CBSE Previous Year Question Papers

Class 10 Science 2018

Time allowed: 3 hours
Maximum marks: 80

  • The question paper comprises five sections, A, B, C, D, and E. You are to attempt all the sections.

  • All questions are compulsory.

  • Internal choice is given in Sections B, C, D and E.

  • Question number 1 and 2 in Section A are one mark questions. They are to be answered in one word or in one sentence.

  • Questions number 3 to 5 in Section B are two marks questions. These are to be answered in about 30 words each.

  • Questions number 6 to 15 in Section C are three-marks questions. These are to be answered in about 50 words each.

  • Question number 16 to 21 in Section D are five-marks questions. These are to be answered in about 70 words each.

  • Questions number 22 to 27 in Section E are based on practical skills. Each question is a two marks question. These are to be answered in brief.

Section – A

Question 1.
A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plant bearing white flowers. What will be the result in F1 progeny? [1]
Answer:
All the progeny of F1 generation will have violet flowers because the violet colour is dominant over the recessive white colour.

Question 2.
Write the energy conversion that takes place in a hydropower plant. [1]
Answer:
In a hydropower plant, the potential energy of the water stored in a dam is converted into kinetic energy then the turbine converts the kinetic energy of falling water into mechanical energy and after that generator converts mechanical energy into electrical energy.

Question 3.
A compound ‘X’ on heating with excess cone, sulphuric acid at 443 K gives an unsaturated compound ‘Y’. ‘X’ also reacts with sodium metal to evolve a colourless gas ‘Z’. Identify ‘X’, ‘Y’ and Z’. Write the equation of the chemical reaction of the formation of ‘Y’ and also write the role of sulphuric acid in the reaction. [2]
Answer:

X = CH3CH2OH-ethanol
Y = CH2 = CH2-ethene
CH3CH2OH + Na → CH3CH2ONa + H2
Z = Hydrogen gas.
Here concentrated sulphuric acid acts as a dehydrating agent, it removes water molecules from ethanol.

Question 4.
(a) Name one gustatory receptor and one olfactory receptor present in human beings.
(b) Write a and b in the given flow chart of the neuron through which information travels as an electrical impulse.
Dendrite → a → b → End Point of Neuron
Answer:
(a) Tongue is the gustatory receptor present in the human body.
(b) Dendrite → Cell body → Axon → End point of Neuron

Question 5.
If the image formed by a spherical mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a labeled ray diagram to support your answer. [2]
Answer:
Convex mirror always forms erect and diminished image.

In the above ray diagram, the image formed is behind the mirror between pole (P) and focus (F), virtual, erect and diminished.

Question 6.
Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, and electricity. [3]
Answer:
Decomposition of zinc carbonate when the heat is applied.

Decomposition of water in presence of electricity:

Decomposition of silver chloride in presence of light:

Question 7.
2 ml of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with a dilute solution of a strong acid?
OR
The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses. [3]
Answer:

When a burning candle is brought towards the hydrogen gas, it burns with the pop sound and the candle will go off.
When Zinc metal reacts with a dilute solution of strong acid then zinc chloride and hydrogen gas will be formed.

OR
The salt commonly used to make tasty and crispy pakoras is baking soda i.e., sodium hydrogen carbonate (NaHCO3).
Chemical equation for its formation:

Uses of Baking Soda:

  1. It is used in the fire extinguisher.

  2. It is also used as an ingredient of antacid.

Question 8.
(a) Why are most carbon compounds poor conductors of electricity?
(b) Write the name and structure of a saturated compound in which the carbon atoms have arranged a ring. Give the number of single bonds present in this compound. [3]
Answer:
(a) Electricity is conducted by moving electrons. But carbon forms covalent bonds by sharing of electrons. It does not have free electrons.
(b) Cyclohexane is a saturated compound in which carbon atoms are arranged in a ring.

There are 6 single bonds present in this compound.

Question 9.
Name the hormones secreted by the endocrine glands and specify one function of each: [3]

  1. Thyroid

  2. Pituitary

  3. Pancreas

Answer:

  1. The thyroid gland secretes thyroxine. It regulates the metabolism and blood pressure of human beings.

  2. The pituitary secretes growth hormone. It helps the bones and other body organs to grow properly.

  3. The pancreas secretes insulin. Insulin helps to lower blood sugar level.

Question 10.
Write one main difference between the asexual and sexual mode of reproduction. Which species is likely to have comparatively better chances of survival the one reproducing asexually or the one reproducing sexually ? Give reasons to justify your answer. [3]
Answer:
Asexual reproduction involves only one parent and the offsprings produced are clones and similar copies of their parents whereas sexual reproduction involves two parents and the offsprings produced are different from their parents. Offsprings produced by sexual reproduction have better chances of survival.

Sexual reproduction leads to variation because it leads to the formation of offspring by the combination of DNA from both parents, so the species will have better adaptability and better survival rate.

Question 11.
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.
OR
What is meant by power of a lens ? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens. [3]
Answer:
The two laws of refraction are:
1. The Incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
2. The ratio of the sine of angle of incidence
(i) to the sine of angle of refraction (r) at a point in a medium is constant, i.e., = Constant
Absolute refractive index: It is the ratio of the speed of light in vacuum (or air) to the speed of light in the given medium.

OR
The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. It is also defined as the reciprocal of its focal length in metres.
The S.I. unit of power is dioptres (D).
Focal length of a convex lens is positive.

So, lens having a focal length of 40 cm is convex.
Focal length of a concave lens is negative.

So, lens having focal length -20 cm is concave.

Question 12.
Show how would you join three resistors, each of resistance 9 Ω so that the equivalent resistance of the combination is (a) 13.5 Ω (b) 6 Ω?
OR
(a) Write Joule’s law of heating.
(b) Two lamps, one rated 100 W; 220 V, and the other 60 W; 220 V, are connected in parallel to electric mains supply. Find the current drawn by two bulbs from the line, if the supply voltage is 220 V. [3]
Answer:
(a) To get an equivalent resistance of 13.5 Ω, the resistances should be connected as shown in the figure given below:

(b) To get an equivalent resistance of 6 Ω, the resistances should be connected as shown in the figure:

OR
(a) According to Joule’s law of heating, the heat produced in a wire is directly proportional to
1. square of current (I2),
2. resistance of wire (R),
3. time (t) for which current is passed.
Thus, the heat produced in the wire by current I in time ‘t’ is
H ∝ I2 Rt
H = K I2 Rt
but K = 1, H = I2 Rt
(b) We know that, P = VI

Question 13.
(a) List the factors on which the resistance of a conductor in the shape of wire depends.
(b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity ? Give reason.
(c) Why are alloys commonly used in electrical heating devices ? Give reason. [3]
Answer:
(a) Resistance of a conductor depends directly on its length, inversely proportional to the area of cross section, directly proportional to the temperature and it depends on the nature of the conductor.
(b) Metals have free electrons which can move and conduct electricity, whereas glass does not have free electrons which can flow freely to conduct electricity.
(c) The resistivity of an alloy is generally higher than that of pure metal. Alloys do not oxidize (bum) readily at higher temperatures. Therefore, conductors of electric heating devices, such as toasters and electric irons, are made of an alloy rather than pure metal.

Question 14.
Students in a school listened to news read in the morning assembly that the mountains of garbage in Delhi, suddenly exploded and various vehicles got buried under it. Several people were also injured and there was a traffic jam all around. In the brain storming session the teacher also discussed this issue and asked the students to find out a solution to the problem of garbage. Finally they arrived at two main points—one is self management of the garbage we produce and the second is to generate less garbage at individual level.
(a) Suggest two measures to manage the garbage we produce.
(b) As an individual what can we do to generate the least garbage ? Give two points.
(c) List two values the teacher instilled in his students in this episode. [3]
Answer:
(a) 1. Segregate the garbage produced into biodegradable and non- biodegradable waste.
2. Stop using plastic bags instead of it more of cotton and jute bags should be used.
(b) 1. We should generate less garbage as much as possible.
2. Recycling and reuse of materials should be adopted in order to minimise the waste.

Question 15.
What is the dam ? Why do we seek to build large dams? While building large dams, which three main problems should particularly be addressed to maintain peace among local people? Mention them. [3]
Answer:
A dam is a barrier constructed to hold back water and raise its level forming a reservoir used to generate electricity or as a water supply.
Large dams are built in order to generate electricity from water supply.
While making large dams, social problems should be addressed. Proper rehabilitation should be provided to people.
Judicious use of people’s money should be done to ensure economic prosperity. Environmental problems, excessive cutting of trees, biological diversity loss should be checked.

Question 16.
(a) Write the steps involved in the extraction of pure metals in middle of the activity series from carbonate ores.
(b) How is copper extracted from its sulfide ore? Explain the various steps supported by chemical equations. Draw labelled diagram for electrolytic refining of copper. [5]
Answer:
(a) First the carbonate ore of a metal is heated in absence of air. This process is called calcination.

(b) Copper is extracted from sulphide ore ,by the process of roasting. It is done in the presence of air:

Question 17.
(a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all three attempts.
(b) Name the scientist who first of all showed that the atomic number of an element is a more fundamental property than its atomic mass.
(c) State Modern Periodic law. [5]
Answer:
(a) 1. Dobereiner:
Advantage: Group three elements having similar properties they were called as the Dobereiner triads. He arranged them in order of increasing atomic mass and the mass of the middle element was equal to the average of the other two elements.
Limitation: He was able to identify only 4 triads but he failed in his attempt to group nitrogen, phosphorus and arsenic because atomic mass of phosphorus is not the average of the other two.
2. Newland:
Advantages: He arranged the elements in horizontal rows in order of increasing atomic mass. The property of every 8th element was similar to the 1st element.
Limitation: His arrangement was only applicable to lighter elements.
3. Mendeleev:
Advantage: He arranged the elements in order of their increasing atomic mass and he corrected the mass of beryllium from 4.5 to 13.5 u.
Limitation: Position of hydrogen and isotopes were not justified in his periodic table.
(b) Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.
(c) Modem periodic law states that “the properties of elements are a periodic function of their atomic number. Repetition of properties is due to the same outer electronic configuration.”

Question 18.
(a) Mention any two components of blood.
(b) Trace the movement of oxygenated blood in the body.
(c) Write the function of valves present in between atria and ventricles.
(d) Write one structural difference between the composition of artery and veins.
OR
(a) Define excretion.
(b) Name the basic filtration unit present in the kidney.
(c) Draw excretory system in human beings and label the following organs of excretory system which perform the following functions:
(i) forms urine.
(ii) is a long tube which collects urine from kidney.
(iii) Store urine until it is passed out. [5]
Answer:
(a) Red blood cells and white blood cells are the components of blood.
(b) The oxygenated blood goes into the left atrium from there it goes to the left ventricle and then to all the body organs.
(c) Valves prevents the backflow of blood from the ventricle to the atrium.
(d) Walls of arteries are thick and they carry oxygenated blood whereas walls of veins are thin and they carry deoxygenated blood.
OR
(a) The process of removing toxic waste from the human body is called excretion.
(b) Nephron is the basic filtration unit present in the kidney.

Question 19.
(a) Write the function of following parts in human female reproductive system:
(i) Ovary
(ii) Oviduct
(iii) Uterus
(b) Describe in brief the structure and function of placenta. [5]
Answer:
(a) (i) Ovary: It produces egg for fertilisation. It secretes estrogen and progesterone. Estrogen regulates secondary sexual characters and progesterone controls the thickness of the lining of the uterus.
(ii) Oviduct: It is the site of fertilization and carries eggs or fertilized ovum (zygote) to the uterus.
(iii) Uterus: It helps to nourish the fertilised ovum that will develop into a foetus. It holds the baby until it is ready for birth.
(b) Placenta is a disc shaped structure on uterine wall before implantation of an embryo. It provides oxygen and nutrients to the foetus.

It helps to remove waste also. The placenta is composed of both material tissues and tissue derived from the embryo. The chorion is the embryonic derived portion of the placenta. It is composed of fetal blood vessels and trophoblasts which are organized into finger-like structures called chorionic villi.

Question 20.
(a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
(b) Why do stars twinkle? Explain.
OR
(a) Write the function of each of the following parts of the human eye :
(i) Cornea
(ii) Iris
(iii) Crystalline lens
(iv) Ciliary muscles
(b) Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give a reason to justify your answer. [5]
Answer:
(a) The boy is suffering from myopia.
This defect is caused:
1. due to increase in length of eyeball, and
2. decrease in focal length of eye lens, when the eye is fully relaxed.
Correction: The image of a distant object (i.e., at infinity) is formed in front of the retina of eye suffering from myopia as shown in fig.(a).

As the image of the object lying at infinity is not formed on the retina of the eye, so such object can not be seen clearly by the myopic eye. The far point of such an eye is near to the eye as shown in fig. (b).

This defect can be corrected by using a concave lens (minus powered) of suitable focal length. So, a man suffering from this defect wears spectacles having a concave lens of suitable focal length. The concave lens diverges the rays of light entering the eye from infinity. Hence, this lens makes the rays of light appear to have come from the far point (O’) of the defective eye as shown in figure (c).

(b) The twinkling of a star is due to atmospheric refraction of starlight. The atmospheric refraction occurs in a medium of gradually changing refractive index.

Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. This apparent position of the star is not stationary but keeps on changing slightly, as the physical conditions of the earth’s atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light. As the path of light rays coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers i.e., the star sometimes appear brighter, and at some other time, fainter, which is the twinkling effect.
OR
(a) (i) Cornea: It is a thin membrane, covering the surface of eyeball, through which light enters. It acts as a primary lens, which provides the refraction for light rays entering the eye.
(ii) Iris: It is a dark muscular dia-phragm that controls the size of the pupil and is located just behind the cornea in the eye.
(iii) Crystalline lens: It is converging in nature, made by the jelly¬like proteinaceous material. The focal length of the crystalline lens is changed by the ciliary muscles. Its function is to focus the incoming light rays from the object on the retina using its refractive property.
(iv) Ciliary muscles: It modifies the curvature and thereby the focal length of the eye lens by contract-ing or relaxing itself to focus the image of an object on the retina according to the distance of the object. It also holds the eye lens in position.

(b) At the sunrise, the light from the sun has to travel the longest distance to reach the observer. Therefore, the sun looks almost reddish because only red colour (λb < λr) which is least scattered, is received by our eye and appears to come from the sun. Hence, the appearance of the sun at sunrise, near the horizon looks almost reddish.
This phenomenon will not be observed by an astronaut on the Moon, since there is no atmosphere so no scattering of light takes place, thus the Sun appears dark.

Question 21.
(a) State Fleming’s left-hand rule.
(b) Write the principle of working of an electric motor.
(c) Explain the function of the following parts of an electric motor:
(i) Armature
(ii) Brushes
(iii) Split ring. [5]
Answer:
(a) According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of the magnetic field and the second finger in the direction of the current, then the thumb will point in the direction of motion or the force acting on the conductor.

(b) Principle of the electric motor: When a coil carrying current is placed in a magnetic field, it will experience a force. As a result of this force, the coil begins to rotate.
(c) (i) Armature: It creates a magnetic field and the second role is to generate an electromotive force.
(ii) Brushes: Carbon brushes are used to make contact with the rotating rings of the commutator and through them to supply current to the coil.
(iii) Split ring: Split rings are used to reverse the direction of current flowing through the coil every time the coil just passes the vertical position during a revolution.

Section – B

Question 22.
A student added few pieces of aluminium metal to two test tubes A and B containing aqueous solution of iron sulphate and copper sulphate. In the second part of her experiment, she added iron metal to another test tube C and D containing aqueous solution of aluminium sulphate and copper sulphate. In which test tube or test tubes will she observe colour change? On the basis of this experiment state which one is the most reactive metal and why? [2]
Answer:
Only in test tubes A, B and D she will observe colour change from green to colourless in A because aluminium is more reactive than iron, it will displace iron. She will observe colour change from blue to colourless in test tube B. She will also observe colour change in test tube D from blue to light green. Aluminium is the most reactive metal because it displaces all the other three metals.

Question 23.
What is observed when a solution of sodium sulphate is added to a solution of barium chloride in a test tube? Write an equation for the chemical reaction involved and name the type of reaction in this case. [2]
Answer:

It is an example of a double displacement reaction.

Question 24.
List the steps of preparation of temporary mount of a leaf Peel to observe stomata. [2]
Answer:

  1. Remove a peel from lower surface of leaf using forceps and put it in water.

  2. Put a few drops of safranin stain in a watch glass.

  3. After 2-3 mins, take out the peel and put it on a slide.

  4. Put a drop of glycerine and put a coverslip over the peel.

Question 25.
Name the process by which an amoeba reproduces. Draw the various stages of its reproduction in a proper sequence.
OR
A student is viewing under a microscope a permanent slide showing various stages of asexual reproduction by budding in yeast. Draw a diagram of what he observes in the proper sequence. [2]
Answer:
Amoeba reproduces by binary fission. It results in a division of nucleus followed by division of cytoplasm.

Question 26.
An object of height 4.0 cm is placed at a distance of 30 cm from optical centre ‘O’ of a convex lens of focal length 20 cm. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘O’ and principal focus ‘F’ on the diagram. Also find the approximate ratio of size of image to the size of an object. [2]
Answer:
Given, f = +20 cm, u = -30 cm, h0 = 4 cm
We know that,


Thus, the height or size of the image is 8 cm. The minus sign shows that this height is in the downward direction, that is, the image is formed below the axis.
Ratio of size of image to object = = -2

Object between F1 and 2F1
Image is formed beyond 2F2, real, inverted.

Question 27.
The values of current (I) flowing through a given resistor of resistance (R), for the corresponding values of the potential difference (V) across the resistor are given below: [2]

Plot a graph between current (I) and potential difference (V) and determine the resistance (R) of the resistor.
Answer:
We know that, V = IR

From the graph,
V = 0.5
I = 0.1
0.5 = 0.1 × R
⇒ R = 5 Ω