CBSE Class 10 Science Previous Year Question Paper 2019

Section A

1. If you could use any source of energy for heating your food which one would you prefer? State one reason for your choice.

Ans: If I could use any source of energy for heating food then I would prefer solar energy because:

1. It is available in abundance.
2. Energy from Sun is available to every individual.
3. Solar energy also preserves the natural nutrients of food by cooking it slowly at low temperature.
4. It is economical. [1]

2. Write the function of voltmeter in an electric circuit.

Ans: The function of the voltmeter in a circuit is to measure the voltage drop across any appliance. The voltmeter is always connected parallel in the circuit. [1]

Section B

3. What happens to the image distance in the normal human eye when we decrease the distance of an object, say 10 m to 1 m? Justify your answer.

Ans: If the object distance is greater than 25 cm the image is always formed on the retina as the focal length of the human eye lens gets adjusted depending on the object distance. This special property of human eye is called as power of accommodation. Hence, image distance will remain same if object distance changes from 10 m to 1 m. [1, 2]

4. List two different functions performed by pancreas in our body.

Ans: The pancreas contains both the endocrine and exocrine portions.

• The exocrine region secretes sodium bicarbonate and many digestive enzymes. The pancreatic amylase causes breakdown of starch, pancreatic lipase causes breakdown of lipids, and trypsin causes digestion of proteins.
• The endocrine regions secrete hormones i.e. insulin and glucagon. These hormones regulate the level of glucose in the blood. Insulin decreases while glucagon increases the blood glucose level. [2]

5. How it can be proved that the basic structure of the Modern Periodic Table is based on the electronic configuration of atoms of different elements?

Ans: Modern periodic law states that the physical and chemical properties of an element in the periodic table are the periodic function of the atomic number of that element. Electronic configuration of the elements plays an important role in the placement of elements in the modern periodic table. The valence shell electron of an element decides its position in a particular group or period. For example, if the electronic configuration of an element is 2, 1, it means that the element belongs to group I and 2nd period. Thus, the modern periodic table is based on the electronic configuration of the different elements. [2]

OR

The electronic configuration of an element is 2, 8, 4. State its:
(a) group and period in the Modern Periodic Table.
(b) name and write its one physical property.

Ans: (a) The electronic configuration of the given element is 2,8,4. Since, the number of valence electrons for the given element is 4. Hence, the element belongs to group 14 (10+4) and the 3rd period.
(b) The atomic number of the given element is 14. Hence, the given element is Silicon. Silicon is a metalloid. [2]

Section C

6. How can we help in reducing the problem of waste disposal? Suggest any three methods.

Ans: The three methods we can utilise for reducing the problem of waste disposal are:

1. Segregation of waste into biodegradable and non-biodegradable.
2. Following the principle of 3R- reduce, reuse and recycle.
3. Converting biodegradable waste into useful commodities like energy from biogas, using the compost as fertiliser. [3]

OR

Define an ecosystem. Draw a block diagram to show the flow of energy in an ecosystem.

Ans: Ecosystem refers to the living and non-living components in an area and the interactions between them. Energy flows across the trophic levels following the ten percent law. Only ten percent of the energy available to a trophic level is passed on to the next trophic level. For example, if 10000 J is available to producers, 1000 J is available to primary consumers, 100 J to secondary consumers, and 10 J to tertiary consumers. [3, 4]

7. List three advantages each of:
(i) exploiting resources with short term aims, and
(ii) using a long term perspective in managing our natural resources.

Ans: (i) Advantages of exploiting resources with short term aims:

• The current basic needs of the population can easily be fulfilled.
• Fast industrialisation.
• Variety of useful products can be made to make life easy. [4]

(ii) Advantages of long term perspective in managing our natural resources:

• Sustainable management of resources.
• The resources can be used efficiently by the present as well as future generations.
• The pollution and the degradation of the environment can be reduced. [5]

8. What is a rainbow? Draw a labelled diagram to show the formation of a rainbow.

Ans: The rainbow is a natural phenomenon in which white sunlight splits into beautiful colours by water droplets, which remain suspended in air after the rain. The formation involves dispersion, refraction, and internal reflection of sunlight by the water droplets. [5]

9. Nervous and hormonal systems together perform the function of control and coordination in human beings. Justify this statement with the help of an example.

Ans: In human beings, control and coordination is brought about by both nervous system and endocrine system. Nervous system works by generation and transmission of electrical impulse while the endocrine system works by secreting chemical messengers called hormones. These systems complement the action of each other. For example, when an emergency stimulus is detected by the nervous system, the stimulus is received and analysed by CNS that send message to effectors. At the same time, the sympathetic nervous system activates adrenal gland to release adrenaline that prepares body by increasing heart rate, blood pressure, etc. Thus, both systems interact to produce an effective response. [6]

10. Trace the sequence of events which occur when a bright light is focused on your eyes.

Ans: When bright light is focussed on our eyes, the message goes to the brain. The brain reverts back the message by motor neuron which contracts the pupil. The sequence is: Receptor → Sensory neuron → Brain → Motor Neuron → Eye muscle contracts. [6, 7]

11. What is photosynthesis? Explain its mechanism.

Ans: Photosynthesis is a process in which the energy of light is utilised to produce simple organic compounds in autotrophs. The energy of sunlight is captured by chlorophyll. This energy is utilised for photolysis of water molecules (releasing oxygen) and reduction of carbon dioxide to simple carbohydrates or sugars. Reaction: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. [7, 8]

12. Name the plant Mendel used for his experiment. What type of progeny was obtained by Mendel in F1 and F2 generations when he crossed the tall and short plants? Write the ratio he obtained in F2 generation plants.

Ans: Mendel used pea plant (Pisum sativum). When Mendel crossed tall and short plants, the progeny obtained in F1 generation were all tall. When the F1 plants were selfed, the F2 generation showed three tall and one dwarf plant. The genotypic ratio of F2 generation is 1:2:1 (TT : Tt: Tt: tt) and the phenotypic ratio is 3:1 (Tall: Dwarf). [8, 9]

OR

List two differences between acquired traits and inherited traits by giving an example of each.

Ans: Acquired Traits: Somatic variations developed due to environmental factors/use and disuse; develop during lifetime; not transferred to offspring. Example: Learning dance, muscular body.
Inherited Traits: Genetic variations developed due to reshuffling of genetic material; transferred from parents to offspring. Example: Eye colour, curly hair. [9, 10]

13. 2 g of silver chloride is taken in a china dish and the china dish is placed in sunlight for sometime. What will be your observation in this case? Write the chemical reaction involved in the form of a balanced chemical equation. Identify the type of chemical reaction.

Ans: When silver chloride (AgCl) is kept in sunlight, it breaks down into Ag and Cl₂. The color changes from white to grey. Reaction: 2AgCl(s) → 2Ag(s) + Cl₂(g). This is a photochemical decomposition reaction. [10]

OR

Identify the type of reactions taking place in each of the following cases and write the balanced chemical equation for the reactions.
(a) Zinc reacts with silver nitrate to produce zinc nitrate and silver.
(b) Potassium iodide reacts with lead nitrate to produce potassium nitrate and lead iodide.

Ans: (a) Displacement reaction: Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(b) Double displacement reaction: 2KI(aq) + Pb(NO₃)₂(aq) → 2KNO₃(aq) + PbI₂(s) [10]

14. Based on the group valency of elements write the molecular formula of the following compounds giving justification for each:
(i) Oxide of first group elements.
(ii) Halide of the elements of group thirteen, and
(iii) Compound formed when an element A of group 2 combines with an element B of group seventeen.

Ans: (i) M₂O: First group elements have valency 1, Oxygen has valency 2. Example: Na₂O.
(ii) MX₃: Group 13 elements have valency 3, Halogens have valency 1. Example: AlCl₃.
(iii) AB₂: Element A (Group 2) has valency 2, Element B (Group 17) has valency 1. Example: MgCl₂. [10-12]

15. Explain the following:
(a) Sodium chloride is an ionic compound which does not conduct electricity in solid state where as it does conduct electricity in molten state as well as in aqueous solution.
(b) Reactivity of aluminium decrease if it is dipped in nitric acid.
(c) Metals like calcium and magnesium are never found in their free state in nature.

Ans: (a) In solid state, ions are not free to move. In molten state and aqueous solution, ions are free to move and conduct electricity.
(b) Nitric acid is a strong oxidizing agent. It forms a layer of aluminium oxide on the metal which prevents further reaction, decreasing reactivity.
(c) Calcium and Magnesium are highly reactive metals, so they readily react with other elements and are not found free in nature. [12]

Section D

16. (a) With the help of a suitable circuit diagram prove that the reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.
(b) In an electric circuit, two resistors of 12 Ω each are joined in parallel to a 6 V battery. Find the current drawn from the battery.

Ans: (a) In a parallel circuit, current I divides into I₁, I₂... while voltage V is same across each. Since I = I₁ + I₂ + ... and I = V/R, we get V/R_eq = V/R₁ + V/R₂ + ... which simplifies to 1/R_eq = 1/R₁ + 1/R₂ + ... [13, 14]
(b) 1/R_net = 1/12 + 1/12 = 2/12 = 1/6. So, R_net = 6 Ω. Current I = V/R = 6/6 = 1 A. [14]

OR

An electric lamp of resistance 20 Ω and a conductor of resistance 4 Ω are connected to a 6 V battery as shown in the circuit. Calculate:
(a) the total resistance of the circuit,
(b) the current through the circuit,
(c) the potential difference across the (i) electric lamp and (ii) conductor, and
(d) power of the lamp.

Ans: (a) Total Resistance R = 20 Ω + 4 Ω = 24 Ω.
(b) Current I = V/R = 6 V / 24 Ω = 0.25 A.
(c) (i) V_lamp = I × R_lamp = 0.25 × 20 = 5 V. (ii) V_conductor = 0.25 × 4 = 1 V.
(d) Power P = I²R = (0.25)² × 20 = 1.25 W. [14, 15]

17. (a) Draw magnetic field lines produced around a current carrying straight conductor passing through cardboard. Name, state and apply the rule to mark the direction of these field lines.
(b) 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 wire carrying constant current? Justify your answer.

Ans: (a) The Right Hand Thumb Rule is used. If the thumb points in the direction of current, the fingers wrap in the direction of magnetic field lines (concentric circles).
(b) The strength of the magnetic field decreases as we move away from the wire because the magnetic field strength is inversely proportional to the distance from the wire. [15, 16]

18. An object is placed at a distance of 60 cm from a concave lens of focal length 30 cm.
(i) Use lens formula to find the distance of the image from the lens.
(ii) List four characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case.
(iii) Draw ray diagram to justify your answer of part (ii).

Ans: (i) u = -60 cm, f = -30 cm. 1/v - 1/u = 1/f => 1/v = 1/f + 1/u = -1/30 - 1/60 = -3/60 = -1/20. v = -20 cm.
(ii) Image is formed at 20 cm in front of the lens. Nature: Virtual. Size: Diminished. Orientation: Erect.
(iii) [Ray diagram showing virtual, erect, diminished image between F and O]. [17, 18]

19. Define pollination. Explain the different types of pollination. List two agents of pollination? How does suitable pollination lead to fertilization?

Ans: Pollination is the transfer of pollen grains from anther to stigma. Types: Self-pollination (same plant) and Cross-pollination (different plant). Agents: Insects, wind. Pollination brings pollen to stigma, leading to germination of pollen tube which carries male gametes to the ovule for fertilization. [18, 19]

OR

Identify the given diagram. Name the parts 1 to 5. What is contraception? List three advantage of adopting contraceptive measures.

Ans: Diagram is Female Reproductive System. 1-Oviduct, 2-Ovary, 3-Uterus, 4-Cervix, 5-Vagina. Contraception includes methods to prevent pregnancy. Advantages: Prevention of unwanted pregnancies, family planning, prevention of STDs. [19, 20]

20. Write the chemical formula and name of the compound which is the active ingredient of all alcoholic drinks. List its two uses. Write chemical equation and name of the product formed when this compound reacts with-(i) Sodium metal (ii) hot concentrated sulphuric acid

Ans: Ethanol (CH₃CH₂OH). Uses: Medical wipes/sanitizers, preservatives. (i) With Sodium: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa (Sodium Ethoxide) + H₂.
(ii) With conc. H₂SO₄: CH₃CH₂OH → CH₂=CH₂ (Ethene) + H₂O. [20, 21]

OR

What is methane? Draw its electron dot structure. Name the type of bonds formed in this compound. Why are such compounds: (i) poor conductors of electricity? and (ii) have low melting and boiling points? What happens when this compound burns in oxygen?

Ans: Methane (CH₄) is a saturated hydrocarbon. Bonds: Covalent bonds. (i) Poor conductors because they have no free electrons (covalent bonds). (ii) Low melting/boiling points due to weak intermolecular forces. Burning: CH₄ + 2O₂ → CO₂ + 2H₂O. [21, 22]

21. Write the main difference between an acid and a base. With the help of suitable examples explain the term neutralization and the formation of – (i) acidic, (ii) basic and (iii) neutral salts

Ans: Acid turns blue litmus red, releases H+ ions. Base turns red litmus blue, releases OH- ions. Neutralization is the reaction between acid and base to form salt and water.
(i) Acidic salt: Strong acid + Weak base (e.g., NH₄Cl).
(ii) Basic salt: Weak acid + Strong base (e.g., CH₃COONa).
(iii) Neutral salt: Strong acid + Strong base (e.g., NaCl). [22, 23]

Section E

22. In the experimental set up to show that "CO2 is given out during respiration", name the substance taken in the small test tube kept in the conical flask. State its function and the consequence of its use.

Ans: The substance is lime water (or KOH in some contexts, but text mentions lime water for detection). Lime water turns milky in presence of CO₂, confirming its release. [23]

23. A student is observing the temporary mount of a leaf peel under a microscope. Draw labelled diagram of the structure of stomata as seen under the microscope.

Ans: [Diagram of stomata showing stomatal pore, chloroplast, and guard cells]. [23]

OR

Draw a labelled diagram in proper sequence to show budding in hydra.

Ans: [Sequence showing bud growing, tentacles developing, and bud detaching]. [24]

24. List four precautions which a student should observe while determining the focal length of a given convex lens by obtaining image of a distant object on a screen.

Ans: 1. Fix lens vertically.
2. Lens and screen should be in line with scale.
3. Record position only when sharp image forms.
4. No obstacles in path. [24]

25. While studying the dependence of potential difference (V) across a resistor on the current (I) passing through it... He got a straight line graph passing through the origin. What does the straight line signify? Write the method of determining the resistance of the resister using this graph.

Ans: Straight line signifies the resistor follows Ohm's Law (V ∝ I). Resistance R = Slope of the graph = V/I. [24, 25]

OR

What would you suggest to a student if while performing an experiment he finds that the pointer/needle of the ammeter and voltmeter do not coincide with the zero marks on the scales when the circuit is open?

Ans: This is zero error. Subtract this error from the final readings to get accurate values. [26]

26. In three test tubes A, B, and C, three different liquids namely, distilled water, underground water and distilled water in which a pinch of calcium sulphate is dissolved, respectively are taken. Equal amount of soap Answer is added to each test tube and the contents are shaken. In which test tube will the length of the foam (lather) be longest? Justify your answer.

Ans: Test tube A (distilled water) will have the longest foam because it is soft water. Tube B and C contain ions (Ca²⁺, Mg²⁺) making them hard water, which forms scum instead of lather. [26, 27]

27. Blue litmus is added to two test tubes A and B containing dilute HCl and NaOH Answer respectively. In which test tube a colour change will be observed? State the colour change and give its reason.

Ans: Test tube A (HCl) will change colour from blue to red because acids turn blue litmus red. [28]

OR

What is observed when 2 mL of dilute hydrochloric acid is added to 1 g of sodium carbonate taken in a clean and dry test tube? Write chemical equation for the reaction involved.

Ans: Brisk effervescence is observed due to evolution of CO₂ gas. Equation: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. [28]

Go to Link