CBSE EXAMINATION PAPER-2023
SCIENCE
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 37 questions. All questions are compulsory.
- This question paper is divided into 5 sections.
- Section A – questions number 1 to 3 are case based questions
- Section B – questions number 4 to 17 are multiple choice questions
- Section C – questions number 18 to 25 are very short answer
- Section D – questions number 26 to 33 are short answer
- Section E – questions number 34 to 37 are long answer
- There is no overall choice given in the question paper. However, an internal choice has been provided in few questions.
- Use of calculator is NOT allowed.
Section A
On the basis of reactivity metals are grouped into three categories -
(i) Metals of low reactivity
(ii) Metals of medium reactivity
(iii) Metals of high reactivity
Therefore metals are extracted in pure form from their ores on the basis of their chemical properties.
Metals of high reactivity are extracted from their ores by electrolysis of the molten ore.
Metals of low reactivity are extracted from their sulphide ores, which are converted into their oxides. The oxides of these metals are reduced to metals by simple heating.
(1) Name the process of reduction used for a metal that gives vigorous reaction with air and water both.
(2) Carbon cannot be used as a reducing agent to obtain aluminium from its oxide? Why?
(3) Describe briefly the method to obtain mercury from cinnabar. Write the chemical equation for the reactions involved in the process.
(4) Differentiate between roasting and calcination giving chemical equation for each.
All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
(1) In humans, how many chromosomes are present in a Zygote and in each gamete?
(2) A few reptiles rely entirely on environmental cues for sex determination. Comment.
(3) "The sex of a child is a matter of chance and none of the parents are considered to be responsible for it." Justify it through flow chart only.
(4) Why do all the gametes formed in human females have an X chromosome?
A student took three concave mirrors of different focal lengths and performed the experiment to see the image formation by placing an object at different distances with these mirrors as shown in the following table.
(1) List two properties of the image formed in Case I.
(2) In which one of the cases given in the table, the mirror will form real image of same size and why?
(3) Name the type of mirror used by dentists. Give reason why do they use such type of mirrors.
(4) Look at the table and identify the situation (object distance and focal length) which resembles the situation in which concave mirrors are used as shaving mirrors? Draw a ray diagram to show the image formation in this case.
Section B
Metal oxides generally react with acids, but few oxides of metal also react with bases. Such metallic oxides are :
I. MgO
II. ZnO
III. Al₂O₃
IV. CaO
In the given diagram of a closed stomata (1), (2), (3) and (4) respectively are
Walking in a straight line and riding a bicycle are the activities which are possible due to a part of the brain. Choose the correct location and name of this part from the given table:
A student wants to obtain an erect image of an object using a concave mirror of 10 cm focal length. What will be the distance of the object from mirror?
Bronze is an alloy of
In an experiment with pea plants, a pure tall plant (TT) is crossed with a pure short plant (tt). The ratio of pure tall plants to pure short plants in F₂ generation will be —
Choose the correct order of the stages of binary fission in Leishmania.
Section C
State whether the given chemical reaction is a redox reaction or not. Justify your answer.
MnO₂ + 4 HCl → MnCl₂ + 2 H₂O + Cl₂
List two differences between the movement of leaves of a sensitive plant and the movement of a shoot towards light.
What happens at synapse between two neurons? State briefly.
Give the name of the enzyme present in the fluid in our mouth cavity. State the gland which produces it. What would happen to the digestion process if this gland stops secreting this enzyme?
Let the resistance of an electrical device remain constant, while the potential difference across its two ends decreases to one fourth of its initial value. What change will occur in the current through it? State the law which helps us in solving the above stated question.
A light ray enters from medium A to medium B as shown in the figure.
(a) Which one of the two media is denser w.r.t. other medium? Justify your answer.
(b) If the speed of light in medium A is vₐ and in medium B is vᵦ, what is the refractive index of B with respect to A?
(a) Medium B is denser than medium A because the light ray bends towards the normal when it enters medium B from medium A. This bending towards the normal shows that light travels slower in medium B, which means medium B is optically denser than medium A.
(b) The refractive index of medium B with respect to medium A is the ratio of the speed of light in medium A to the speed of light in medium B. Therefore, refractive index of B with respect to A = vₐ / vᵦ.
(a) A ray of light starting from diamond is incident on the interface separating diamond and water. Draw a labelled ray diagram to show the refraction of light in this case.
(b) Absolute refractive indices of diamond and water are 2.42 and 1.33 respectively. Find the value of refractive index of water w.r.t. diamond.
(a) When a ray of light travels from diamond (higher refractive index 2.42) to water (lower refractive index 1.33), it bends away from the normal at the interface. The ray starts inside the diamond, strikes the surface separating diamond and water, and refracts into water bending away from the normal line because light is moving from a denser to a rarer medium. The diagram should include the diamond region, water region, normal at the interface, incident ray inside diamond, and refracted ray inside water bending away from normal.
(b) Refractive index of water with respect to diamond = (Absolute refractive index of water) / (Absolute refractive index of diamond) = 1.33 / 2.42 ≈ 0.55.
State the rule to determine the direction of a (a) magnetic field produced around a straight conductor carrying current and (b) force experienced by a current carrying straight conductor placed in a magnetic field which is perpendicular to it.
Section D
Explain the process of transport of oxygenated and deoxygenated blood in a human body.
A substance 'X' is used as a building material and is insoluble in water. When it reacts with dilute HCl, it produces a gas which turns lime water milky.
(i) Write the chemical name and formula of 'X'.
(ii) Write chemical equations for the chemical reactions involved in the above statements.
(ii) When calcium carbonate reacts with dilute hydrochloric acid (HCl), it produces carbon dioxide gas, calcium chloride, and water. The chemical equation is:
CaCO3 + 2HCl → CaCl2 + CO2 + H2O
The carbon dioxide gas (CO2) produced turns lime water milky due to the formation of calcium carbonate precipitate. The reaction is:
CO2 + Ca(OH)2 → CaCO3 + H2O
This shows that calcium carbonate is insoluble in water and produces carbon dioxide gas on reaction with dilute HCl, which turns lime water milky.
A metal 'M' on reacting with dilute acid liberates a gas 'G'. The same metal also liberates gas 'G' when reacts with a base.
(i) Write the name of gas 'G'.
(ii) How will you test the presence of this gas?
(iii) Write chemical equations for the reactions of the metal with
(1) an acid and (2) a base.
(i) The gas 'G' liberated on reacting the metal 'M' with both dilute acid and base is hydrogen gas (H₂).
(ii) To test for hydrogen gas, bring a burning matchstick near the mouth of the test tube containing the gas. If the gas is hydrogen, it will burn with a 'pop' sound.
(iii) The chemical reactions are:
1. Reaction with acid (e.g., dilute hydrochloric acid):
Metal + Acid → Salt + Hydrogen gas
For metal 'M' reacting with dilute HCl: M + 2HCl → MCl₂ + H₂ ↑
2. Reaction with base (e.g., sodium hydroxide in case of amphoteric metals like zinc or aluminum):
Metal + Base + Water → Complex salt + Hydrogen gas
Example with zinc and sodium hydroxide:
Zn + 2NaOH + 2H₂O → Na₂[Zn(OH)₄] + H₂ ↑
Name the gland and the hormone secreted by it in scary situations in human beings. List any two responses shown by our body when this hormone is secreted into the blood.
In the given diagram
(1) Name the parts labelled A, B, and C.
(ii) Write the functions of A and C.
(iii) Reflex arcs have evolved in animals? Why?
(i) The parts labelled in the diagram are: A - Receptor, B - Sensory Neuron, C - Motor Neuron.
(ii) Functions of A and C:
A (Receptor) detects the stimulus like heat or pain from the environment.
C (Motor Neuron) carries the message from the spinal cord to the effector (muscle) to produce a response such as pulling the hand away.
(iii) Reflex arcs have evolved in animals because the brain's thinking process is not fast enough to respond to sudden harmful stimuli. Reflex actions provide a quick, automatic response by bypassing the brain, allowing immediate reaction to danger. Many animals also lack a complex nervous system for thinking, so reflex arcs enable them to survive by reacting quickly to stimuli.
With the help of an appropriate example, justify that some of the chemical reactions are determined by
(a) Change in temperature,
(b) Evolution of a gas, and
(c) Change in colour
Give chemical equation for the reaction involved in each case.
Chemical equation: CaO + H2O -> Ca(OH)2 + heat
(b) Evolution of a gas: When zinc reacts with dilute hydrochloric acid, hydrogen gas is evolved which is a sign of chemical change.
Chemical equation: Zn + 2HCl -> ZnCl2 + H2(gas)
(c) Change in colour: When iron reacts with copper sulfate solution, the blue color of copper sulfate fades and reddish-brown copper metal is deposited showing a color change.
Chemical equation: Fe + CuSO4 -> FeSO4 + Cu (deposited)
State reasons for Myopia. With the help of ray diagrams, show the
(a) image formation by a myopic eye, and
(b) correction of myopia using an appropriate lens.
Reasons for Myopia: Myopia or nearsightedness occurs when the eye ball is longer than normal or the cornea is too curved. This causes light rays from distant objects to focus in front of the retina instead of on it, making distant objects appear blurry.
(a) Image formation by a myopic eye: In a myopic eye, parallel rays of light from distant objects converge and meet before the retina. Hence, the image formed is blurred on the retina since the focus is in front of it.
(b) Correction of myopia using a concave lens: Myopia is corrected by using a concave (diverging) lens in front of the eye. The concave lens diverges the incoming rays before they enter the eye, moving the effective focus back so the rays focus exactly on the retina. This corrects the blurred vision and allows the person to see distant objects clearly.
(a) Write the percentage of (i) solar energy captured by the autotrophs and (ii) energy transferred from autotrophs to the next level in a food chain.
(b) What are trophic levels? Why do different food chains in an ecosystem not have more than four to five trophic levels? Give reason.
(a) (i) Autotrophs capture only a small fraction of the total solar energy available. Although the exact percentage is not explicitly stated in the given context, it is generally understood that only about 1% to 5% of solar energy is captured by autotrophs (producers) through photosynthesis.
(ii) About 10% of the energy stored in the autotrophs is transferred to the next trophic level in the food chain, which is usually the herbivores or primary consumers.
(b) Trophic levels refer to the different steps or levels in a food chain, representing organisms that share the same function in the feeding process. The first trophic level consists of autotrophs (producers) that capture solar energy and produce organic matter. The second level consists of herbivores (primary consumers) that feed on producers. Higher levels include secondary consumers (small carnivores) and tertiary consumers (larger carnivores).
Food chains generally do not have more than four to five trophic levels because at each step there is a loss of energy, mainly as heat. Only about 10% of the energy is transferred from one level to the next, so the amount of usable energy decreases significantly as we move up the trophic levels. After four or five levels, there is too little energy left to support higher trophic levels, limiting the length of the food chain.
Section E
(i) A compound ‘A’ with a molecular formula of C₂H₄O₂ reacts with a base to give salt and water. Identify ‘A’, state its nature and the name of the functional group it possesses. Write the chemical equation for the reaction involved.
(ii) When the above stated compound ‘A’ reacts with another compound ‘B’ having molecular formula C₂H₆O in the presence of an acid, a sweet-smelling compound ‘C’ is formed.
(1) Identify ‘B’ and ‘C’.
(2) State the role of acid in this reaction.
(3) Write the chemical equation for the reaction involved.
C2H4O2 + NaOH → CH3COONa + H2O
(b)(1) The compound ‘B’ with molecular formula C2H6O is ethanol (CH3CH2OH). When ethanoic acid (‘A’) reacts with ethanol (‘B’) in the presence of an acid catalyst, a sweet-smelling compound ‘C’ called ethyl ethanoate (an ester) is formed along with water.
(2) The acid acts as a catalyst in this esterification reaction. It helps in speeding up the reaction without being consumed.
(3) The chemical equation for this esterification reaction is:
CH3COOH + CH3CH2OH → CH3COOCH2CH3 + H2O (in presence of acid catalyst)
(i) Name the compound formed when ethanol is heated at 443 K in the presence of conc. H₂SO₄ and draw its electron-dot structure. State the role of conc. H₂SO₄ in this reaction.
(ii) What is hydrogenation? Explain it with the help of a chemical equation. State the role of this reaction in industry.
Electron-dot structure of ethene (C2H4):
Each carbon atom shares two electrons to form a double bond between them and also forms single bonds with two hydrogen atoms each.
(ii) Hydrogenation is the process of adding hydrogen (H2) to unsaturated compounds like alkenes to convert them into saturated compounds like alkanes.
For example, ethene reacts with hydrogen in the presence of a catalyst (like nickel) to form ethane: C2H4 + H2 --(Ni catalyst)--> C2H6.
This reaction is important in industry for converting vegetable oils (which are unsaturated fats) into saturated fats (like margarine), thereby improving texture and shelf life.
(i) What is meant by resistance of a conductor? Define its SI unit.
(ii) List two factors on which the resistance of a rectangular conductor depends.
(iii) How will the resistance of a wire be affected if its
(1) length is doubled, and
(2) radius is also doubled?
Give justification for your answer.
(i) Resistance of a conductor is the property of the conductor that opposes or resists the flow of electric current through it. It is denoted by R and is measured in ohms (Ω). The SI unit of resistance is ohm, defined as the resistance of a conductor in which a current of one ampere flows when a potential difference of one volt is applied across its ends.
(ii) The resistance of a rectangular conductor depends on the following two factors: (1) Length of the conductor – resistance is directly proportional to length, so if length increases, resistance increases. (2) Cross-sectional area of the conductor – resistance is inversely proportional to the cross-sectional area, so if area increases, resistance decreases.
(iii) (1) If the length of the wire is doubled, the resistance also doubles. This is because resistance R is directly proportional to the length (L), so R ∝ L.
(2) If the radius of the wire is doubled, its resistance decreases to one-fourth of the original resistance. This is because the cross-sectional area A of the wire depends on the radius r as A = πr². Doubling the radius increases the area by four times, and since resistance is inversely proportional to area, resistance becomes one-fourth.
Thus, the factors affecting resistance follow the relationships: R ∝ L and R ∝ 1/A, where A depends on radius.
In an electric circuit three bulbs of 100 W each are connected in series to a source. In another circuit set of three bulbs of the same wattage are connected in parallel to the same source.
(i) Will the bulb in the two circuits glow with the same brightness? Justify your answer.
(ii) Now, let one bulb in both the circuits get fused. Will the rest of the bulbs continue to glow in each circuit? Give reason for your answer.
(i) Brightness of bulbs in series and parallel circuits:
When three bulbs of 100 W each are connected in series to a source, the same current flows through each bulb. Because of series connection, total resistance is the sum of individual resistances, which reduces the current through the bulbs. Consequently, each bulb receives less current and glows with less brightness than its rated brightness.
In contrast, when three bulbs of the same rating are connected in parallel across the same source, each bulb receives the full voltage of the source. Hence, each bulb draws the current it requires according to its rating and glows with full brightness.
Therefore, bulbs connected in parallel glow brighter than those connected in series under the same voltage supply.
(ii) Effect of one fused bulb in series and parallel circuits:
If one bulb gets fused in the series circuit, the circuit becomes open because the connection is broken at that point. As a result, the current stops flowing through the circuit and all bulbs stop glowing.
However, if one bulb gets fused in the parallel circuit, only that particular branch is broken, but the other two parallel branches remain connected. Therefore, current can still flow through the other bulbs, and they will continue to glow.
Hence, bulbs in series stop glowing if one bulb fuses, while bulbs in parallel continue to glow except the fused bulb.
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