CBSE EXAMINATION PAPER-2025
SCIENCE
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 35 questions. All questions are compulsory.
- This question paper is divided into 5 sections.
- Section A – questions number 1 to 1 are case based questions
- Section B – questions number 2 to 16 are multiple choice questions
- Section C – questions number 17 to 24 are very short answer
- Section D – questions number 25 to 31 are short answer
- Section E – questions number 32 to 35 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 our homes, we receive the supply of electric power through a main supply also called mains, either supported through overhead electric poles or by underground cables. In our country the potential difference between the two wires (live wire and neutral wire) of this supply is 220 V.
(1) Write the colours of the insulation covers of the line wires through which supply comes to our homes.
(2) What should be the current rating of the electric circuit (220 V) so that an electric iron of 1 kW power rating can be operated ?
(3) List two precautions to be taken to avoid electrical accidents. State how these precautions prevent possible damage to the circuit/appliance.
(4) What is the function of the earth wire ? State the advantage of the earth wire in domestic electric appliances such as electric iron.
Section B
Electrolysis of water is a decomposition reaction. The mass ratio (M_H : M_O) of hydrogen and oxygen gases liberated at the electrodes during electrolysis of water is:
The products formed when Aluminium and Magnesium are burnt in the presence of air respectively are:
The following table shows the pH values of four solutions A, B, C and D on a pH scale:
The solutions A, B, C and D respectively are of a
Consider the following reactions:
(i) Dilute hydrochloric acid reacts with sodium hydroxide.
(ii) Magnesium oxide reacts with dilute hydrochloric acid.
(iii) Carbon dioxide reacts with sodium hydroxide.
It is found that in each case:
The metals obtained from their molten chlorides by the process of electrolytic reduction are:
The correct/true statement(s) for a bisexual flower is/are:
(i) They possess both stamen and pistil.
(ii) They possess either stamen or pistil.
(iii) They exhibit either self-pollination or cross-pollination.
(iv) They cannot produce fruits on their own.
The percentage of solar energy which is not converted into food energy by the leaves of green plants in a terrestrial ecosystem is about:
Assertion (A): Decomposition reactions are generally endothermic reactions.
Reason (R): Decomposition of organic matter into compost is an exothermic process.
Assertion (A): A human child bears all the basic features of human beings.
Reason (R): It looks exactly like its parents, showing very little variations.
Assertion (A): No two magnetic field lines are found to cross each other.
Reason (R): The compass needle cannot point towards two directions at the point of intersection of two magnetic field lines.
Assertion (A): The amount of ozone in the atmosphere began to drop sharply in the 1980s.
Reason (R): The oxygen atoms combine with molecular oxygen to form ozone.
Section C
A student performs the following experiment in his school laboratory.
List two observations to justify that in this experiment a chemical change has taken place.
(i) The transport system in plants is relatively slower than in animals. Give reasons.
(ii) State the role of phloem in the transport of materials in plants.
(i) The transport system in plants is slower than in animals because plants rely on passive transport mechanisms like diffusion and transpiration pull, which are naturally slower processes. Additionally, plants have rigid cell walls and large vacuoles that limit quick movement, and the transport occurs mainly through xylem and phloem which are less dynamic than animal circulatory systems.
(ii) Phloem is responsible for transporting food, especially sugars produced by photosynthesis in the leaves, to all parts of the plant. It consists of sieve tubes and companion cells that help move nutrients from the leaves to roots, stems, and growing parts, ensuring energy supply throughout the plant.
A wire of resistance R is cut into three equal parts. If these three parts are then joined in parallel, calculate the total resistance of the combination so formed.
Section D
(a) ‘‘Displacement reactions also play a key role in extracting metals in the middle of the reactivity series.’’ Justify this statement with two examples.
(b) Why can metals high up in the reactivity series not be obtained by reduction of their oxides by carbon ?
(b) Metals that are high up in the reactivity series, such as sodium, calcium, and aluminium, cannot be obtained by reduction of their oxides using carbon because these metals form very stable oxides and carbon is not reactive enough to reduce them. Instead, more reactive metals like aluminium are used as reducing agents to extract metals lower in the series. High-reactivity metals require other extraction methods like electrolysis to be obtained in pure form.
Plants have neither a nervous system nor muscles, even then they respond to stimuli. For example, the leaves of chhui-mui (touch-me-not) plant when touched begin to fold up and droop.
(a) How is the information communicated in ‘‘touch-me-not’’ plants ?
(b) What enables the plant cells to bring out the observable response ?
(c) Differentiate the movement mentioned above from the movement of tendrils in a pea plant.
(b) The plant cells respond because they can quickly lose water from specialized cells at the base of the leaflets called pulvini cells. The loss of water changes the turgor pressure inside these cells, causing the leaves to fold and droop.
(c) The movement of the touch-me-not plant is a rapid response caused by changes in cell turgor pressure without growth, and it happens quickly after the stimulus. In contrast, the movement of tendrils in a pea plant is a slow movement that involves growth and bending towards support (a directional growth response called tropism), which helps the plant climb.
(a) What are chromosomes ?
(b) Explain in brief how stability of DNA content of a species is ensured in sexually reproducing organisms ?
(a) What are chromosomes?
Chromosomes are thread-like structures located inside the nucleus of cells. They are made of DNA and proteins. Chromosomes carry genetic information in the form of genes, which determine the hereditary traits passed from parents to offspring. Each chromosome consists of two identical sister chromatids joined together after replication.
(b) How is the stability of DNA content ensured in sexually reproducing organisms?
In sexually reproducing organisms, stability of DNA content is maintained through the process of meiosis. Male and female gametes (sperm and egg) are produced with half the number of chromosomes (haploid). When these gametes fuse during fertilization, they restore the full number of chromosomes (diploid) in the new organism. This ensures that each generation has the same chromosome number and maintains genetic stability across generations.
Draw ray diagrams to show the nature, position and relative size of the image formed by a convex mirror when the object is placed
(i) at infinity and
(ii) between infinity and pole P of the mirror.
(i) When the object is at infinity, the rays coming from the object are parallel to the principal axis. After reflection from the convex mirror, these rays diverge as if they are coming from the focus (F) behind the mirror. The image formed is virtual, highly diminished, erect, and located at the focus of the mirror behind the pole.
(ii) When the object is placed between infinity and the pole (P) of the convex mirror, the rays diverge after reflection. By tracing the reflected rays backwards, they appear to come from a point behind the mirror. The image formed is virtual, erect, diminished compared to the object, and located between the focus (F) and the pole (P) behind the mirror.
Thus, in both cases, the image formed by a convex mirror is virtual, erect, and smaller than the object, with its position changing depending on the location of the object.
(a) Write the relationship between resistivity and resistance of a cylindrical conductor of length l and area of cross-section A. Hence derive the SI unit of resistivity.
(b) Why are alloys used in electrical heating devices ?
(a) The resistance R of a cylindrical conductor depends on its length l, area of cross-section A, and the material's resistivity ρ. The relationship is given by R = ρ (l / A). This means resistance is directly proportional to length and resistivity, and inversely proportional to the area of cross-section. Here, resistivity ρ is a property of the material which quantifies how strongly it opposes the flow of electric current.
From the formula R = ρ (l/A), we can express resistivity as ρ = R (A/l).
Since resistance R is measured in ohms (Ω), length l in meters (m), and area A in square meters (m²), the SI unit of resistivity is ohm-meter (Ω·m).
(b) Alloys are used in electrical heating devices because they generally have higher resistivity than pure metals. This higher resistivity allows them to generate heat more efficiently when current passes through. Additionally, alloys do not oxidize or burn easily at high temperatures, making them durable and safe materials for devices like electric irons and toasters.
(i) Name two metals which react violently with cold water. List any three observations which a student notes when these metal are dropped in a beaker containing water.
(ii) Write a test to identify the gas evolved (if any) during the reaction of these metals with water.
(i) Two metals which react violently with cold water are sodium and potassium.
When sodium or potassium is dropped into a beaker containing cold water, a student may observe the following:
- Fizzing or bubbling due to the release of gas.
- The metal piece may start to move or float on the water surface.
- In the case of potassium, it often catches fire with a lilac-colored flame.
(ii) The gas evolved during the reaction of these metals with water is hydrogen. To test for hydrogen gas, bring a burning splinter near the mouth of the test tube containing the collected gas. If the gas is hydrogen, it will burn with a characteristic pop sound.
Section E
(i) Write the functions of the following parts of the human female reproductive system:
(I) Ovary,
(II) Fallopian tube,
(III) Uterus.
(ii) State briefly two contraceptive methods used by human males.
(i) Functions of parts of the female reproductive system:
(I) Ovary: The ovaries are two small, oval-shaped organs located on each side of the uterus. Their main functions are to produce female sex hormones like estrogen and progesterone, which regulate the menstrual cycle and help develop female secondary sexual characteristics. The ovaries also produce and release eggs (ova) through a process called ovulation.
(II) Fallopian tube: Also called the oviduct, the fallopian tubes are narrow tubes that connect the ovaries to the uterus. Their primary function is to transport the released egg from the ovary to the uterus. Fertilization, where sperm meets the egg, usually takes place inside the fallopian tubes.
(III) Uterus: The uterus is a hollow, pear-shaped organ where a fertilized egg implants and develops into a fetus. Each month, its inner lining thickens in preparation to support a fertilized egg. If fertilization does not occur, this lining sheds during menstruation.
(ii) Two contraceptive methods used by human males:
1. Use of condoms: Condoms are a barrier method worn over the penis during sexual intercourse to prevent sperm from entering the female reproductive tract, thereby avoiding pregnancy.
2. Sterilization (vasectomy): This is a surgical procedure where the vas deferens (sperm-carrying tubes) are cut or sealed to prevent sperm from mixing with semen, resulting in permanent contraception.
(i) The power of a lens ‘X’ is – 2·5 D. Name the lens and determine its focal length in cm. For which eye defect of vision will an optician prescribe this type of lens as a corrective lens ?
(ii) ‘‘The value of magnification ‘m’ for a lens is – 2.’’ Using new Cartesian Sign Convention and considering that an object is placed at a distance of 20 cm from the optical centre of this lens, state :
(I) the nature of the image formed;
(II) size of the image compared to the size of the object;
(III) position of the image, and
(IV) sign of the height of the image.
(iii) The numerical values of the focal lengths of two lenses A and B are 10 cm and 20 cm respectively. Which one of the two will show higher degree of convergence/divergence ? Give reason to justify your answer.
(i) The power of the lens X is –2.5 D. Since the power is negative, the lens is a concave lens. The focal length f of the lens is related to its power P by the formula: f = 100 / P (in cm). Substituting P = –2.5, we get f = 100 / (–2.5) = –40 cm. The negative focal length confirms it is a concave lens. This type of lens is prescribed by an optician to correct myopia (nearsightedness), which occurs either due to the eye lens having too short a focal length or the eyeball being too long. The concave lens diverges the rays before they enter the eye, enabling the image to form on the retina.
(ii) Given magnification m = –2 and object distance u = –20 cm (object distances are negative as per new Cartesian sign convention).
- (I) Nature of the image: Since magnification is negative, the image formed is real and inverted.
- (II) Size of the image: Magnification of –2 means the image is twice the size of the object.
- (III) Position of the image: Magnification m = v/u, so v = m × u = –2 × (–20 cm) = +40 cm. The positive image distance indicates the image is formed on the opposite side of the lens, confirming it is real.
- (IV) Sign of the height of the image: Since magnification is negative, the height of the image is negative, indicating the image is inverted relative to the object.
(iii) Lens A has focal length f_A = 10 cm and lens B has focal length f_B = 20 cm. The degree of convergence or divergence of a lens is inversely proportional to its focal length. The lens with a smaller focal length will have a higher degree of convergence (if convex) or divergence (if concave). Therefore, lens A with 10 cm focal length has a higher degree of convergence/divergence than lens B with 20 cm focal length because 10 cm is smaller than 20 cm, so lens A bends the light rays more strongly than lens B.
(i) Differentiate between self-pollination and cross-pollination.
(ii) Identify A, B and C in the diagram given below and write one function of each.
Self-pollination occurs when pollen from the anther of a flower lands on the stigma of the same flower or another flower on the same plant. This means the pollen and stigma belong to the same plant. It helps in reproduction but results in less genetic variation.
Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of a flower on a different plant of the same species. This process increases genetic diversity and helps plants adapt to changing environments.
In brief:
- Self-pollination: Pollen → stigma (same flower or same plant).
- Cross-pollination: Pollen → stigma (different flower on a different plant).
(ii) Identification and Functions of A, B, and C in the diagram:
Assuming the diagram shows parts of a flower involved in pollination:
A: Anther
Function: Produces and releases pollen grains, which contain the male reproductive cells.
B: Stigma
Function: It is the sticky part on top of the pistil where pollen grains land and germinate.
C: Pollen grains
Function: They carry the male reproductive cells (sperm cells) necessary for fertilization of the ovule.
These parts play vital roles in the process of pollination and fertilization in flowering plants.
(i) Draw a ray diagram to show the refraction of a ray of light through a rectangular glass slab when it falls obliquely from air into glass.
(ii) State Snell’s law of refraction of light.
(iii) Differentiate between the virtual images formed by a convex lens and a concave lens on the basis of :
(I) object distance, and
(II) magnification.
When a ray of light traveling from air falls obliquely on the surface of a rectangular glass slab, it bends towards the normal because glass is denser than air. After passing through the slab, the ray bends away from the normal as it enters air again. The ray inside the slab is parallel to the incident ray but laterally shifted.
(ii) Snell’s Law of Refraction:
Snell’s law states that the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant for the two given media. Mathematically, it is expressed as:
sin(i) / sin(r) = constant = refractive index
(iii) Difference between Virtual Images formed by Convex and Concave Lens:
(I) Object Distance:
- Convex Lens: Virtual image is formed when the object is placed between the optical centre and the principal focus (object distance less than focal length).
- Concave Lens: Virtual image is formed for any object position since it always forms virtual images.
(II) Magnification:
- Convex Lens: The virtual image formed is magnified (larger than the object).
- Concave Lens: The virtual image formed is diminished (smaller than the object).
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