2018 Class 10 Science Paper CBSE: Previous Year Questions with Answers

CBSE Class 10 Science 2018 Board Paper – Questions and Solutions

Class 10 Science 2018 Previous Year Question Paper with Answers

Science , 2018

Series : TYM Set – 1

Please check that this question paper contains 12 printed pages.

Code number given on the right hand side of the question paper should be written on the title page of the answer-book by the candidate.

Please check that this question paper contains 27 questions.

Please write down the Serial Number of the question before attempting it.

15 minute time has been allotted to read this question paper. The question paper will be distributed at 10.15 a.m. From 10.15 a.m. to 10.30 a.m., the students will read the question paper only and will not write any answer on the answer-book during this period.

Time allowed : 3 hours Maximum Marks : 80

General Instructions :

(i) The question paper comprises two Sections, A and B. You are to attempt both the sections.

(ii) All questions are compulsory.

(iii) All questions of Section A and Section B are to be attempted separately.

(iv) There is an internal choice in three questions of three marks each, two questions of five marks each in Section A and in one question of two marks in Section B.

(v) Question numbers 1 and 2 in Section A are one-mark questions. They are to be answered in one word or in one sentence.

(vi) Question numbers 3 to 5 in Section A are two-marks questions. These are to be answered in about 30 words each.

(vii) Question numbers 6 to 15 in Section A are three-marks questions. These are to be answered in about 50 words each.

(viii) Question numbers 16 to 21 in Section A are five-marks questions. These are to be answered in about 70 words each.

(ix) Question numbers 22 to 27 in Section B are based on practical skills. Each question is a two-marks question. These are to be answered in brief.

Section – A

Q1. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F₁ progeny ? [1M]

Answer: All the F₁ progeny will bear violet flowers, because violet colour is dominant over white.

Q2. Write the energy conversion that takes place in a hydropower plant. [1M]

Answer: In a hydropower plant, potential energy of stored water is converted into kinetic energy, which then changes into mechanical energy (turbine) and finally into electrical energy (generator).

Q3. A compound ‘X’ on heating with excess conc. 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 formation of ‘Y’ and also write the role of sulphuric acid in the reaction. [2M]

Answer: ‘X’ is ethanol, ‘Y’ is ethene and ‘Z’ is hydrogen gas.

Reaction: ( at 443 K)

Role of sulphuric acid: It acts as a dehydrating agent, removing water from ethanol

Q4. (a) Name one gustatory receptor and one olfactory receptor present in human beings.

(b) Write a and b in the given flow chart of neuron through which information travels as an electrical impulse. [2M]

Dendrite

→

End point of Neuron

Answer: (a) Gustatory receptor: Taste buds (on tongue)
Olfactory receptor: Olfactory receptors (in nose)

(b) Dendrite → Cell body (a)→ Axon (b) → End point of neuron

Q5. 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 labelled ray diagram to support your answer. [2M]

Answer: The mirror is a convex mirror.A convex mirror always forms an erect and diminished image for all object positions.

Ray diagram of the convex mirror :

Q6. 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. [3M]

Answer: Decomposition reactions need energy to break down compounds:

(i) Heat (Thermal decomposition) : CaCO₃ → CaO + CO₂

(ii) Light (Photolysis) : 2AgCl → 2Ag + Cl₂ (in presence of sunlight)

(iii) Electricity (Electrolysis) : 2H₂O → 2H₂ + O₂ (by passing electric current)

Q7. 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 dilute solution of a strong acid.

Answer: The reaction: Zn + 2NaOH → Na₂ZnO₂ + H₂

Pass the gas through soap solution to form bubbles, then bring a burning matchstick near it. It burns with a ‘pop’ sound, confirming hydrogen gas.
Hydrogen gas is also evolved when zinc reacts with a dilute strong acid.

Q7. 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. [3M]

Answer: The salt is sodium hydrogen carbonate (baking soda).

Formation reaction: NaCl + NH₃ + H₂O + CO₂ → NaHCO₃ + NH₄Cl

Uses: (i) Making pakoras soft and crispy (as baking soda)

(ii) As an antacid to relieve acidity

Q8. (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 are arranged in a ring. Give the number of single bonds present in this compound. [3M]

Answer: (a) Most carbon compounds are poor conductors of electricity because they do not have free ions or electrons to carry electric current.

(b) The compound is cyclohexane (C₆H₁₂).
Structure:

Number of single bonds: 18 single bonds (6 C–C and 12 C–H bonds).

Q9. Name the hormones secreted by the following endocrine glands and specify one function of each : (a) Thyroid (b) Pituitary (c) Pancreas [3M]

Answer: (a) Thyroid gland – Hormone: Thyroxine
Function: Regulates metabolism of the body.

(b) Pituitary gland – Hormone: Growth hormone
Function: Controls growth and development of the body.

Q10. Write one main difference between 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 reason to justify your answer. [3M]

Answer: Asexual reproduction involves only one parent and produces identical offspring, while sexual reproduction involves two parents and leads to variation.

Species reproducing sexually have better chances of survival because variations help them adapt to changing environmental conditions, increasing their survival and evolution.

Q11. 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. [3M]

Answer: The laws of refraction of light are : (i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media.

The absolute refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in that medium.

Consider a ray of light travelling from medium 1 to medium 2. Let and be the speeds of light in medium 1 and medium 2, respectively. The refractive index of medium 2 with respect to medium 1 is . Similarly, the refractive index of medium 1 with respect to medium 2 is . If medium 1 is vacuum or air, the refractive index of medium 2 with respect to vacuum is called the absolute refractive index, given by , where c is light speed in vacuum.

Q11. 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.

Answer: The power of a lens is defined as the reciprocal of its focal length .

The SI unit of power of a lens is dioptre (D).

For focal length 40 cm : Here,

We have,

Nature: Convex lens (since focal length is positive)
Thus the power of lens is +2.5 D .

For focal length – 20 cm : Here,

We have,

Nature: Concave lens (since focal length is negative)
Thus the power of lens is – 5 D .

Q12. Show how would you join three resistors, each of resistance 9 Ω so that the equivalent resistance of the combination is (i) 13.5 Ω (ii) 6Ω ? [3M]

Answer: (i) The circuit diagram :

Here,

We have,

(ii) The circuit diagram :

We have,

12.(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.

Answer: Joule’s law of heating states that the heat produced in a conductor is directly proportional to the square of the current flowing through it, the resistance of the conductor, and the time for which the current flows.

H=I²Rt

where H is heat produced, I is current, R is resistance, and ttt is time.

(b) Here, ,,

We have,

For 100 W lamp:

For 60 W lamp:

Therefore, the total current drawn by the two bulbs is 0.72 A.

13. (a) List the factors on which the resistance of a conductor in the shape of a 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. [3M]

Answer: (a) The resistance of a conductor in the shape of a wire depends on:
(i) its length,
(ii) its area of cross-section, and
(iii) the nature (material) of the conductor.

(b) Metals are good conductors of electricity because they have a large number of free electrons which can move easily and carry electric current. Glass is a bad conductor because it does not have free electrons for the flow of current.

(c) Alloys are commonly used in electrical heating devices because they have high resistivity and high melting point. They produce more heat and do not burn easily at high temperatures.

14. Students in a school listened to the news read in the morning assembly that the mountain of garbage in Delhi, suddenly exploded and various vehicles got buried under it. Several people were also injured and there was 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. [3M]

Answer: (a) Two measures to manage the garbage we produce are:

(i) Segregating garbage into biodegradable and non-biodegradable waste.

(ii) Recycling and reusing materials like paper, plastic, glass and metal.

(b) As individuals, we can generate the least garbage by:

(i) Avoiding the use of disposable items and plastic bags.

(ii) Using reusable products and buying only necessary items.

(i) Environmental awareness and cleanliness.

(iii) Sense of responsibility towards society and nature.

15. What is a 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. [3M]

Answer: A dam is a barrier built across a river to store water and control its flow.

Large dams are built for irrigation, generation of hydroelectricity, flood control and water supply for domestic and industrial purposes.

While building large dams, the following three problems should be addressed to maintain peace among local people:

(i) Proper rehabilitation and resettlement of displaced people.

(ii) Protection of forests, wildlife and the environment.

(iii) Fair distribution and proper use of water resources among different regions and people.

16. (a) Write the steps involved in the extraction of pure metals in the middle of the activity series from their carbonate ores.

(b) How is copper extracted from its sulphide ore ? Explain the various steps supported by chemical equations. Draw labelled diagram for the electrolytic refining of copper. [5M]

Answer: (a) The following steps are involved in the extraction of pure metals in the middle of the activity series from their carbonate ores:

Concentration of ore → Metals of medium reactivity → Carbonate ore → calcinations →Oxide of metal → Reduction to metal → Purification of metal.

(b) Copper is extracted from its sulphide ore by the following steps:

Roasting: The concentrated ore is heated strongly in the presence of air to convert sulphide ore into copper oxide.

Reduction: Copper oxide reacts with copper sulphide to form copper metal.

Electrolytic Refining:

In electrolytic refining of copper, impure copper is made the anode and pure copper is made the cathode. Copper sulphate solution is used as the electrolyte. On passing electric current, pure copper from the anode dissolves into the solution and gets deposited on the cathode. Insoluble impurities settle at the bottom as anode mud.

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 the three attempts.

(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.

(c) State Modern periodic law. [5M]

Answer: (a) Advantages and limitations of early classifications of elements

Dobereiner’s Triads :

Advantage: It showed that elements with similar properties could be grouped together.

Limitation: Only a few triads could be identified and all elements could not be arranged into triads.

Newlands’ Law of Octaves :

Advantage: It showed periodicity in properties of elements arranged in increasing order of atomic masses.

Limitation: It was applicable only up to calcium and failed for heavier elements.

Mendeleev’s Periodic Table :

Advantage: It left gaps for undiscovered elements and predicted their properties correctly.

Limitation: Position of hydrogen was not fixed and isotopes could not be explained.

(b) The scientist was Henry Moseley.

(c) Modern periodic law : The physical and chemical properties of elements are periodic functions of their atomic numbers.

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.

Answer: (a) The two components of blood are plasma and red blood cells (RBCs)

(b) Oxygenated blood flows from lungs → pulmonary veins → left atrium → left ventricle → aorta → arteries → capillaries of body tissues.

(c) The function of the valves present between the atria and ventricles is to prevent the backflow of blood into the atria during ventricular contraction, ensuring that blood flows only in one direction from the atria to the ventricles.

(d) Arteries have thick, muscular and elastic walls, while veins have thinner walls with less muscle and elasticity.

18. (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 following functions :

(i) form urine. (ii) is a long tube which collects urine from kidney. (iii) store urine until it is passed out. [5M]

Answer: (a) Excretion is the process of removal of metabolic waste products like urea, carbon dioxide and excess water from the body.

(b) The basic filtration unit of the kidney is the nephron.

(i) forms urine → Kidney

(ii) long tube which carries urine from kidney to urinary bladder → Ureter

(iii) stores urine until it is passed out → Urinary bladder

(iv) carries urine out of the body → Urethra

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. [5M]

Answer: (a) (i) Ovary: Produces female gametes (ova) and secretes hormones like oestrogen and progesterone.

(ii) Oviduct (Fallopian tube): Carries the egg from ovary to uterus and is the site of fertilisation.

(iii) Uterus: It is the site where the embryo gets implanted and develops into a foetus.

(b) The description of the structure and function of the placenta is :

Structure: The placenta is a special disc-like tissue embedded in the uterine wall. It contains villi on the embryo’s side, while on the mother’s side there are blood spaces that surround these villi, providing a large surface area.

Function: The placenta allows glucose and oxygen to pass from the mother to the embryo. It also enables waste substances generated by the developing embryo to be removed by transferring them into the mother’s blood.

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.

Answer: (a) The boy is suffering from Myopia (also known as short-sightedness or near-sightedness).

The possible causes of this defect:

(i) Elongation of the eyeball

(ii) Excessive curvature of the cornea or lens.

This defect is corrected by using a concave lens (diverging lens) of appropriate power. The concave lens diverges the light rays slightly before they enter the eye, so that the rays appear to come from a farther distance and are then focused exactly onto the retina, allowing the student to see distant objects clearly.

(b) Stars twinkle due to atmospheric refraction of light. As starlight enters Earth's atmosphere, it passes through layers of varying density and temperature, causing continuous bending of light. This makes the star’s apparent position and brightness change rapidly. Hence, stars appear to twinkle when seen from Earth in night sky clearly.

20. (a) Write the function of each of the following parts of 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 reason to justify your answer.

Answer: (a) (i) Cornea: It is the transparent front layer of the eye. It helps in protecting the eye and also refracts (bends) light entering the eye.

(ii) Iris: It is the coloured part of the eye. It controls the size of the pupil and regulates the amount of light entering the eye.

(iii) Crystalline lens: It focuses light rays onto the retina by changing its shape, helping in clear vision of objects at different distances.

(iv) Ciliary muscles: These muscles control the shape of the lens and help in accommodation for near and distant vision.

(b) Early in the morning, the Sun appears reddish because sunlight has to travel a longer distance through the atmosphere. During this path, shorter wavelength colours like blue and violet are scattered away by air molecules, while the longer wavelength red light reaches our eyes, making the Sun look red.

No, this phenomenon will not be observed by an astronaut on the Moon because the Moon has no atmosphere. So, there is no scattering of sunlight, and the Sun will appear white.

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 [5M]

Answer: (a) Fleming’s left hand rule states: Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, the middle finger points in the direction of the current , then the thumb will point in the direction of the force acting on the conductor.

(b) An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule. This force causes the conductor (armature coil) to rotate.

(c) (i) The armature is the rotating part of the motor consisting of a coil wound on a soft iron core. Its function is to rotate when current passes through it in the presence of a magnetic field, thereby converting electrical energy into mechanical energy.

(ii) Brushes are made of carbon and press against the split rings. Their function is to supply electric current from the external circuit to the rotating coil of the armature while allowing smooth rotation.

(iii) The split ring (also called a commutator) is a ring divided into two halves, each connected to one end of the armature coil. Its function is to reverse the direction of current through the armature coil every half rotation. This reversal ensures that the coil continues to rotate in the same direction.

Section – B

22. A student added few pieces of aluminium metal to two test tubes A and B containing aqueous solutions of iron sulphate and copper sulphate. In the second part of her experiment, she added iron metal to another test tubes C and D containing aqueous solutions 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. [2M]

Answer: Observation of colour change:

Test tube A : Al + FeSO₄ → No change

Since, Al is more reactive than Fe .

Test tube B : Al + CuSO₄ → Colour change occurs

Since, Al is more reactive than Cu .

Test tube C : Fe + Al₂(SO₄)₃ → No reaction .

Since, Al is more reactive than Fe .

Test tube D : Fe + CuSO₄ → Colour change occurs .

Aluminium is the most reactive metal.

So, aluminium displaces both iron and copper from their salt solutions, showing it is more reactive than both Fe and Cu.

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

Answer: When a solution of sodium sulphate is added to a solution of barium chloride, a white precipitate of barium sulphate is formed.

Chemical equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

The reaction is a double displacement reaction and also a precipitation reaction because an insoluble white precipitate is produced.

24. List the steps of preparation of temporary mount of a leaf peel to observe stomata. [2M]

Answer: Steps for preparation of temporary mount of a leaf peel to observe stomata:

(a) Take a fresh leaf and peel off a thin layer from the lower surface using forceps.

(b) Place the peel in a watch glass containing water.

(d) Remove excess stain by washing with water.

(e) Place the peel on a clean glass slide and add a drop of glycerine.

(f) Cover it gently with a cover slip using a needle to avoid air bubbles.

(g) Observe the slide under a microscope to see stomata.

25. Name the process by which an amoeba reproduces. Draw the various stages of its reproduction in a proper sequence.

Answer: The process by which an Amoeba reproduces is called binary fission.

Amoeba reproduces by binary fission. First, its nucleus elongates and divides into two. Then, the cytoplasm constricts in the middle, separating into two equal daughter cells, each with a nucleus. This sequence shows how one parent amoeba forms two new individuals through simple cell division.

26. An object of height 4.0 cm is placed at a distance of 30 cm from the 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 the image to the size of the object. [2M]

Solution: Given,

Using the lens formula ,

Magnification,

Again,

The height of the image is 8 cm .

Ratio of image size to object size,

The image is formed 60 cm from the optical centre on the other side of the lens. The image is real, inverted, and 8.0 cm tall.
The ratio of size of image to size of object is 2:1.

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

V (volts)	0.5	1.0	1.5	2.0	2.5	3.0	4.0	5.0
I (amperes)	0.1	0.2	0.3	0.4	0.5	0.6	0.8	1.0

Plot a graph between current (I) and potential difference (V) and determine the resistance (R) of the resistor.

Answer: Plot potential difference (V) on the X-axis and current (I) on the Y-axis.

The graph obtained is a straight line passing through the origin, showing that current is directly proportional to potential difference.

Using Ohm’s law:

From the graph take a point (5.0 , 1.0) , we have

Therefore, the resistance of the resistor is 5 Ω.