and the screen only
(C) object and the image only
(D) lens and the object and also between the object and the image
Ans: A
(26) Study the following diagram and select the correct statement about the device ‘X’:
(A) Device ‘X’ is a concave mirror of radius of curvature 12 cm
(B) Device ‘X’ is a concave mirror of focal length 6 cm
(C) Device ‘X’ is a concave mirror of focal length 12 cm
(D) Device ‘X’ is a convex mirror of focal length 12 cm
Ans: C
(27) After tracing the path of a ray of light through a glass prism a student marked the angle of incidence (<i), angle of refraction (<r) angle of emergence (<e) and the angle of deviation (<D) as shown in the diagram. The correctly marked angles are:
(A) <i and <r
(B) <I and <e
(C) <i, <e and <D
(D) <i, <r and <e
Ans: B
(28) Four students P, Q, R and S traced the path of a ray of light passing through a glass slab for an angle of incidence 40° and measured the angle of refraction. The values as measured them were 180; 220 ; 250 and 300 respectively. The student who has performed the experiment methodically is
(A) P
(B) Q
(C) R
(D) S
Ans: C
(29) Hard water is not available for an experiment. Some salts are given below:
(I) Sodium chloride
(II) Sodium sulphate
(III) Calcium chloride
(IV) Calcium sulphate
(V) Potassium chloride
(VI) Magnesium sulphate
Select from the following a group of these salts, each member of which may be dissolved in water to make it hard.
(A) I, II, V
(B) I, III, V
(C) III,IV, VI
(D) II, IV, VI
Ans: C
(30) A student adds 2 mL of acetic acid to a test tube containing 2 mL of distilled water. He then shakes the test tube well and leaves it to settle for some time. After about 5 minutes he observes that in the test tube there is:
(A) a clear transparent colourless solution
(B) a clear transparent pink solution
(C) a precipitate settling at the bottom of the test tube
(D) a layer of water over the layer of acetic acid
Ans: D
(31) Four students A, B, C and D reported the following set of organs to be homologous. Who is correct?
(A) Wings of a bat and a butterfly
(B) Wings of a pigeon and a bat
(C) Wings of a pigeon and a butterfly
(D) Forelimbs of cow, a duck and a lizard
Ans: A
(32) A student identified the various parts of an embryo of a gram seed and listed them as given below:
(I) Testa
(II) Plumule
(III) Radicle
(IV) Cotyledon
(V) Tegman
Out of these the actual parts of the embryo are:
(A) I, II, III
(B) II, III, IV
(C) III, IV, V
(D) II, IV, V
Ans: D
(33) To find the image-distance for varying object-distances in case of a convex lens, a student obtains on a screen sharp image of a bright object placed very far from the lens and each time focuses its image of the screen.
(a) In which direction – towards or away from the lens, does he move the screen to focus the objects?
Ans: Away from the lens
(b) What happens to the size of image – does it increase or decrease?
Ans: Increases
(c) What happen when he moves the object very close to the lens?
Ans: No image on the screen
(34) List two observations which you make when you add a pinch of sodium hydrogen carbonate to acetic acid in a test tube. Write chemical equation for the reaction that occurs.
Ans: Two observations ;
- Brisk effervescence
- Evolution of a colourless/ odourless gas.
CH3COOH + NaHCO3 CH3COONa + H2O + CO2
(35) Name the type of asexual reproduction in which two individuals are formed from a single parent and the parental identity is lost. Draw the initial and the final stages of this type of reproduction. State the event with which this reproduction starts.
Ans: Binary Fission