Complete notes, solved papers, and key problems can help you succeed in CBSE 10th Class Science. This great educational content will help you excel in your studies.
Time allowed:3 Hours Maximum Marks:80
SECTION – A
1. Name two industries based on forest produce. [1]
Ans. Timber industries and paper manufacturing industries are based on forest produce.
2. Why are the heating elements of electric toasters and electric irons made of an alloy rather than a pure metal ?[1]
Ans. The resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at higher temperatures. Therefore, conductors of electric heating devices, such as toasters and electric irons, are made up of an alloy rather than pure metal.
SECTION – B
3. Write the molecular formula of ethene and draw its electron dot structure. [2]
Ans. Molecular formula of ethene is C2H4
Electron dot structure of ethene.
4. Given reasons : [2]
(a) Platinum, gold and silver are used to make jewellery.
Ans. Platinum, gold and silver are used to make jewellery because of its bright shiny surface, malleable and ductile nature. These properties are causes of jewellery being very custrous, drawn into wires and sheets to make jewellery designs more efficient.
(b) Metals like sodium and potassium are stored under oil.
Ans. Metals like sodium and potassium are stored under oil because they are very reactive in nature, they react with oxygen present in air. Thus to prevent their oxidation they are kept in the oil.
OR
Silver articles become black when kept in open for some time, whereas copper vessels lose their shiny brown surfaces and gain a green coat when kept in open. Name the substances present in air with which these metals react and write the name of the products formed.
Ans. Silver articles become black when kept in open for some time, whereas copper vessel lose their shiny brown surfaces and gain a green coat when kept in open because silver articles reacts with sulphur compounds such as hydrogen suphide present in the air to form silver sulphide (Ag2S) whereas copper reacts slowly with CO2 and water present in the air to form green coating of mixture of copper carbonate and copper hydroxide.
5. The absolute refractive index of Ruby is 1.7. Find the speed of light in Ruby. The speed of light in vacuum is 3 x 108 m/s. [2]
Ans. We know that,
Refractive index of ruby (μ)
or Speed of light in ruby = 3 x 108/1.7 ms-1
= 1.76 x 108 ms-1
Thus, speed of light in ruby is 1.76 x 108 ms-1.
SECTION – C
6. On heating blue coloured powder of copper (II) nitrate in a boiling tube, black copper oxide, O2 and a brown gas X is formed. [3]
(a) Identify the type of reaction and the gas X.
Ans. Decomposition reaction
The gas X is nitrogen dioxide (NO2)
(b) Write balanced chemical equation of the reaction.
Ans.
(c) Write the pH range of aqueous solution of the gas X.
Ans. Oxides of non-metals are acidic. Therefore aqueous solution of this gas would be acidic. The pH would be less than 7 or 6.9
7. (a) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans. The process of mixing the concentrated acid with water is highly exothermic. So, when a concentrated acid is added to water then heat is easily absorbed by the large amount of water. Thus it is recommended to add acid to water and not water to the acid.
(b) Dry hydrogen chloride gas does not change the colour of dry litmus paper why ?
Ans. Dry hydrogen chloride does not contain any hydrogen ions in it, so it does not show acidic behaviour and thus does not change the colour of dry litmus paper.
OR
How is sodium hydroxide manufactured in industries ? Name the process. In this process a gas X is formed as by-product. This gas reacts with lime water to give a compound Y, which is used as a bleaching agent in the chemical industry. Identify X and Y and write the chemical equation of the reaction involved. [3]
Ans. Sodium hydroxide is manufactured by the electrolysis of concentrated aqueous solution of sodium chloride.
The process of manufacturing sodium hydroxide by electrolysis process is called chloro-alkali process.
Gas X is chlorine gas and compound Y is calcium oxychloride (Bleaching powder).
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Ca(OH)2 + Cl2 → CaCl2 + H2O
8. What are amphoteric oxides ? Give an example. Write balanced chemical equations to justify your answer. [3]
Ans. Those oxides which behave both as acidic and basic oxides are called amphoteric oxides.
Example: Al2O3 (Alumina)
9. What is a homologous series of carbon compounds ? Give an example and list its three characteristics. [3]
Ans. A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by CH2 group.
Example: Alkanes with general formula CnH2n+2
Characteristics :
- (i) All the members of a homologous series can be represented by the same general formula and they have same functional group.
- (ii) Any two adjacent homologues differ by 1 carbon atom and 2 hydrogen atoms in their molecular formulae.
- (iii) The difference in the molecular masses of any two adjacent homologues is 14 u.
10. List in tabular form three distinguishing features between autotrophic nutrition and heterotrophic nutrition. [3]
Ans.
| S. No. | Autotrophic nutrition | Heterotrophic nutrition |
| 1. | In this mode of nutrition organisms make their own food. | In this mode of nutrition the organisms do not make their own food. |
| 2. | CO2 and H2O are required for the preparation of food as raw materials. | They depend On autotrophs for their food either directly or undirectly. |
| 3. | Chlorophyll and sunlight are essentially needed and they store the food in the form of starch. | Chlorophyll and sunlight are not needed and they store the food in the form of glycogen. |
11. What is transpiration ? List its two functions. [3]
Ans. The evaporation of water from the aerial parts of a plant is called transpiration.
Functions of transpiration:
- (i) It helps in the upward movement of water and minerals from root to the aerial parts through the stem and in the absorption.
- (ii) It helps in cooling the plant surface. It helps in the movement of dissolved minerals from root to leaves.
OR
(a) What is translocation ? Why is it essential for plants ?
Ans. The transport of food from leaves to other parts of the plant is called translocation. Translocation is essential for plants because without it food prepared by the leaves cannot reach other parts of the plant for their growth and development.
(b) Where do the substances in plants reach as a result of translocation ?
Ans. The substances in plants reach to other tissues in plants from the leaves, fruits, seeds and other growing organs as a result of translocation.
12. What is carpel ? Write the function of its various parts.[3]
Ans. The flask-shaped organ in the centre of a flower is called carpel. It is also called as female reproductive organ of the plant.
It is made up of three parts :
- (i) Stigma is the top part of carpel and is sticky. So, it receives the pollen from the anther of stamen.
- (ii) Style connects stigma to ovary and acts as the passage for the growth of pollen tube.
- (iii) Ovary contains female gametes of the plant and helps in reproduction, it is the site of fertilization.
13. A student holding a mirror in his hand, directed the reflecting surface of the mirror towards the Sun. He then directed the reflected light on to a sheet of paper held close to the mirror. [3]
(a) What should he do to burn the paper ?
Ans. He should place the sheet of paper at the focus of the mirror to burn the paper.
(b) Which type of mirror does he have ?
Ans. He has a concave mirror.
(c) Will he be able to determine the approximate value of focal length of this mirror from this activity ? Give reason and draw ray diagram to justify your answer in this case.
Ans. Yes, the sheet of paper will start burning at the focus of the mirror which will give approximate value of focal length, i.e., the distance between mirror and the focal point where the sheet of paper starts burning.
A concave mirror forms a real image of the sun.
OR
A 10 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 12 cm. The distance of the object from the lens is 18 cm. Find the nature, position and size of the image formed.
Ans. Given: Height of object, h1 = + 10 cm.
Focal length, f = + 12 cm.
Object distance, u = – 18 cm.
From the lens fomula,
= – 20 cm
The position of image formed is at distance of 36 cm from convex lens.
Since the value of magnification is more than 1 (it is 2), the image formed is larger than object.
The minus sign of magnification shows that image is formed below the principal axis. Hence, the image formed is real and inverted.
14. What are solar cells ? Explain the structure of solar panel. List two principal advantages associated with solar cells.**[3]
15. Write the essential function performed by ozone at the higher levels of the Earth’s atmosphere ? How is it produced ? Name the synthetic chemicals mainly responsible for the drop of amount of ozone in the atmosphere. How can the use of these chemicals be reduced? [3]
Ans. Ozone layer absorbs most of the harmful ultraviolet radiations from the sun to the earth. It is formed high up in the atmosphere by the action of ultraviolet radiation on oxygen gas. Chlorofluorocarbons are the synthetic chemicals responsible for the drop of amount of ozone in the atmosphere.
The use of these chemicals can be reduced by:
- (i) Replacement of chlorofluorocarbons with hydrochlorofluorocarbons because it breaks down more quickly.
- (ii) Safe disposal of old appliances such as refrigerators and freezers.
- (iii) Finding substitute chemicals that are ozone friendly.
SECTION – D
16. (a) List any three observations which posed a challenge to Mendeleev’s Periodic law.**
(b) How does the metallic character of elements vary on moving from**
(i) left to right in a period,
(ii) from top to bottom in a group of the Modern Periodic Table ?
Give reason for your answer. [5]
OR
The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7, electrons respectively in their outermost shells. Write the group numbers in which these elements are placed in the Modern Periodic Table. Write the electronic configuration of the atoms of B and D, and the molecular formula of the compound formed when B and D combine.**
17. (a) Why is the use of iodised salt advisable? Name the disease caused due to deficiency of iodine in our diet and state its one symptom.
Ans. Iodised salt is advisable because iodine is necessary for the formation of thyroxine hormone by thyroid gland. Goitre is the disease caused due to its deficiency.
Symptom: The neck of the person appears to be swollen due to the enlargement of thyroid gland.
(b) How do nerve impulses travel in the body? Explain.
Ans. Two neurons are not joined to one another completely. There is a small gap between a pair of neuron. This gap is called synapse. The nerve impulse are carried out to this gap by the help of neuro transmitter (chemical substance). The conduction of nerve impulse through the synapse takes place in the form of electrical nerve impulse. When a stimulus acts on the receptor, an electrical impulse is produced with the help of chemical reaction. This electrical impulse passes through the synapse and then to the other neuron. Thus, in this way nerve impulses travel in the body.
OR
What is hydrotropism ? Design an experiment to demonstrate this phenomenon. [5]
Ans. The movement of root of plants towards water is called hydrotropism.
Take two glass troughs A and B fill each one of them two-thirds with soil. In trough A plant a tiny seedling figure (a). In trough B plant a similar seedling and also place a small ‘clay pot inside the soil figure (b). Water the soil in trough A daily and uniformly. Do not water the soil in trough B but put some water in the clay pot buried in the soil. Leave both the troughs for a few days.
Now, dig up the seedlings carefully from both the trough without damaging their roots. We will find that the root of seedling in trough A is straight. On the other hand, the root of seedling in trough B is found to be bent to the right side (towards the clay pot containing water) figure (b)This can be explained as follows.
In trough A, the root of seedling gets water from both sides (because the soil is watered uniformly) in trough B, the roots gets water oozing out from the clay pot which is kept on the right side. So, the root of seedling in trough B grows and bends towards the source of water to the right side. The experiment shows that the root of a plant grows towards water. In other words, the root of a plant is positively hydrotropic.
18. (a) What are homologous structures ? Give an example.**
(b) “The sex of a new born 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 sex-determination in human beings. [5]
Ans. The sex of a newborn depends on what happens at the time of fertilization.
(i) If a sperm carrying X chromosome fertilizes the ovum carrying X chromosome, then the girl child will be born and the child will have XX combination of sex chromosomes.
(ii) If a sperm carrying Y chromosome fertilizes the ovum carrying X chromosome, then the child born will be a boy and the child will have XY combination of sex chromosome.
The above presentation clearly shows that it is matter of chance whether the newborn will be boy or girl and none of the parents may be considered responsible for it.
19. When do we consider a person to be myopic or hypermetropic ? List two causes of hypermetropia. Explain using ray diagrams how the defect associated with hypermetropic eye can be corrected. [5]
Ans. Myopia is the defect in vision in which a person cannot see the distant objects clearly whereas in hypermetropia is the defect in which a person cannot see nearby objects clearly.
Hypermetropia is caused due to:
- (i) Decrease in converging power of eye lens.
- (ii) Too short eye ball.
In a hypermetropic eye, the image of near by object lying at normal near point N (at 25 cm) is formed behind the retina.
Hypermetropia and its correction by using convex lens.
Hypermetropic eye can be corrected using convex lenses. When a convex lens of suitable power is placed in front of hypermetropic eye, then the diverging rays of light from the object are converged first by the convex lens used. This form a virtual image of the object at another near point N.
Now, the rays can be easily focused by the eye lens to form an image on retina.
20. (a) How will you infer with the help of an experiment that the same current flows through every part of a circuit containing three resistors in series connected to a battery?
Ans. Let three resistors R1, R2 and R3 are connected in series which are also connected with a battery, an ammeter and a key as shown in figure.
When key is closed, the current starts flowing through the circuit. Take the reading of ammeter. Now change the position of ammeter to anywhere in between the resistors and take its reading. We will observe that in both the cases reading of ammeter will be same showing same current flows through every part of the circuit above.
(b) Consider the given circuit and find the current flowing in the circuit and potential difference across the 15 Ω resistor when the circuit is closed. [5]
Ans. Given,
R1 = 5 Ω, R2 = 10 Ω, R3 = 15 Ω, V = 30 V
Total resistance, R = R1 + R2 + R3
[∵ 5 Ω, 10 Ω and 15 Ω are connected in series]
= 5 + 10 + 15
= 30 Ω
Potential difference, V = 30V
Current in the circuit, I = ?
From Ohm’s law.
∴ Current flowing in the circuit = 1 A
Potential difference across 15 Ω resistors = IR3
= 1A x 15 Ω
= 15 V
OR
(a) Three resistors R1, R2 and R3 are connected in parallel and the combination is connected to a battery, ammeter, voltmeter and key. Draw suitable circuit diagram and obtain an expression for the equivalent resistance of the combination of the resistors.
Ans. Let R1, R2 and R3 are three resistance connected in parallel to one another and R is the equivalent resistance of the circuit. A battery of V volts has been applied across the ends of this combination. When the switch of the key is closed, current I flows in the circuit such that,
From Ohm’s law,
I = V/R …(i)
I1 = V/R1 …(ii)
I2 = V/R2 …(iii)
I3 = V/R3 …(iv)
I = I1 + I2 + I3 …(v)
Putting the values of I, I1, I2 and I3 in equation (v),
(b) Calculate the equivalent resistance of the following network :
Ans. Let RP is the equivalent resistance of resistors connected in parallel.
∴ Equivalent resistance of resistors in parallel :
RP = 10 Ω
Now, equivalent circuit becomes.
∵ 10 Ω and 10 Ω are connected in series.
∴ Equivalent resistance of the circuit
= 10 Ω + 10 Ω
= 20 Ω
21. Draw the pattern of magnetic field lines produced around a current carrying straight conductor passing perpendicularly through a horizontal cardboard. State and apply right hand thumb rule to mark the direction of the field lines. How will the strength of the magnetic field change when the point where magnetic field is to be determined is moved away from the straight conductor ? Give reason to justify your answer. [5]
Ans. Maxwell’s Right Hand Thumb rule states that if current carrying wire is imagined to be held in the right hand so that thumb points in the direction of current, then the direction in which fingers encircle the wire will give the direction of magnetic field lines around the wire. If we hold the current carrying straight wire so that thumbs.
Magnetic field pattern due to a straight current-carrying wire
in upward direction points the direction of current, the direction of magnetic field lines will be anticlockwise. The strength of magnetic field is inversely proportional to the distance of the point of observation from the wire. So, as we move away from the wire the strength of magnetic field decreases.
SECTION – E
22. A teacher provided acetic acid, water, lemon juice, aqueous solution of sodium hydrogen carbonate and sodium hydroxide to students in the school laboratory to determine the pH values of these substances using pH papers. One of the students reported the pH values of the given substances as 3, 12, 4, 8 and 14 respectively. Which one of these values is not correct ? Write its correct value stating the reason. [2]
Ans. The value of pH for water is not correct. The correct value of pH of water is 7 because it has almost equal concentration of H+ and OH– due to which it is neutral.
OR
What would a student report nearly after 30 minutes of placing duly cleaned strips of aluminium, copper, iron and zinc in freshly prepared iron sulphate solution taken in four beakers ?
Ans. Aluminium displaces the iron from iron sulphate and the colour of two solution changes from green to colourless.
No change takes place when copper strip is dipped in iron sulphate solution.
No change will be observed when iron strips are dipped in iron sulphate solution.
The colour of the solution changes from green to colourless when zinc is added to iron sulphate solution.
23. What is observed when a pinch of sodium hydrogen carbonate is added to 2 mL of acetic acid taken in a test tube ? Write chemical equation for the reaction involved in this case. [2]
Ans. CO2 gas is evolved with brisk effervescence when sodium hydrogen carbonate is added to acetic acid.
24. List in proper sequence four steps of obtaining germinating dicot seeds. [2]
Ans.
- (i) The root is formed when radicle of seed grows.
- (ii) The root grows downward into the soil and absorbs water and minerals from the soil.
- (iii) The shoot is formed from the upward growth of plumule.
- (iv) The green leaves are developed when shoot comes above the ground.
OR
After examining a prepared slide under the high power of a compound microscope, a student concludes that the given slide shows the various stages of binary fission in a unicellular organism. Write two observations on the basis of which such a conclusion may be drawn.
Ans. (i) The nucleus of mature cell seems elongated and a grove is formed in cell which divides the nucleus.
(ii) A single parent divides to form two daughter cells. Constriction appears due to the division of the cytoplasm.
25. List four precautions which a student should observe while preparing a temporary mount of a leaf peel to show stomata in his school laboratory. [2]
Ans.
- (i) Freshly plucked leaf should be taken for epidermal peel.
- (ii) Hold the slide by its edges.
- (iii) Peel should be cut to a proper size.
- (iv) The peel should be allowed to dry.
26. Draw the path of a ray of light when it enters one of the faces of a glass slab at an angle of nearly 45°, Label on it (i) angle of refraction, (ii) angle of emergence and (iii) lateral displacement. [2]
Ans.
OR
A student traces the path of a ray of light through a glass prism as shown in the diagram, but leaves it incomplete and unlabelled. Redraw and complete the diagram. Also label on it ∠i, ∠e, ∠r, and ∠D.
Ans.
27. The current flowing through a resistor connected in a circuit and the potential difference developed across its ends are as shown in the diagram by milliammeter and voltmeter readings respectively : [2]
(a) What are the least counts of these meters ?
Ans. 10 mA and 0.1 V
(b) What is the resistance of the resistor ?
Ans. V = 2.4 volt, I = 250 mA = 0.25A
From Ohm’s law.
