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 2 are case based questions
- Section B – questions number 3 to 16 are multiple choice questions
- Section C – questions number 17 to 23 are very short answer
- Section D – questions number 24 to 29 are short answer
- Section E – questions number 30 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
Many pure metals like copper, iron and gold are very soft and as such are considered unsuitable for certain uses. Metallic objects around us such as cooking utensils, statues, ornaments, guns etc. are actually not made up of pure metals. Instead of pure metals, alloys are used in the design of most of the useful objects. Making alloys enhances the basic properties of a metal which is the primary constituent (metal) of an alloy.
(1) Name an alloy used for welding two wires together in an electric circuit. Write its major constituents.
[1 Marks](2) How does electrical conductivity and melting point of a metal change when it is converted to its alloy by mixing a small amount of an element in it?
[1 Marks](3) What is stainless steel ? How is it prepared ? Write one important property which makes it more useful in making cooking utensils as compared to its primary metal.
(4) What are alloys ? How is ‘Brass’ (an alloy) prepared ?
(1) Name the movement which causes ‘X’ and ‘Y to grow downwards and upwards respectively. (Refer above figure)
(2) Write the name of a hormone that plays a major role in (i) falling of leaves (ii) rapid cell division
(3) Leaves of the sensitive plant move very quickly in response to ‘touch’. How is this stimulus of touch communicated and explain how the movement takes place.
(4) Name the plant hormone which is synthesized at the shoot tip. How does this hormone helps the plant to bend towards light ?
Section B
You have three aqueous solutions A, B and C as given below:
A – Potassium nitrate
B – Ammonium chloride
C – Sodium carbonate
The ascending order of the pH of these solutions is:
Select from the following a statement which is not true about burning of magnesium ribbon in air :
The colour of the solution observed after about 1 hour of placing iron nails in copper sulphate solution is
Juice of tamarind turns blue litmus to red. It is because of the presence of a chemical compound called
The water of crystallization is present in
(i) Bleaching Powder (ii) Plaster of Paris (iii) Washing Soda (iv) Baking Soda
A tall pea plant with round seeds (TTRR) is crossed with a short pea plant with wrinkled seeds (ttrr). The F₁ generation will be:
A pair of endocrine glands located in the human brain is
The basic filtration unit of the excretory system in human beings is :
In human alimentary canal, the digestive juice secreted by the gastric glands are
The part of human eye which controls the amount of light entering into it.
Consider the following food chain:
Grass → Grasshopper → Frog → Snake → Eagle
If the amount of energy available at third trophic level is 50 kJ, the available energy at the producer level was:
Assertion (A) : Carbon and its compounds are our major sources of fuels.
Reason (R) : Most of the carbon compounds on burning release a large amount of heat and light.
Assertion (A): In the common domestic circuits the earth wire is connected to a metallic plate buried deep inside the earth.
Reason (R) : Earth wire ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, so the user may not get a severe electric shock.
Assertion (A) : Food web is a network of several food chains operating in an ecosystem.
Reason (R) : Food web decreases the stability of an ecosystem.
Section C
A light green coloured solution of sulphate salt of metal ‘P’ is taken in a beaker, a rod of another metal “Q’ is put in this solution as shown the following figures :
Identify the metals ‘P’ and ‘Q’ and write its chemical equation for the reaction that occurs. State the conclusion of this reaction in terms of reactivity series of metals.
(a) How is brain protected in our body ?
(b) A doctor finds in one of his patients that he is not maintaining a proper posture and balance of his body. State the region of brain and also the part of brain which is responsible for it.
“Proteins control the expression of various characters.” Explain this statement by taking an example of “tallness” as a characteristic in plants.
Explain the mechanism of inheritance used by sexually reproducing organisms to ensure the stability of DNA of the species.
Study the figure in which the path of a ray of light going from Medium 1 to Medium 2 is shown.
(a) Out of the two Media — Medium 1 and Medium 2, in which is the speed of light more ?
(b) State reason of bending of the refracted ray away from the normal.
(c) Express refractive index of Medium 2 with respect to Medium 1 in terms of speed of light in two media.
(b) The refracted ray bends away from the normal because the speed of light increases as it moves from Medium 1 to Medium 2. This change in speed causes bending due to refraction.
(c) The refractive index of Medium 2 with respect to Medium 1 is given by n = v1 / v2, where v1 is the speed of light in Medium 1 and v2 is the speed of light in Medium 2.
Section D
State the chemical property in each case on which the following uses of baking soda are based upon:
(i) as an anti-acid,
(ii) as a constituent in making baking powder,
(iii) in soda-acid fire extinguishers.
Write chemical equations to show what happens when an acid reacts with
(i) metal,
(ii) base,
(iii) carbonate.
Write the name of the main product formed in each case.
When an acid reacts with a metal, it forms a salt and hydrogen gas.
Example: 2HCl + Zn → ZnCl2 + H2
The main product formed is a salt.
(ii) Acid + Base:
When an acid reacts with a base, it gives a salt and water. This is called neutralization.
Example: HCl + NaOH → NaCl + H2O
The main product formed is a salt.
(iii) Acid + Carbonate:
When an acid reacts with a carbonate, it produces a salt, carbon dioxide, and water.
Example: 2HCl + Na2CO3 → 2NaCl + CO2 + H2O
The main product formed is a salt.
A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below :
Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations) :
(a) What is the focal length of the convex lens used ? Give reason to justify your answer.
(b) Which one of the sets of observations is not correct and why ?
(c) Draw ray diagram to show image formation for any correct set of observation.
(b) Any set of observations where image characteristics contradict the properties of convex lens images is not correct. For example, if an image is described as virtual and formed on the screen, it is incorrect because virtual images cannot be projected on a screen. Also, if magnification or position does not follow the lens formula's logical pattern, that set is wrong.
(c) Ray diagram for object placed beyond 2F (twice the focal length):
- Draw the convex lens vertical line with focal points F on both sides.
- Place the candle flame beyond 2F.
- Draw one ray parallel to principal axis which after refraction passes through the focal point on other side.
- Draw second ray passing through the optical centre which goes straight without deviation.
- The rays converge at a point between F and 2F on the other side forming a real, inverted and smaller image on the screen.
This ray diagram corresponds to a correct observation set with object beyond 2F.
A person uses lenses of +2.0 D power in his spectacles for the correction of his vision.
(a) Name the defect of vision the person is suffering from.
(b) List two causes of this defect.
(c) Determine the focal length of the lenses used in the spectacles.
(b) Two causes of hypermetropia are: (i) The eyeball may be too small from front to back. (ii) The eye lens may have a focal length that is too long.
(c) The power P of a lens is related to its focal length f by the formula P = 100 / f where f is in cm. Given power P = +2.0 D, so focal length f = 100 / 2 = 50 cm = 0.50 m. Hence, the focal length of the lens used is +0.50 meter.
(a) Explain the statement “Potential difference between two points is 1 volt”.
(b) What do the symbols given below represent in an electric circuit ? Write one function of each.
(a) The statement “Potential difference between two points is 1 volt” means that 1 joule of work is done to move 1 coulomb of electric charge from one point to the other in the circuit. Essentially, this means the electric potential energy difference between these two points is equivalent to 1 volt. It indicates how much energy each unit charge gains or loses while moving between those points.
(b) In an electric circuit, different symbols represent various components:
Voltmeter (V): This instrument measures the voltage or potential difference between two points in the circuit. It helps us know how much electric potential energy per unit charge is available between those points.
Key (K): A key is a type of switch that can open or close the circuit. When closed, it allows current to flow; when opened, it stops the current, controlling the working of the circuit.
Section E
Name an alcohol and a carboxylic acid having two carbon atoms in their structures. Draw their structures and state how this alcohol can be converted into a carboxylic acid. What happens when these two compounds react in the presence of an acid ? Write chemical equations for the reactions involved in the two cases mentioned above.
Structure of Ethanol:
CH3 - CH2 - OH
Carboxylic acid with two carbon atoms: Ethanoic acid (Acetic acid) (CH3COOH)
Structure of Ethanoic acid:
CH3 - COOH
(b) Conversion of ethanol into ethanoic acid:
Ethanol can be oxidized to ethanoic acid using an oxidizing agent such as potassium dichromate (K2Cr2O7) in acidic medium (dilute H2SO4). This is an oxidation reaction.
Chemical equation:
CH3CH2OH + [O] --(K2Cr2O7/H2SO4)--> CH3COOH + H2O
(c) Reaction between ethanol and ethanoic acid in presence of acid catalyst:
When ethanol reacts with ethanoic acid in presence of acid catalyst (usually concentrated H2SO4), an esterification reaction occurs forming an ester called ethyl ethanoate and water.
Chemical equation:
CH3COOH + CH3CH2OH --(H2SO4)--> CH3COOCH2CH3 + H2O
This reaction is reversible and acid catalyst helps in increasing the rate of reaction.
Summary:
- Ethanol is an alcohol with two carbon atoms.
- Ethanoic acid is a carboxylic acid with two carbon atoms.
- Ethanol can be oxidized to ethanoic acid.
- Ethanol and ethanoic acid react in presence of acid to form ester (ethyl ethanoate) and water.
(a) Define Puberty. List any two changes seen in boys at the time of puberty.
(b) Why are testes in human males located outside the abdominal cavity in scrotum ?
(c) List any three techniques of contraception used by humans. Which one of these is not meant for males ?
(b) The testes in human males are located outside the abdominal cavity in the scrotum because sperm formation requires a temperature slightly lower than the normal body temperature. The scrotum keeps the testes cooler, which helps in the production of healthy sperm.
(c) Three techniques of contraception used by humans are condoms, oral contraceptive pills, and copper-T (intrauterine device). Among these, oral contraceptive pills are not meant for males; they are used by females to prevent pregnancy.
(a) Name the part performing following functions in human female reproductive system :
(i) production of eggs
(ii) site of fertilization
(iii) site of implantation
(iv) entry of the sperms
(b) What changes are observed in the uterus :
(i) subsequent to implantation of zygote and
(ii) if an egg does not get fertilized ?
(i) Production of eggs: The ovary is responsible for producing eggs or ova.
(ii) Site of fertilization: Fertilization occurs in the fallopian tube (oviduct) where the sperm meets the egg.
(iii) Site of implantation: The fertilized egg implants itself in the lining of the uterus called the endometrium.
(iv) Entry of sperms: Sperms enter the female reproductive system through the vagina.
(b) Changes observed in the uterus:
(i) Subsequent to implantation of zygote: The uterine lining thickens and becomes enriched with blood vessels to nourish the developing embryo. The uterus prepares to support pregnancy by becoming thick and spongy.
(ii) If an egg does not get fertilized: The thickened uterine lining breaks down and is shed from the body as menstrual blood. This process is called menstruation. The uterus returns to its normal state waiting for the next cycle.
What are soaps ? Write the structure of a soap molecule. Explain the cleansing action of a soap. Why are soaps not considered suitable for washing clothes in a region where water is hard ? How is this problem overcomed ?
(b) Structure of Soap Molecule : A soap molecule has a long hydrocarbon chain which is hydrophobic (water repelling) and an ionic carboxylate group (–COO-Na+) which is hydrophilic (water loving).
(c) Cleansing Action : Soap molecules have two different ends: the hydrophobic tail dissolves in grease and oil, while the hydrophilic head remains in water. When soap is added to water containing dirt, the hydrophobic tails attach to the oil and grease, while the hydrophilic heads remain in water. This forms spherical structures called micelles where the dirt is trapped inside. These micelles are suspended in water and can be washed away, cleaning the clothes.
(d) Soap in Hard Water : Hard water contains Ca2+ and Mg2+ ions which react with soap to form insoluble compounds called scum. This scum reduces the cleaning ability of soaps and makes washing ineffective.
(e) Solution : This problem is overcome by using synthetic detergents instead of soaps. Detergents do not form scum in hard water and clean effectively.
What are magnetic field lines ? How is the direction of magnetic field at a point determined ? Draw the pattern of magnetic field lines of the magnetic field produced by a current carrying circular loop. Mark on it the direction of (i) current and (ii) magnetic field lines.
Name the two factors on which the magnitude of the magnetic field due to a current carrying coil depends.
Magnetic field lines are imaginary lines used to represent the magnetic field around a magnet or a current carrying conductor. These lines show the direction of the magnetic force and help visualize the strength and shape of the magnetic field. The magnetic field is stronger where the lines are closer and weaker where they are farther apart.
(b) How is the direction of magnetic field at a point determined?
The direction of the magnetic field at any point is given by the tangent to the magnetic field line at that point. Around a current carrying conductor, the magnetic field direction can be found by using the right-hand thumb rule. If you curl the fingers of your right hand in the direction of the current, your thumb points the direction of the magnetic field.
(c) Pattern of magnetic field lines produced by a current carrying circular loop:
The magnetic field lines form concentric closed loops around the wire. Inside the circular loop, the field lines are almost straight and parallel, showing a uniform magnetic field. Outside the loop, the lines spread out and loop back to the other side. On the diagram, the direction of current is marked as arrows along the circular wire (usually anticlockwise or clockwise), and the direction of magnetic field lines is shown by arrows forming loops around and inside the coil.
(d) Two factors on which the magnitude of magnetic field due to a current carrying coil depends:
1. Magnitude of the current flowing through the coil.
2. Number of turns in the coil.
More current or more turns will increase the magnetic field produced by the coil.
Why can’t two magnetic field lines cross each other ? Draw magnetic field lines showing the direction of the magnetic field due to a current carrying long straight solenoid. State the conclusion which can be drawn from the pattern of magnetic field lines inside the solenoid.
Name any two factors on which the magnitude of the magnetic field due to this solenoid depends.
(b) The magnetic field lines due to a current carrying long straight solenoid are shown as closely spaced, nearly parallel lines inside the solenoid indicating a strong and uniform magnetic field. Outside the solenoid, the field lines spread out and loop from one end to the other, similar to the field of a bar magnet.
(c) The conclusion drawn from the pattern of magnetic field lines inside the solenoid is that the magnetic field inside the solenoid is strong and uniform. The solenoid behaves like a bar magnet with a north and south pole.
(d) Two factors on which the magnitude of the magnetic field due to the solenoid depends are:
1. The number of turns of the coil (N) in the solenoid.
2. The current (I) passing through the solenoid.
Other factors include the length of the solenoid and the presence of a magnetic core.
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