CBSE Class 10 Science Board Paper 2025 — Set 1 Solutions

Answer: Budding in Hydra starts with a small bulge called a bud forming on the side of the parent Hydra. This bud grows gradually, developing tiny tentacles similar to those of the parent. Eventually, the bud fully develops into a miniature Hydra and detaches from the parent body, becoming an independent organism. The stages include the initial bud formation, bud growth with tentacle development, and final detachment.

Answer: Plugging any leak in a blood vessel is essential not only to prevent blood loss but also to maintain blood pressure. Loss of pressure due to leakage can reduce the efficiency of the heart’s pumping system, affecting blood circulation. The component of blood responsible for plugging these leaks is called platelets. Platelets circulate throughout the body and immediately gather at the site of injury. They help in clotting the blood by sticking together and forming a plug that seals the wound to stop bleeding and prevent further loss of blood and pressure.

Answer: Given: Object distance u = -60 cm (since the object is in front of the lens), Focal length f = -30 cm (concave lens has negative focal length). Using the lens formula, 1/f = 1/v - 1/u. Substituting values: 1/(-30) = 1/v - 1/(-60). Simplifying, 1/v = 1/(-30) + 1/60 = (-2 + 1)/60 = -1/60. Therefore, v = -60 cm. The negative sign means the image is formed on the same side as the object, at 60 cm from the lens. So, the image is virtual, upright, and reduced in size, located 60 cm in front of the lens.

Answer: Electric power is the rate at which electrical energy is consumed or converted into another form of energy in an electric circuit. The SI unit of electric power is watt (W). We say that the power consumed in an electric circuit is 1 watt when the energy consumption rate is one joule per second. Practically, this means a device consuming 1 ampere of current at a potential difference of 1 volt is using power equal to 1 watt.

Answer: Excessive use of chemicals and pesticides in agriculture causes environmental problems. These chemicals are sprayed on plants or used to treat seeds and soil. When used excessively, they contaminate soil and water bodies as they get washed away by rain. This pollution harms many plant and animal species because these chemicals are often poisonous. Overusing fertilizers destroys soil fertility by killing valuable micro-organisms and reducing organic matter. It also leads to water pollution. Therefore, using too many chemicals in farming adversely affects the environment.

Answer: Two observations that justify that a chemical change has taken place in the experiment are: (1) There is a change in colour of the substance, indicating the formation of a new substance. (2) A gas is evolved during the experiment, which shows a chemical reaction is occurring. These observations confirm that a chemical change has occurred as physical changes do not involve such changes.

Answer:

(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.

Answer: When a wire of resistance R is cut into three equal parts, each part will have resistance equal to R/3 since resistance is directly proportional to length. Joining these three parts in parallel, the reciprocal of the total resistance (R_total) is the sum of the reciprocals of each of the three resistances: 1/R_total = 1/(R/3) + 1/(R/3) + 1/(R/3). Simplifying, 1/R_total = 3/(R/3) = 9/R. Therefore, R_total = R/9. Hence, the total resistance when the three parts are joined in parallel is one-ninth of the original resistance R.

Answer: (a) Displacement reactions are important in extracting metals, especially those in the middle of the reactivity series. In these reactions, a more reactive metal displaces a less reactive metal from its compound. For example, when aluminium powder is heated with manganese dioxide, aluminium displaces manganese, producing pure manganese metal and aluminium oxide. Another example is zinc displacing copper from copper sulfate solution. Such reactions help in obtaining pure metals from their compounds.

(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.

Answer: To understand the conditions under which iron rusts, we can perform a simple activity. Take three test tubes labeled A, B, and C, and place clean iron nails in each of them. In test tube A, pour water; in test tube B, pour water and add some oil to prevent air from reaching the water; in test tube C, leave it dry without water. Observe these test tubes over a few days. The nail in test tube A will start rusting because it is exposed to both water and air, which are necessary for rusting. The nail in test tube B will not rust because the oil layer prevents air from reaching the water, so rusting does not occur. The nail in test tube C will also not rust as there is no moisture present, even though air is present. From this activity, we learn that iron rusts only when both air and water are present. Rusting is a chemical reaction where iron reacts with oxygen and moisture to form hydrated iron oxide, which weakens the iron. Therefore, air (oxygen) and water (moisture) are essential conditions for rust to form on iron articles.

Answer: (a) In touch-me-not plants, when the leaves are touched, the information about the stimulus is communicated from cell to cell by electrical and chemical signals. Even though plants do not have nervous tissue, these signals travel through the cells to reach the parts of the leaf that respond.

(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.

Answer:

(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.

Answer:

(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.

Answer:

(a) To find the total resistance of the circuit, first calculate the equivalent resistance of the resistors in parallel. For the 10 Ω and 15 Ω resistors in parallel, the combined resistance Rₚₐᵣₐₗₗₑₗis found using the formula: 1/Rₚₐᵣₐₗₗₑₗ = 1/10 + 1/15. This gives Rₚₐᵣₐₗₗₑₗ = 6 Ω. If this parallel combination is in series with other resistors, add their resistances to Rₚₐᵣₐₗₗₑₗ to get the total resistance Rₜₒₜₐₗ

(b) Using the total resistance found, calculate the total current drawn from the source using Ohm's law: Iₜₒₜₐₗ = V / Rₜₒₜₐₗ , where V is the voltage of the source.

(c) The potential difference across the parallel combination of 10 Ω and 15 Ω resistors is equal to the voltage drop across Rₚₐᵣₐₗₗₑₗ. Since resistors in parallel have the same voltage across them, this value can be found by calculating Iₜₒₜₐₗ multiplied by the resistance of the series resistors before the parallel section, then subtracting from the total voltage to get the voltage across the parallel resistors.

Answer:

(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.

Answer:

(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.

Answer:

(i) Two isomeric structures of Butene (C₄H₈) are:

1. 1-Butene:CH₂=CH–CH₂–CH₃ (Double bond between first and second carbon atoms)
2. 2-Butene: CH₃–CH=CH–CH₃ (Double bond between second and third carbon atoms)

(ii) Names of the given structures:

(I) Structure 1 is 1-Butene
(II) Structure 2 is 2-Butene

(iii) Chemical equations and conditions for reactions:

(I) Ethanol undergoes complete oxidation:
Equation: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Condition: Presence of a catalyst like copper or heating with oxygen
This reaction is combustion and complete oxidation of ethanol producing carbon dioxide and water.

(II) Propene undergoes hydrogenation:
Equation:CH₃–CH=CH₂ + H₂ → CH₃–CH₂–CH₃
Condition: Presence of a catalyst such as finely divided nickel, at room temperature
Hydrogenation is the addition of hydrogen to the double bond converting an alkene to an alkane.

(III) Ethanoic acid reacts with ethanol:
Equation: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Condition: Presence of concentrated sulfuric acid as a catalyst and heating
This is an esterification reaction, where ethyl acetate (an ester) and water are formed.

Answer:

(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.

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.

Answer:

Part (i):
The carbon compound X mentioned is ethanol (C₂H₅OH), which is a good solvent. When ethanol reacts with sodium, it produces sodium ethoxide (Y) and hydrogen gas (Z). Here, sodium ethoxide is the product Y, and hydrogen gas is Z. Hydrogen gas (Z) is used in the hydrogenation process to convert vegetable oil into vegetable ghee.

The reaction is:
2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂

Part (ii):
(I) When ethanol burns in oxygen/air, it undergoes complete combustion to form carbon dioxide and water.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

(II) When ethanol is heated at 443 K in excess concentrated sulfuric acid, it undergoes dehydration to form ethene (an unsaturated hydrocarbon) and water.
C₂H₅OH ⟶(conc. H₂SO₄, 443 K) C₂H₄ + H₂O
(III) When ethanol reacts with acidified potassium dichromate, it gets oxidized to form ethanoic acid (acetic acid). The color of the solution changes from orange to green.
3C₂H₅OH + 2K₂Cr₂O₇ + 8H₂SO₄ → 3CH₃COOH + 2Cr₂(SO₄)₃ + 2K₂SO₄ + 11H₂O

Answer: (i) Difference between Self-pollination and Cross-pollination:

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.

Answer: (i) Ray Diagram for Refraction through a Rectangular Glass Slab:
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).