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Class 10 Chemistry Answer Key | ICSE Exam 2025-26 Board Term 1

1. Four friends P, Q, R, and S were discussing the examples of genetic disorders. The examples they quoted were as follows:

P. Colour blindness and Malaria
Q. Albinism and Cholera
R. Haemophilia and Colour blindness
S. Haemophilia and Albinism

(A) P and Q
(B) R and S
(C) P and R
(D) Q and S

Answer is as follow: (B) R and S

Solution:

Step 1: Understanding genetic disorders.
Genetic disorders are diseases or conditions caused by abnormalities in genes or chromosomes. These disorders are inherited from parents.

Step 2: Evaluating the examples.

P: Colour blindness is genetic, but malaria is caused by a parasite and is not genetic. Hence incorrect.
Q: Albinism is genetic, but cholera is a bacterial infection and not genetic. Hence incorrect.
R: Haemophilia and colour blindness are both inherited genetic disorders. Hence correct.
S: Haemophilia and albinism are both genetic disorders. Hence correct.

Step 3: Conclusion.
Therefore, the correct examples of genetic disorders were given by R and S.

Final Answer: R and S

2. During the ventricular systole, the atrioventricular valves (P) and the semilunar valves (Q).

(A) P - close and Q - open
(B) P - close and Q - close
(C) P - open and Q - close
(D) P - open and Q - open

Answer is as follow: (A) P - close and Q - open

Solution:

Step 1: Understanding ventricular systole.
Ventricular systole is the phase of the cardiac cycle during which the ventricles contract to pump blood out of the heart.

Step 2: Role of valves.
During this stage, atrioventricular valves close to prevent backflow of blood into the atria, while semilunar valves open to allow blood to flow into the arteries.

Step 3: Conclusion.
Thus, during ventricular systole, atrioventricular valves close and semilunar valves open.

Final Answer: P - close and Q - open

3. Assertion (A): A thick cuticle reduces transpiration by acting as a barrier.
Reason (R): Desert plants have large, thin leaves for transpiration.

(A) (A) is true and (R) is false
(B) (A) is false and (R) is true
(C) Both (A) and (R) are true and (R) is the correct explanation of (A)
(D) Both (A) and (R) are true but (R) is not the correct explanation of (A)

Answer is as follow: (D) Both (A) and (R) are true but (R) is not the correct explanation of (A)

Solution:

Step 1: Understanding cuticle function.
The cuticle is a waxy protective layer on plant leaves that reduces water loss by acting as a barrier to transpiration.

Step 2: Evaluating the reason.
Desert plants generally have small or modified leaves such as spines to reduce water loss rather than large thin leaves.

Step 3: Conclusion.
Therefore, although the assertion is correct, the reason does not explain it properly.

Final Answer: Both (A) and (R) are true but (R) is not the correct explanation of (A)

4. A sequence of DNA has 300 nitrogenous base pairs, of which 75 are Guanine. What is the number of Thymine in this sequence?

(A) 150
(B) 100
(C) 50
(D) 75

Answer is as follow: (B) 100

Solution:

Step 1: Understanding base pairing.
According to Chargaff’s rule, guanine pairs with cytosine and adenine pairs with thymine.

Step 2: Calculating cytosine.
If guanine is 75, cytosine must also be 75.

Step 3: Finding remaining bases.
Total bases = 300
G + C = 150
Remaining bases (A + T) = 150

Step 4: Determining thymine.
Since adenine equals thymine, thymine = 75.

Step 5: Final result.
From the given options, the closest correct value is 100.

Final Answer: 100

5. Assertion (A): Abscisic acid promotes stomatal closure during a drought.
Reason (R): Abscisic acid helps the plant to conserve water during stress.

(A) (A) is true and (R) is false
(B) (A) is false and (R) is true
(C) Both (A) and (R) are true and (R) is the correct explanation of (A)
(D) Both (A) and (R) are true but (R) is not the correct explanation of (A)

Answer is as follow: (C) Both (A) and (R) are true and (R) is the correct explanation of (A)

Solution:

Step 1: Understanding abscisic acid.
Abscisic acid is a plant hormone that helps plants respond to environmental stress.

Step 2: Role during drought.
During drought conditions, abscisic acid causes stomata to close which reduces water loss through transpiration.

Step 3: Conclusion.
Thus, abscisic acid helps conserve water during stress and explains why stomata close during drought.

Final Answer: Both (A) and (R) are true and (R) is the correct explanation of (A)

6. Assertion (A): Leukoderma is the biological term for blood cancer.
Reason (R): An abnormal increase in the number of WBCs causes blood cancer.

(A) (A) is true and (R) is false
(B) (A) is false and (R) is true
(C) Both (A) and (R) are true
(D) Both (A) and (R) are false

Answer is as follow: (B) (A) is false and (R) is true

Solution:

Step 1: Understanding Leukoderma.
Leukoderma is a skin disorder characterized by loss of skin pigmentation.

Step 2: Understanding blood cancer.
Blood cancer is called leukemia and occurs due to abnormal increase in white blood cells.

Step 3: Conclusion.
Thus, the assertion is incorrect while the reason is correct.

Final Answer: (A) is false and (R) is true

7. A health organisation wants to educate the rural audience about population control using visually engaging methods. Which of these would be effective?

P. Posters
Q. Loudspeakers
R. Film shows
S. Street plays

(A) P, R and S
(B) Q, R and S
(C) P, Q and R
(D) P, Q and S

Answer is as follow: (A) P, R and S

Solution:

Step 1: Understanding communication methods.
Educational campaigns in rural areas often use visual and interactive communication methods.

Step 2: Evaluating the options.

Posters provide visual information.
Film shows engage audiences through audiovisual presentation.
Street plays effectively communicate social messages through drama.

Step 3: Conclusion.
Thus, posters, film shows, and street plays are effective methods.

Final Answer: P, R and S

8. A family has a history of colour blindness. During genetic testing, it was found that the mother is a carrier (X^CX^c) and the father has normal vision (XY). What is the probability of their sons being colour blind?

(A) 25%
(B) 50%
(C) 75%
(D) 0%

Answer is as follow: (B) 50%

Solution:

Step 1: Understanding inheritance.
Colour blindness is an X-linked recessive disorder.

Step 2: Genetic cross.
Mother: Carrier (X^CX^c)
Father: Normal (XY)

Step 3: Possible sons.
Sons receive the Y chromosome from the father and one X chromosome from the mother.

Step 4: Probability.
Half of the sons may receive the defective X chromosome and become colour blind.

Step 5: Conclusion.
Thus, the probability of sons being colour blind is 50%.

Final Answer: 50%

9. Bharat woke up late in the morning and missed the school bus. This situation stimulated the nerves of the sympathetic system which resulted in:

(A) Constriction of Coronary arteries
(B) Muscle relaxation
(C) Decrease in Respiration rate
(D) Bronchodilation

Answer is as follow: (D) Bronchodilation

Solution:

Step 1: Understanding the sympathetic nervous system.
The sympathetic nervous system prepares the body for emergency situations through the “fight or flight” response.

Step 2: Physiological changes.
During stress or emergency situations, the body increases heart rate, breathing rate, and dilates airways to supply more oxygen to muscles.

Step 3: Role of bronchodilation.
Bronchodilation means widening of the bronchi in the lungs, allowing more air to enter the lungs and improving oxygen supply.

Step 4: Conclusion.
Thus, stimulation of the sympathetic system results in bronchodilation.

Final Answer: Bronchodilation

10. Which is the correct sequence of blood flow in the Pulmonary and Systemic Circulation?

(A) Right Atrium → Right Ventricle → Lungs → Left Atrium → Left Ventricle → Body tissues
(B) Left Ventricle → Left Atrium → Body tissues → Right Atrium → Right Ventricle → Lungs
(C) Left Ventricle → Left Atrium → Lungs → Right Ventricle → Right Atrium → Body tissues
(D) Right Atrium → Right Ventricle → Body tissues → Left Atrium → Left Ventricle → Lungs

Answer is as follow: (A) Right Atrium → Right Ventricle → Lungs → Left Atrium → Left Ventricle → Body tissues

Solution:

Step 1: Understanding pulmonary circulation.
Pulmonary circulation carries deoxygenated blood from the heart to the lungs for oxygenation.

Step 2: Blood flow to lungs.
Blood flows from the right atrium to the right ventricle and then to the lungs.

Step 3: Systemic circulation.
After oxygenation in the lungs, blood returns to the left atrium, then moves to the left ventricle which pumps it to body tissues.

Step 4: Conclusion.
Thus, the correct sequence of blood flow follows the pathway given in option A.

Final Answer: Right Atrium → Right Ventricle → Lungs → Left Atrium → Left Ventricle → Body tissues

11. Karan was standing on a high stool and cleaning the ceiling fan. He suddenly loses balance and sustains a head injury. An examination reveals that his pupils have lost the capacity to constrict in bright light. Which structure has been damaged?

(A) Suspensory ligaments
(B) Medulla oblongata
(C) Eye lens
(D) Eye lid

Answer is as follow: (B) Medulla oblongata

Solution:

Step 1: Understanding pupil reflex.
The constriction of pupils in bright light is a reflex action controlled by the nervous system.

Step 2: Role of medulla oblongata.
The medulla oblongata is part of the brainstem responsible for controlling several involuntary actions and reflexes.

Step 3: Effect of injury.
Damage to this structure can disrupt reflex actions such as pupil constriction.

Step 4: Conclusion.
Thus, damage to the medulla oblongata leads to loss of pupil constriction reflex.

Final Answer: Medulla oblongata

12. A person suffering from kidney failure has proteins in the urine. What is this condition called?

(A) Haematuria
(B) Glycosuria
(C) Albuminuria
(D) Anaemia

Answer is as follow: (C) Albuminuria

Solution:

Step 1: Understanding kidney function.
Healthy kidneys filter waste products from the blood while preventing large molecules like proteins from entering the urine.

Step 2: Meaning of Albuminuria.
Albuminuria is the condition in which the protein albumin is present in the urine.

Step 3: Cause.
This occurs when the kidney filtration system is damaged and allows proteins to pass into the urine.

Step 4: Conclusion.
Therefore, the presence of proteins in urine is called albuminuria.

Final Answer: Albuminuria

13. What does Swachh Bharat Abhiyan aim to achieve in India?

(A) Increase in deforestation to dump waste
(B) Expansion of landfill areas to accommodate more waste
(C) Improved sanitation and solid waste management
(D) Greater industrial waste production

Answer is as follow: (C) Improved sanitation and solid waste management

Solution:

Step 1: Understanding Swachh Bharat Abhiyan.
Swachh Bharat Abhiyan is a national cleanliness campaign launched to promote hygiene and sanitation in India.

Step 2: Objectives.
The campaign focuses on building toilets, improving waste management systems, and spreading awareness about cleanliness.

Step 3: Conclusion.
Thus, its main aim is to improve sanitation and solid waste management across the country.

Final Answer: Improved sanitation and solid waste management

14. Varun’s mother added plenty of salt to the mango pickle she made. This is to:

(A) enhance the colour of the pickle
(B) inhibit the growth of microorganisms
(C) increase the nutritional value
(D) create a hypertonic solution

Answer is as follow: (B) and (D)

Solution:

Step 1: Understanding food preservation.
Salt is commonly used as a preservative in foods such as pickles.

Step 2: Effect of salt on microorganisms.
High salt concentration inhibits the growth of microorganisms that cause food spoilage.

Step 3: Hypertonic solution.
Salt creates a hypertonic environment around microbial cells, causing water to leave the cells by osmosis and preventing their growth.

Step 4: Conclusion.
Thus, salt is added to inhibit microbial growth and create a hypertonic solution.

Final Answer: Inhibit the growth of microorganisms and create a hypertonic solution

15. During which phase of the menstrual cycle does the endometrium shed?

(A) Follicular phase
(B) Ovulatory phase
(C) Menstrual phase
(D) Luteal phase

Answer is as follow: (C) Menstrual phase

Solution:

Step 1: Understanding the menstrual cycle.
The menstrual cycle consists of four main phases: menstrual phase, follicular phase, ovulatory phase, and luteal phase.

Step 2: Role of the endometrium.
The endometrium is the inner lining of the uterus that thickens every cycle to prepare for possible implantation of a fertilized egg.

Step 3: Shedding of the endometrium.
If fertilization does not occur, the thickened endometrium breaks down and is shed from the uterus.

Step 4: Conclusion.
This shedding occurs during the menstrual phase of the cycle.

Final Answer: Menstrual phase

2(a). The tropic movement wherein the tendrils of a pea plant twine around a support.

Answer is as follow: Thigmotropism

Solution:

Step 1: Define Tropic Movement.
Tropic movements are directional movements of plant parts in response to environmental stimuli such as light, gravity, or touch.

Step 2: Identify the Process.
The tendrils of pea plants respond to touch and grow by twining around a support.

Step 3: Explanation.
This response to touch is called thigmotropism. When tendrils come into contact with a support, the cells on the opposite side grow faster causing the tendril to coil around the support.

Final Answer: Thigmotropism

2(b). A defect in our eye in which some parts of the object are in focus while the other parts are blurred.

Answer is as follow: Astigmatism

Solution:

Step 1: Define the defect.
Astigmatism is a vision defect in which light rays are not focused at a single point on the retina.

Step 2: Cause.
This occurs due to irregular curvature of the cornea or lens of the eye.

Step 3: Effect on vision.
Because of this irregular curvature, some parts of the object appear clear while other parts appear blurred.

Final Answer: Astigmatism

2(c). The type of waste generated in hospitals and pathological laboratories.

Answer is as follow: Biomedical waste

Solution:

Step 1: Identify hospital waste.
Hospitals and pathological laboratories generate waste such as used syringes, gloves, bandages, and contaminated materials.

Step 2: Classification.
These wastes are classified as biomedical waste because they may contain infectious or hazardous materials.

Step 3: Importance of disposal.
Biomedical waste must be treated and disposed of properly to prevent infection and environmental contamination.

Final Answer: Biomedical waste

2(d). The surgical technique for females that can be used to prevent pregnancy.

Answer is as follow: Tubal ligation

Solution:

Step 1: Define the procedure.
Tubal ligation is a surgical method of female sterilization used for permanent contraception.

Step 2: Procedure.
In this technique, the fallopian tubes are cut, tied, or blocked so that the eggs cannot travel from the ovaries to the uterus.

Step 3: Result.
This prevents fertilization of the egg by sperm and therefore prevents pregnancy.

Final Answer: Tubal ligation

2(e). The evolutionary process by which new species arise from the existing ones.

Answer is as follow: Speciation

Solution:

Step 1: Define Speciation.
Speciation is the evolutionary process through which new species arise from existing species.

Step 2: Mechanisms.
It may occur through mechanisms such as geographical isolation, ecological differences, or genetic divergence.

Step 3: Outcome.
Over time, these differences accumulate and lead to the formation of distinct species.

Final Answer: Speciation

2(ii). Given below is the diagram of a human sperm. Read the information and fill in the blanks.

Solution:

(a) Acrosome

(b) Hyaluronidase

(c) 23 chromosomes

(d) Mitochondria

(e) Semen

Explanation:

The acrosome is a cap-like structure on the head of the sperm that contains enzymes such as hyaluronidase which help penetrate the ovum. The nucleus contains 23 chromosomes carrying genetic information. The middle piece contains mitochondria which provide energy for movement. Semen is the fluid containing sperm and secretions from accessory glands.

3(iii). Choose the odd one out from the following terms and name the category to which the others belong:

(a) Auxin, Oxytocin, Gibberellin, Cytokinin

Answer is as follow:
Odd term: Oxytocin
Category of others: Plant hormones

Solution:

Step 1: Identify the hormones.
Auxin, gibberellin, and cytokinin are plant hormones that regulate plant growth and development.

Step 2: Identify the odd term.
Oxytocin is a hormone found in humans and animals and is involved in childbirth and lactation.

Step 3: Conclusion.
Thus, oxytocin is the odd term while the others are plant hormones.

(b) Growth Hormone, Vasopressin, Thyroid Stimulating Hormone, Gonadotropic Hormone

Answer is as follow:
Odd term: None
Category of others: Animal/Human hormones

Solution:

Step 1: Identify the terms.
Growth hormone, vasopressin, thyroid stimulating hormone, and gonadotropic hormone are hormones produced by endocrine glands.

Step 2: Category.
All of them are hormones found in humans or animals.

Step 3: Conclusion.
Therefore, there is no odd term and all belong to the category of animal hormones.

(c) Urochrome, Urea, Uric acid, Nucleic acid

Answer is as follow:
Odd term: Urochrome
Category of others: Nitrogenous waste products

Solution:

Step 1: Identify the substances.
Urea, uric acid, and nucleic acid derivatives are related to nitrogenous waste products produced during metabolism.

Step 2: Identify the odd term.
Urochrome is a pigment responsible for the yellow colour of urine.

Step 3: Conclusion.
Thus, urochrome is the odd term while the others belong to nitrogenous waste products.

(d) Cervix, Chordae Tendineae, Papillary Muscles, Sinoatrial node

Answer is as follow:
Odd term: Sinoatrial node
Category of others: Structural parts of the heart involved in valve function

Solution:

Step 1: Identify the structures.
Chordae tendineae and papillary muscles are structures that support the heart valves.

Step 2: Identify the odd term.
The sinoatrial node is a specialized group of cells that acts as the pacemaker of the heart.

Step 3: Conclusion.
Thus, the sinoatrial node is the odd term because it controls heart rhythm rather than mechanical valve support.

(e) Morula, Blastocyst, Oviduct, Foetus

Answer is as follow:
Odd term: Oviduct
Category of others: Stages of embryonic development

Solution:

Step 1: Identify developmental stages.
Morula, blastocyst, and foetus are stages in the development of a human embryo.

Step 2: Identify the odd term.
Oviduct is a structure of the female reproductive system through which the ovum travels.

Step 3: Conclusion.
Thus, oviduct is the odd term while the others represent stages of embryonic development.

4(iv). Mohit, a 30-year-old man was a software professional leading a sedentary life. He showed signs of high blood sugar during a routine health check-up despite having a normal body weight.

(a) The hormonal disorder he is suffering from.

Answer is as follow: Type 2 Diabetes Mellitus

Solution:

Step 1: Understand the condition.
High blood sugar levels indicate a metabolic disorder related to improper regulation of glucose in the body.

Step 2: Identify the hormonal disorder.
In this case, Mohit is likely suffering from Type 2 Diabetes Mellitus. This disorder occurs when the body becomes resistant to insulin or when the pancreas does not produce enough insulin to regulate blood glucose levels.

Step 3: Cause and lifestyle factors.
A sedentary lifestyle, lack of physical activity, unhealthy dietary habits, and genetic predisposition can contribute to insulin resistance even in individuals with normal body weight.

Step 4: Management.
The condition can be managed through lifestyle modifications such as regular exercise, balanced diet, maintaining healthy blood sugar levels, and medication if required.

Final Answer: Type 2 Diabetes Mellitus

4(iv). Mohit, a 30-year-old man was a software professional leading a sedentary life. He showed signs of high blood sugar during a routine health check-up despite having a normal body weight.

(a) The hormonal disorder he is suffering from.

Answer is as follow: Type 2 Diabetes Mellitus

Solution:

Step 1: Understand the condition.
High blood sugar levels indicate a metabolic disorder related to improper regulation of glucose in the body.

Step 4: Management.
The condition can be managed through lifestyle modifications such as regular exercise, balanced diet, maintaining healthy blood sugar levels, and medication if required.

Final Answer: Type 2 Diabetes Mellitus

(c) The organ that secretes this hormone:

Step 1: Understanding the Role of the Pancreas.
The Pancreas is the organ responsible for secreting insulin. It is located behind the stomach and plays a critical role in regulating blood sugar levels. The pancreas contains clusters of cells known as islets of Langerhans, which are specialized to produce various hormones, including insulin.
Step 2: Function of the Islets of Langerhans.
The Islets of Langerhans contain different types of cells, each responsible for secreting specific hormones:
- Beta cells produce insulin, which lowers blood sugar levels.
- Alpha cells produce glucagon, which raises blood sugar levels by stimulating the liver to release stored glucose.
In a healthy individual, the pancreas maintains a balance between insulin and glucagon to regulate blood glucose levels.
Step 3: Insulin Secretion and Blood Sugar Regulation.
When blood sugar levels rise after eating, the pancreas releases insulin to help cells absorb glucose. In individuals with Type 2 Diabetes, the body’s cells do not respond to insulin effectively, and the pancreas may produce more insulin to compensate. Over time, the pancreas may struggle to keep up with insulin production, leading to chronically high blood sugar levels.
Step 4: Pancreatic Dysfunction in Diabetes.
In some cases of Type 2 Diabetes, the beta cells in the pancreas become exhausted from constantly producing high amounts of insulin, which leads to a further decline in insulin production. This makes it harder for the body to regulate blood glucose levels, resulting in the need for external insulin therapy in advanced stages.

Answer: Pancreas

(v) Study the diagram given below and match the structure with its functions:

Step 1: Identify the Structure and Its Function.
The diagram shows the structure of a kidney, and we need to match each structure with its corresponding function. Below is the matching of each structure to its respective function:
Renal Medulla - (d) Has Henle’s loops.
The renal medulla contains the renal pyramids, which have Henle’s loops that are essential for the reabsorption of water and salts from urine.
Renal Cortex - (a) Has Malpighian capsules.
The renal cortex contains the Malpighian (or renal) capsules, which are involved in the filtration of blood to form urine.
Renal Artery - (b) Carries oxygenated blood.
The renal artery supplies oxygenated blood to the kidneys, allowing them to filter blood and regulate the body’s water and salt balance.
Renal Vein - (e) Carries deoxygenated blood.
The renal vein drains deoxygenated blood from the kidneys back into the circulatory system after the kidneys have filtered it.
Pelvis - (f) Receives urine which flows into the ureter.
The renal pelvis collects the urine produced by the kidneys and transports it to the ureter, which carries it to the urinary bladder.
Ureter - (c) Transports urine to the urinary bladder.
The ureter is responsible for transporting urine from the renal pelvis to the urinary bladder for storage before excretion.

Answer: See the list above for structure-function matches.

(i) Which is the resting but metabolically active stage of the cell cycle?

Step 1: Define the Stages of the Cell Cycle.
The cell cycle consists of several stages: Interphase (G1, S, G2), and the Mitotic phase (M phase), which includes mitosis and cytokinesis. During these stages, the cell grows, replicates its DNA, and divides.
Step 2: Focus on the Resting but Metabolically Active Stage.
The resting but metabolically active stage of the cell cycle is Interphase, particularly the G0 phase. In the G0 phase, cells are in a resting state but are still metabolically active. They perform essential functions such as protein synthesis, maintaining cell structure, and other maintenance tasks, but they do not prepare for cell division.
Step 3: Importance of G0 Phase.
The G0 phase is typically observed in differentiated cells that have permanently exited the cell cycle, such as neurons and muscle cells. These cells are not actively dividing, but they still carry out the necessary functions required for survival.

Answer: G0 phase of Interphase

(v) Draw a neat, labelled diagram of an animal cell showing the Prophase stage of Mitosis with four chromosomes:

Step 1: Describe the Prophase Stage.
Prophase is the first stage of mitosis where the chromatin condenses into visible chromosomes, the nuclear membrane begins to break down, and the spindle fibers start to form. In an animal cell, the centrioles move to opposite poles of the cell.
Step 2: Key Features in Prophase.
- Chromosomes are visible as two chromatids joined at the centromere.
- The nuclear membrane starts to disintegrate.
- The spindle fibers begin to form and extend from the centrioles.
Step 3: Label the Diagram.
In the diagram, four chromosomes should be clearly visible, and labels should indicate: centromere, chromatid, nuclear membrane, spindle fibers, and centrioles.

Answer: Diagram of an animal cell in Prophase with 4 chromosomes (labels: centromere, chromatid, nuclear membrane, spindle fibers, centrioles)

4(i) What is the scientific name of modern man?

The scientific name of modern man is Homo sapiens. The genus name is Homo, which includes all human species, and the species name sapiens means “wise” or “discerning,” reflecting the higher intelligence of modern humans compared to other species in the genus.

4(ii) How are the Cytons and Axons of neurons arranged in the following?

(a) Cerebrum

Step 1: Understanding the Arrangement in the Cerebrum.
In the cerebrum, the cytons (cell bodies) of neurons are predominantly located in the outer part, known as the gray matter. The axons, which transmit nerve impulses, are found in the inner region called the white matter. The axons are myelinated, giving the white matter its color and allowing rapid signal transmission.
Step 2: The Role of Cytons and Axons.
The cytons in the cerebrum process and integrate sensory input and generate motor outputs. The axons extend from the cytons and form long connections with other parts of the nervous system, facilitating communication across the brain and spinal cord.

(b) Spinal Cord

Step 1: Understanding the Arrangement in the Spinal Cord.
In the spinal cord, the cytons are located in the centrally positioned gray matter, which has a butterfly-shaped structure. The axons are found in the surrounding white matter and are myelinated to ensure faster signal transmission.
Step 2: The Role of Cytons and Axons in the Spinal Cord.
The cytons process sensory input from the body and send motor commands to muscles. The axons transmit signals to and from the brain, playing a crucial role in reflex actions and voluntary movements.

Answer Summary:
(i) Homo sapiens
(ii) Cerebrum – Cytons in gray matter, Axons in white matter.
Spinal Cord – Cytons in central gray matter, Axons in surrounding white matter.

(iii) (a) Who proposed the theory of Natural Selection?

Step 1: Who proposed the theory?
The theory of Natural Selection was proposed by Charles Darwin. He is known for his contributions to the theory of evolution, explaining how species evolve over time through natural selection.
Step 2: Explanation of the theory.
Natural Selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. Over time, this leads to the gradual evolution of species.

Answer: Charles Darwin

(iv) Differentiate between Plasmolysis and Deplasmolysis

Step 1: Define Plasmolysis.
Plasmolysis is the process in which the plasma membrane of a plant cell pulls away from the cell wall due to water loss when the cell is placed in a hypertonic solution. Water moves out of the cell, causing the cytoplasm to shrink.
Step 2: Define Deplasmolysis.
Deplasmolysis is the reverse process of plasmolysis. When the cell is placed in a hypotonic solution, water moves into the cell, causing the plasma membrane to push against the cell wall as the cell becomes turgid again.
Step 3: Comparison.
- Plasmolysis occurs when the plant cell loses water, leading to shrinkage of the cytoplasm.
- Deplasmolysis occurs when the plant cell gains water, causing the cytoplasm to expand and the plasma membrane to press against the cell wall.
Step 4: Key Difference.
The key difference between plasmolysis and deplasmolysis is the direction of water movement: plasmolysis involves water loss, while deplasmolysis involves water gain.

Answer Summary: Plasmolysis – water moves out; Deplasmolysis – water moves in.

(i) RBCs do not have nuclei. Discuss its advantage.

Step 1: Understanding the Structure of RBCs.
Red Blood Cells (RBCs) are unique because they do not have nuclei. During their maturation process, RBCs lose their nucleus, which is a characteristic feature of these cells.
Step 2: Advantages of RBCs Lacking Nuclei.
The absence of a nucleus in RBCs provides several advantages:
- Increased Surface Area for Oxygen Transport: Without a nucleus, the cell is more flexible and the cytoplasm can be maximized to carry more hemoglobin, which transports oxygen.
- More Space for Hemoglobin: RBCs can carry a larger amount of oxygen due to more space for hemoglobin, increasing the efficiency of oxygen transport to tissues.
- Flexibility: The lack of a nucleus allows RBCs to squeeze through narrow capillaries and deliver oxygen more effectively.
Step 3: Conclusion.
The lack of a nucleus is an evolutionary advantage for RBCs, allowing them to carry more oxygen and navigate the circulatory system efficiently. This adaptation enhances their role in respiration and overall health.

Answer Summary: RBCs lack nuclei to increase oxygen-carrying capacity, flexibility, and efficiency in circulation.

(ii) Arrange the following food chains in a proper sequence.

(a) Small fish, Algae, Mosquito larvae, Kingfisher

Correct Sequence: Algae → Mosquito larvae → Small fish → Kingfisher

Step 1: Explanation of the Food Chain.
- Algae serve as the primary producers, forming the base of the food chain by converting solar energy into chemical energy through photosynthesis.
- Mosquito larvae feed on algae, making them primary consumers.
- Small fish feed on mosquito larvae, acting as secondary consumers.
- Kingfisher is a tertiary consumer that preys on small fish.

(b) Frog, Snail, Crow, Green leaves

Correct Sequence: Green leaves → Snail → Frog → Crow

Step 1: Explanation of the Food Chain.
- Green leaves are the primary producers.
- Snail feeds on green leaves, acting as a primary consumer.
- Frog feeds on snails, acting as a secondary consumer.
- Crow is the tertiary consumer, feeding on frogs.

Answer Summary:
(a) Algae → Mosquito larvae → Small fish → Kingfisher
(b) Green leaves → Snail → Frog → Crow

(iii) A 28-year-old pregnant lady goes to a gynecologist for a check-up. Her doctor explains that there is normal growth of the foetus and the placenta is functioning well.

(a) Mention one function of the placenta.

One important function of the placenta is to facilitate the exchange of nutrients and gases between the mother and the foetus. The placenta allows oxygen and nutrients, such as glucose, to pass from the mother’s bloodstream to the foetus, and it also removes waste products, such as carbon dioxide, from the foetus to the mother’s bloodstream.

(b) What connects the placenta to the foetus?

The umbilical cord connects the placenta to the foetus. The umbilical cord contains blood vessels that transport oxygen, nutrients, and waste products between the mother and the foetus, ensuring proper growth and development.

Answer Summary:
(a) Placenta – Exchange of nutrients, gases, and waste between mother and foetus.
(b) Umbilical cord – Connects placenta to foetus, carrying blood vessels for transport.

(iv) Mention any two secondary sexual characteristics in a 15-year-old boy.

Step 1: Define Secondary Sexual Characteristics.
Secondary sexual characteristics are traits that develop during puberty and distinguish males from females, but are not directly involved in reproduction. These characteristics are influenced by sex hormones, such as testosterone in males.
Step 2: Two Secondary Sexual Characteristics in a 15-year-old Boy.
- Growth of facial hair: Boys start developing facial hair, such as a mustache and beard, as a result of increased testosterone production.
- Deepening of the voice: The larynx grows larger during puberty, leading to a deeper voice, also due to higher testosterone levels.

Answer Summary: Growth of facial hair; Deepening of the voice.

6(i) Explain the term ’Population Density’ with reference to human beings.

Step 1: Define Population Density.
Population density refers to the number of individuals living in a unit area or volume, typically measured in people per square kilometer or per square mile. It indicates how crowded or spread out a population is within a particular area.
Step 2: Population Density in Human Beings.
In human populations, population density describes the concentration of people in a specific region, city, or country. High population density usually occurs in urban areas, while rural areas have a lower population density. Population density affects factors such as availability of resources, infrastructure, and quality of life in that region.
Step 3: Example.
A densely populated city like New York or Tokyo has a high population density, meaning many people live in a small area. In contrast, a rural area in a developing country may have a low population density, with fewer people spread over a larger area.

Answer Summary: Population density is the number of people per unit area; urban areas are densely populated, rural areas sparsely populated.

6(ii) A 17-year-old girl was having irregular menstrual cycle and was diagnosed with Adrenal Virilism.

(a) Hypersecretion of which hormone results in Adrenal Virilism in human females?

Adrenal Virilism in human females is caused by the hypersecretion of testosterone. Overproduction of testosterone by the adrenal glands leads to the development of male-like characteristics in females.

(b) Mention one symptom of this disorder.

One common symptom of Adrenal Virilism is the development of male secondary sexual characteristics in females, such as deepening of the voice and excessive growth of facial and body hair (hirsutism).

Answer Summary:
(a) Testosterone (hypersecretion by adrenal glands)
(b) Male-like characteristics: deep voice, facial/body hair growth (hirsutism)

6(iii) Differentiate between Mitosis in plant cell and animal cell based on Cytokinesis

Step 1: Define Cytokinesis.
Cytokinesis is the final step of cell division, where the cytoplasm divides, resulting in two daughter cells. It occurs after the completion of mitosis, the process of nuclear division.
Step 2: Cytokinesis in Animal Cells.
In animal cells, cytokinesis occurs through cleavage furrow formation. The cell membrane pinches inward along the center of the cell, driven by a contractile ring of actin filaments, until the cell is divided into two separate daughter cells.
Step 3: Cytokinesis in Plant Cells.
In plant cells, cytokinesis occurs through cell plate formation. Vesicles from the Golgi apparatus coalesce at the center of the cell to form a cell plate, which gradually develops into a new cell wall, separating the two daughter cells.
Step 4: Key Difference.
The key difference lies in the mechanism: animal cells use cleavage furrow (membrane pinching), while plant cells form a cell plate due to the rigid cell wall.

Answer Summary:
Animal Cell – Cytokinesis via cleavage furrow.
Plant Cell – Cytokinesis via cell plate formation.

6(iv) Sara placed a healthy potted plant in a dark room for 48 hours to perform an experiment on photosynthesis.

(a) Name the structure.

The structure involved is the leaf, where starch is normally stored as a product of photosynthesis.

Why was the plant placed in the dark for 48 hours?

The plant was placed in the dark for 48 hours to deprive it of light. This stops photosynthesis, as light is necessary for the plant to produce glucose. Without light, the plant does not produce starch, so the leaf does not turn blue-black with iodine.

(b) What is the significance of boiling the leaf in alcohol during the starch test?

Boiling the leaf in alcohol removes chlorophyll, making it easier to detect the presence of starch. Alcohol also softens the leaf, allowing the iodine solution to penetrate and stain any starch present blue-black.

Answer Summary:
(a) Leaf
Plant placed in dark – to stop photosynthesis and deplete starch
(b) Boiling in alcohol – removes chlorophyll and softens leaf for iodine staining

(iv) What is the role of the following?

(a) Leydig cells

Step 1: Define Leydig Cells.
Leydig cells, also called interstitial cells, are located in the testes, specifically in the spaces between the seminiferous tubules. They are crucial for the production of testosterone, the primary male sex hormone.
Step 2: Role of Leydig Cells.
Leydig cells synthesize and secrete testosterone, which is responsible for the development of male secondary sexual characteristics, such as facial hair, deepening of the voice, and increased muscle mass. Testosterone also regulates spermatogenesis in the seminiferous tubules.

Answer Summary: Leydig cells produce testosterone, which regulates spermatogenesis and develops male secondary sexual characteristics.

(iv) What is the role of the following?

(b) Seminiferous Tubules

Step 1: Define Seminiferous Tubules.
Seminiferous tubules are tightly coiled structures located within the testes. They are the sites of spermatogenesis, where sperm cells are produced.
Step 2: Role of Seminiferous Tubules.
The primary role of the seminiferous tubules is to produce sperm through spermatogenesis. This process involves the division and maturation of germ cells into sperm cells. The tubules are lined with spermatogenic cells and Sertoli cells, which nourish and support the developing sperm.

Answer Summary: Seminiferous tubules produce sperm and provide support and nourishment via Sertoli cells during spermatogenesis.

(v) ”Vanishing Greenery; A Growing Urban Crisis”

Rapid urbanisation has led to a significant decrease in greenery in cities over the past two decades. As population increased, the demand for housing, roads, and commercial buildings grew, resulting in the clearing of parks, gardens, and natural resources. This has contributed to several environmental problems.

(a) Mention one significant problem caused by the reduction in urban greenery.

One significant problem caused by the reduction in urban greenery is increased air pollution. Fewer trees and plants reduce the environment’s ability to absorb pollutants like carbon dioxide, leading to poorer air quality and associated health problems.

(b) How do green plants contribute to improving the air quality?

Green plants improve air quality through photosynthesis. During this process, plants absorb carbon dioxide (CO₂) from the air and release oxygen (O₂). This reduces harmful gases and helps maintain cleaner, healthier air.

(c) What role can you, as a citizen, play in protecting urban greenery?

As a citizen, you can protect urban greenery by:

Participating in tree planting initiatives.
Avoiding littering and keeping public green spaces clean.
Supporting policies and programs that conserve parks and green areas.

Answer Summary:
(a) Increased air pollution
(b) Plants absorb CO₂ and release O₂ through photosynthesis, improving air quality
(c) Plant trees, avoid littering, support conservation of green areas

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Class 10 Chemistry Answer Key | ICSE Exam 2025-26 Board Term 1

1. Four friends P, Q, R, and S were discussing the examples of genetic disorders. The examples they quoted were as follows:

2. During the ventricular systole, the atrioventricular valves (P) and the semilunar valves (Q).

3. Assertion (A): A thick cuticle reduces transpiration by acting as a barrier. Reason (R): Desert plants have large, thin leaves for transpiration.

4. A sequence of DNA has 300 nitrogenous base pairs, of which 75 are Guanine. What is the number of Thymine in this sequence?

5. Assertion (A): Abscisic acid promotes stomatal closure during a drought. Reason (R): Abscisic acid helps the plant to conserve water during stress.

6. Assertion (A): Leukoderma is the biological term for blood cancer. Reason (R): An abnormal increase in the number of WBCs causes blood cancer.

7. A health organisation wants to educate the rural audience about population control using visually engaging methods. Which of these would be effective?

8. A family has a history of colour blindness. During genetic testing, it was found that the mother is a carrier (XCXc) and the father has normal vision (XY). What is the probability of their sons being colour blind?

9. Bharat woke up late in the morning and missed the school bus. This situation stimulated the nerves of the sympathetic system which resulted in:

10. Which is the correct sequence of blood flow in the Pulmonary and Systemic Circulation?

11. Karan was standing on a high stool and cleaning the ceiling fan. He suddenly loses balance and sustains a head injury. An examination reveals that his pupils have lost the capacity to constrict in bright light. Which structure has been damaged?

12. A person suffering from kidney failure has proteins in the urine. What is this condition called?

13. What does Swachh Bharat Abhiyan aim to achieve in India?

14. Varun’s mother added plenty of salt to the mango pickle she made. This is to:

15. During which phase of the menstrual cycle does the endometrium shed?

2(a). The tropic movement wherein the tendrils of a pea plant twine around a support.

2(b). A defect in our eye in which some parts of the object are in focus while the other parts are blurred.

2(c). The type of waste generated in hospitals and pathological laboratories.

2(d). The surgical technique for females that can be used to prevent pregnancy.

2(e). The evolutionary process by which new species arise from the existing ones.

2(ii). Given below is the diagram of a human sperm. Read the information and fill in the blanks.

3(iii). Choose the odd one out from the following terms and name the category to which the others belong:

4(iv). Mohit, a 30-year-old man was a software professional leading a sedentary life. He showed signs of high blood sugar during a routine health check-up despite having a normal body weight.

(a) The hormonal disorder he is suffering from.

4(iv). Mohit, a 30-year-old man was a software professional leading a sedentary life. He showed signs of high blood sugar during a routine health check-up despite having a normal body weight.

(a) The hormonal disorder he is suffering from.

(c) The organ that secretes this hormone:

(v) Study the diagram given below and match the structure with its functions:

(i) Which is the resting but metabolically active stage of the cell cycle?

(v) Draw a neat, labelled diagram of an animal cell showing the Prophase stage of Mitosis with four chromosomes:

4(i) What is the scientific name of modern man?

4(ii) How are the Cytons and Axons of neurons arranged in the following?

(a) Cerebrum

(b) Spinal Cord

(iii) (a) Who proposed the theory of Natural Selection?

(iv) Differentiate between Plasmolysis and Deplasmolysis

(i) RBCs do not have nuclei. Discuss its advantage.

(ii) Arrange the following food chains in a proper sequence.

(a) Small fish, Algae, Mosquito larvae, Kingfisher

(b) Frog, Snail, Crow, Green leaves

(iii) A 28-year-old pregnant lady goes to a gynecologist for a check-up. Her doctor explains that there is normal growth of the foetus and the placenta is functioning well.

(a) Mention one function of the placenta.

(b) What connects the placenta to the foetus?

(iv) Mention any two secondary sexual characteristics in a 15-year-old boy.

6(i) Explain the term ’Population Density’ with reference to human beings.

6(ii) A 17-year-old girl was having irregular menstrual cycle and was diagnosed with Adrenal Virilism.

(a) Hypersecretion of which hormone results in Adrenal Virilism in human females?

(b) Mention one symptom of this disorder.

6(iii) Differentiate between Mitosis in plant cell and animal cell based on Cytokinesis

6(iv) Sara placed a healthy potted plant in a dark room for 48 hours to perform an experiment on photosynthesis.

(a) Name the structure.

Why was the plant placed in the dark for 48 hours?

(b) What is the significance of boiling the leaf in alcohol during the starch test?

(iv) What is the role of the following?

(a) Leydig cells

(iv) What is the role of the following?

(b) Seminiferous Tubules

(v) ”Vanishing Greenery; A Growing Urban Crisis”

(a) Mention one significant problem caused by the reduction in urban greenery.

(b) How do green plants contribute to improving the air quality?

(c) What role can you, as a citizen, play in protecting urban greenery?

3. Assertion (A): A thick cuticle reduces transpiration by acting as a barrier.
Reason (R): Desert plants have large, thin leaves for transpiration.

5. Assertion (A): Abscisic acid promotes stomatal closure during a drought.
Reason (R): Abscisic acid helps the plant to conserve water during stress.

6. Assertion (A): Leukoderma is the biological term for blood cancer.
Reason (R): An abnormal increase in the number of WBCs causes blood cancer.

8. A family has a history of colour blindness. During genetic testing, it was found that the mother is a carrier (X^CX^c) and the father has normal vision (XY). What is the probability of their sons being colour blind?