CBSE EXAMINATION PAPER-2025
SCIENCE
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 36 questions. All questions are compulsory.
- This question paper is divided into 5 sections.
- Section A – questions number 1 to 2 are case based questions
- Section B – questions number 3 to 16 are multiple choice questions
- Section C – questions number 17 to 25 are very short answer
- Section D – questions number 26 to 32 are short answer
- Section E – questions number 33 to 36 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
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types — ‘X’ chromosomes and ‘Y chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
(1) What are chromosomes?
[1 Marks](2) Why is the pair of sex chromosomes in human males called mismatched pair?
[1 Marks](3) Show with the help of a flow chart that the statistical probability of getting a boy or a girl is 50 : 50.
[2 Marks](4) Mention two examples of animals where sex is not determined genetically like human beings. Describe in brief the method of sex determination in these animals.
In order to obtain magnetic field lines around a bar magnet, a student performed an experiment using a magnetic compass and a bar magnet. The magnet was placed on a sheet of white paper fixed on a drawing board. Using magnetic needle he obtained on the paper a pattern of magnetic field lines (as shown below) around the bar magnet.
(1) By convention, the field lines emerge from the north pole and merge at the south pole. Why? Give reason.
[1 Marks](2) State the relationship between strength of the magnetic field and the degree of closeness of the field lines.
(3) (i) No two field lines can ever intersect each other. Give reason. (ii) The magnetic field in a given region is uniform. Draw a diagram to represent it.
(4) Draw the pattern of the magnetic field lines through and around a current carrying solenoid. What does the pattern of field lines inside the solenoid represent ?
Section B
A common feature observed in the crystals of washing soda, copper sulphate, gypsum and ferrous sulphate is that all:
The values of a, b, c, and d in the following balanced chemical equation are respectively:
a Pb(NO₃)₂ ⟶(heat) b PbO + c NO₂ + d O₂
Which among the following is not a neural action controlled by the part of the human brain labelled ‘X’ in the figure above?
Hunger is not a neural action controlled by the mid-brain and hind-brain, but rather a sensation associated with a center in the fore-brain. The other options—blood pressure, salivation, and vomiting—are involuntary actions controlled by the medulla in the hind-brain.
The examples of natural and manmade (artificial) ecosystems are respectively:
The correct answer is 'Forests and ponds'. Forests and ponds are natural ecosystems formed by nature, while gardens and crop fields are human-made (artificial) ecosystems. The context specifically mentions that lakes, forests, and ponds are natural ecosystems.
Assertion (A): Silver chloride turns grey in sunlight.
Reason (R): Decomposition of silver chloride into silver and chlorine takes place by sunlight.
Assertion (A): The embryo gets nutrition from the mother’s blood with the help of a tissue called placenta.
Reason (R): Placenta is a disc embedded in the uterine wall.
Section C
A crystalline substance of green colour ‘X’ emits gases of characteristic odour when heated over a flame. It first loses water and changes colour. On further heating, it decomposes and produces a solid compound Y.
(a) Identify ‘X’ and ‘Y.
(b) State the change in colour observed when ‘X’ is heated.
Give reasons:
(a) The male reproductive organ responsible for formation of germ cells is located outside the abdominal cavity.
(b) The roles of the glands, present along the path of the vas-deferens, are very significant.
How is lymph formed ? State its important function.
(a) Identify ‘X’ in the figure of human nephron shown below. What role does it play in the process of urine formation ?
(b) Why some substances are selectively reabsorbed from the initial filtrate of urine, as it flows along the tabular part of nephron ?
(b) Some substances are selectively reabsorbed because they are necessary for the body’s functions, such as glucose and salts. The body retains these useful substances while waste products remain in the tubule to be excreted as urine.
The values of absolute refractive indices of kerosene and water are 1.44 and 1.33 respectively. Compare the two media on the basis of their
(a) optical density
(b) mass density
(c) relative speed of propagation of light.
What do you infer on the basis of above comparisons?
(b) Mass density: Water has a higher mass density than kerosene. Although kerosene is optically denser, its mass density is less than water.
(c) Relative speed of propagation of light: Light travels slower in kerosene than in water because higher refractive index means lower light speed.
Inference: A medium having higher optical density does not necessarily have higher mass density. Optical density depends on how much light slows down in the medium, not on mass density directly.
1. Electric iron: The heating effect is used to heat the iron plate, allowing clothes to be ironed and smoothed.
2. Fuse: The fuse wire melts due to heating effect when there is an excessive current, protecting the circuit from damage.
1/Rtotal = 1/R1 + 1/R2 + 1/R3
Given, R1 = 2 ohms, R2 = 4 ohms, R3 = 6 ohms
So,
1/Rtotal = 1/2 + 1/4 + 1/6 = 6/12 + 3/12 + 2/12 = 11/12
Therefore,
Rtotal = 12/11 ohms which is approximately 1.09 ohms.
Hence, the total resistance of the parallel combination is about 1.09 ohms.
(a) Why are the organisms of first trophic level important in any food chain ?
(b) Justify the following statement :
‘The flow of energy in an ecosystem is unidirectional.’
(b) The flow of energy in an ecosystem is unidirectional because energy enters the ecosystem through producers and passes on to consumers. At each trophic level, some energy is lost as heat, so energy cannot flow back to the previous level, making the energy flow one-way only.
Establish the relationship between the commercial unit of electric energy and the SI unit of electric energy.
1 kW h = 1000 watts * 3600 seconds = 3,600,000 joules (J).
The SI unit of electric energy is the joule (J).
Therefore, 1 unit (1 kW h) = 3.6 * 106 joules (J).
Section D
(b) Washing soda is obtained from baking soda (NaHCO3) by heating it strongly. Baking soda on heating decomposes to form sodium carbonate, carbon dioxide gas, and water. The sodium carbonate formed is then recrystallised to get washing soda which contains 10 molecules of water of crystallisation.
(c) Two uses of washing soda are:
1. It is used in glass, soap, and paper industries.
2. It is used for removing permanent hardness of water.
Observe the following diagram showing an experiment to determine the conditions under which a metal ‘M’ corrodes.
List your observations in each of the three cases A, B, and C with reasons, if the metal ‘M’ is generally protected against corrosion by the method of galvanisation.
(b) In case B, where metal M is exposed only to air but no moisture, there is no visible corrosion. This is because moisture is required for the electrochemical reactions during corrosion.
(c) In case C, where metal M is exposed to moisture but no air, corrosion is very slow or negligible. Although moisture is present, lack of oxygen limits the corrosion process.
Reason for protection by galvanisation: Galvanisation protects the metal by coating it with a layer of zinc, which is more reactive than iron or steel. Zinc corrodes first, acting as a sacrificial anode and protecting the underlying metal even if the zinc layer is damaged. This prevents rusting of the metal M and increases its life.
(a) Write the name and one function of respiratory pigment found in human beings.
(b) Why do lungs always contain a residual volume of air ?
(c) Why is ATP known as energy currency of the living beings ?
(a) Define fertilisation.
(b) What happens to Zygote, Ovule, Ovary and Stamens after fertilisation in a flowering plant ?
(b) After fertilisation in a flowering plant: The zygote develops into an embryo which forms the new plant. The ovule develops into a seed that contains the embryo. The ovary enlarges and develops into a fruit which protects the seed and helps in its dispersal. The stamens wither and fall off as they have completed their role of producing pollen.
The power of a lens is 0.25 D. Based on this information, find out
(a) The type of lens and its focal length.
(b) The eye defect for which it may be used as a corrective lens.
(c) The nature and size of the image formed by this lens when an object is placed between F and 2F from the optical centre of this lens.
(b) A convex lens is used to correct farsightedness or hypermetropia. This defect occurs when the eye cannot focus on near objects properly because the focal length of the eye lens is too long or the eyeball is shorter. So, a convex lens helps to converge light rays and form the image on the retina.
(c) When an object is placed between F and 2F of a convex lens, the image formed is real, inverted, magnified, and located beyond 2F on the opposite side of the lens. The size of the image is larger than the object.
(a) “The third wire of earth connection is very important in domestic electric appliances.” Justify this statement.
(b) List two precautions to be taken to avoid the overloading of domestic electric circuits.
(b) Two precautions to avoid overloading of domestic electric circuits are: 1. Do not connect too many appliances to a single socket or circuit to prevent excessive current flow. 2. Use electrical appliances within their rated power limits and ensure proper wiring and fuses are installed for safety.
(a) Show the formation of Aluminium Nitride (AIN) by the transfer of electrons. [ At. no. of Al = 13; At. no. of N= 7]
(b) “Ionic compounds are solids and are generally brittle and break into pieces when pressure is applied.” Give reason to justify the statement.
Al → Al3+ + 3e-
N + 3e- → N3-
The ionic bond between Al3+ and N3- forms AlN.
(b) Ionic compounds are solids due to the strong electrostatic force of attraction between positive and negative ions. These ions are arranged in a regular, repeating pattern known as a crystal lattice. When pressure is applied, layers of ions shift and similar charges repel each other causing the crystal to break into pieces. Hence, ionic compounds are generally brittle and break upon applying pressure.
Section E
(a) What is meant by the term homologous series of carbon compounds ? Write molecular formula of any two consecutive members of homologous series of ketones.
(b) Write chemical equation of the reactions of ethanoic acid with
(i) Sodium hydroxide and
(ii) Ethanol (in the presence of an acid); giving the name of the products in each case.
(c) Draw the structure of the molecule of benzene.
Two consecutive members of ketones are propanone (C3H6O) and butanone (C4H8O).
(b) (i) Reaction of ethanoic acid with sodium hydroxide:
CH3COOH + NaOH → CH3COONa + H2O
Product formed is sodium ethanoate (CH3COONa) and water.
(ii) Reaction of ethanoic acid with ethanol in presence of acid catalyst:
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Product formed is ethyl ethanoate (an ester) and water.
(c) The structure of benzene is a hexagonal ring of six carbon atoms with alternating double bonds, each carbon atom bonded to one hydrogen atom.
H
|
H-C=C-H
|
C C
|
H H
(a) Analyse the given situations and interpret the possible reason for each :
(i) Iodine deficiency in diet increases the possibility of a disease of swollen neck in a person.
(ii) Some people in population may have very short heights (dwarfs).
(iii) Thick facial hairs develop in boys at the age of 10-12 years.
(b) Explain two reasons which necessitate the need of chemical communication in multicellular organisms.
(ii) Very short height or dwarfism in some people may be due to hormonal imbalance, especially insufficient production of growth hormone by the pituitary gland during childhood.
(iii) Thick facial hairs develop in boys around 10-12 years of age due to the increase in sex hormones like testosterone during puberty, which leads to the development of secondary sexual characteristics.
(b) Two reasons for the need of chemical communication in multicellular organisms are:
1. Coordination: Different cells, tissues, and organs need to work together in a coordinated manner to perform complex functions, which is achieved by chemical messengers like hormones.
2. Regulation: Chemical signals regulate growth, development, metabolism, and response to environmental changes by sending messages from one part of the body to another.
(a) Differentiate between voluntary and involuntary action.
(b) Define reflex action. With the help of a flow diagram, show the correct sequence of path of Nerve impulse from place of its origin.
Voluntary actions are those that are consciously controlled and executed by an individual. They involve the brain's decision-making processes and include activities such as walking, writing, or speaking. In contrast, involuntary actions are automatic and occur without conscious thought. These actions, like breathing or digestion, are regulated by the autonomic nervous system and ensure the body functions without requiring active control.
Reflex action is an involuntary and nearly instantaneous movement in response to a stimulus, typically following a reflex arc. When a sensory nerve detects a stimulus, such as heat, it sends signals to the spinal cord, where interneurons transmit the signal to motor neurons, leading to a quick response, such as pulling your hand away from a hot surface.
To summarize the flow of a nerve impulse in a reflex action:
Stimulus (e.g., heat)
Sensory neuron (detects stimulus)
Interneuron (in spinal cord processes the signal)
Motor neuron (sends signal to muscles)
Effector (muscles respond)
This pathway allows for rapid reactions that protect the body from harm.
Analyse the following observation table showing variation of image distance (v) with object distance (u) in case of a convex lens and answer the questions that follow without doing any calculations :
(a) Determine the focal length of the lens. Give reason for your answer.
(b) Find magnification of the image formed in Observation No. 3.
(c) The numerical value of magnifications in cases of observation 1 and 2 is same. List two differences in the images formed in these two cases.
(b) Magnification (m) is defined as m = image distance (v) / object distance (u). In Observation No. 3, substitute the values of v and u in this formula to find magnification. The sign of magnification also tells about the nature of the image (real or virtual).
(c) Although the numerical values of magnifications in observations 1 and 2 are same, the images differ as follows: (1) In Observation 1, the image is real and inverted, whereas in Observation 2, the image is virtual and erect. (2) In Observation 1, the image is formed on the opposite side of the lens, whereas in Observation 2, the image is formed on the same side as the object. Thus the size may be same but the nature and position of images are different.
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