CBSE · Class 9 Science · One-mark questions

Assertion and Reason, Class 9 Science — 20 questions with explanations

An assertion–reason question gives you two statements and asks whether each is true and whether the second explains the first. Below are twenty CBSE Class 9 Science questions with full explanations — deliberately five of each answer type, so you meet the tricky (b) case as often as the easy (a) one.

Questions
20
Each worth
1 mark
Balance
5 of each option
Answers
All explained

The three checks, in order

  1. 1 Is A true? Decide this with R covered up. R has no vote here.
  2. 2 Is R true? Again on its own — a true statement in the wrong place is still true.
  3. 3 Only if both are true: does R explain A, or is it just another correct fact standing nearby? This third check is the whole difficulty of the question type.

Standard options: (a) both true, R explains A · (b) both true, R does not explain A · (c) A true, R false · (d) A false, R true. Read them on the paper each time — boards sometimes swap (c) and (d).

Twenty questions, with the reasoning spelled out

Cover the answers, work through all twenty, then check. If you get most of the (a)s right and trip on the (b)s, that is the normal pattern — and exactly what this set is built to fix.

Q1 Assertion & Reason 1 mark · Ch 1: Exploration: Entering the World of Secondary Science

Assertion (A): The effectiveness of a surgical mask in filtering viruses depends on concepts from physics (particle motion), chemistry (polymer properties), and biology (virus size). Reason (R): Real-world problems often require an integrated approach combining multiple branches of science, as natural phenomena do not adhere to artificial academic boundaries.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (a)

(a) Both A and R are true and R is the correct explanation of A. Explanation: The mask's function is a complex phenomenon involving filtration (physics), material structure (chemistry), and biological agents. The reason correctly explains that such complexity necessitates an interdisciplinary approach, as science divisions are human constructs for organization.

Q2 Assertion & Reason 1 mark · Ch 1: Exploration: Entering the World of Secondary Science

Assertion (A): Asking about measurable data and past patterns makes a prediction scientifically testable. Reason (R): Questions with simple yes/no answers are usually not useful for scientific testing.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (b)

(b) Both A and R are true but R is NOT the correct explanation of A. Explanation: Both statements are true. While yes/no questions are less useful, the reason a prediction is testable is specifically because it relies on measurable data (Assertion A), not merely because it avoids yes/no formats.

Q3 Assertion & Reason 1 mark · Ch 1: Exploration: Entering the World of Secondary Science

Assertion (A): Disproving the viral claim that 'food becomes harmful during an eclipse' requires asking scientific questions about physical, chemical, or biological mechanisms. Reason (R): Scientific disproof relies on emotional appeals and anecdotal evidence to counter popular beliefs, rather than looking for measurable data.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (c)

(c) A is true but R is false. Explanation: The assertion is correct; scientific inquiry seeks mechanistic explanations. The reason is false because science relies on evidence, data, and logic, not emotional appeals or anecdotes. Disproving a claim requires demonstrating the lack of a causal mechanism (e.g., no temperature change significant enough to spoil food).

Q4 Assertion & Reason 1 mark · Ch 1: Exploration: Entering the World of Secondary Science

Assertion (A): When scientific predictions do not match observations, it is considered a weakness of the scientific method. Reason (R): Science is reliable because it is open to being corrected by nature, allowing scientists to re-examine assumptions and improve theories based on evidence.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (d)

(d) A is false but R is true. Explanation: The assertion is false because the text states that such failures are the 'greatest strength' of science, not a weakness. The reason is true as it correctly describes the self-correcting nature of science based on evidence.

Q5 Assertion & Reason 1 mark · Ch 2: Cell: the Building Block of Life

Assertion (A): When a Rhoeo leaf peel is placed in a 20% sugar solution, the space between the cell membrane and cell wall increases. Reason (R): Water moves out of the cell into the surrounding hypertonic solution, causing the protoplasm to shrink.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (a)

(a) Both A and R are true and R is the correct explanation of A. Explanation: The 20% sugar solution is hypertonic relative to the cell sap. Water leaves the cell via osmosis, causing the protoplasm to shrink away from the rigid cell wall.

Q6 Assertion & Reason 1 mark · Ch 2: Cell: the Building Block of Life

Assertion (A): Smooth Endoplasmic Reticulum (SER) does not have ribosomes on its surface. Reason (R): SER is involved in the synthesis and storage of fats (lipids) and hormones.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (b)

(b) Both A and R are true but R is NOT the correct explanation of A. Explanation: While both statements are true, the absence of ribosomes defines the 'smooth' structure, and lipid synthesis is its function; the function does not explain the structural absence of ribosomes directly.

Q7 Assertion & Reason 1 mark · Ch 2: Cell: the Building Block of Life

Assertion (A): Liver cells of a person who consumes alcohol regularly exhibit a significantly higher volume of Smooth Endoplasmic Reticulum (SER) compared to a non-drinker. Reason (R): SER is primarily involved in the synthesis of proteins and their transport to the Golgi apparatus.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (c)

(c) A is true but R is false. Explanation: While A is true because SER detoxifies poisons and drugs (like alcohol), R is false because SER synthesizes lipids and detoxifies; Rough ER (RER) is involved in protein synthesis.

Q8 Assertion & Reason 1 mark · Ch 3: Tissues in Action

Assertion (A): Platelets play a crucial role in stopping bleeding at the site of an injury. Reason (R): Platelets release fibrinogen, which is converted into fibrin threads that form a mesh to trap blood cells and form a clot.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (a)

(a) Both A and R are true and R is the correct explanation of A. Explanation: Upon injury, platelets aggregate and release factors that convert soluble fibrinogen into insoluble fibrin, creating a net that stabilizes the platelet plug and stops hemorrhage.

Q9 Assertion & Reason 1 mark · Ch 3: Tissues in Action

Assertion (A): The adult human skeleton constitutes approximately 12-15% of total body weight. Reason (R): Muscle mass constitutes a larger percentage of body weight, varying significantly between males and females, compared to bone mass.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (b)

(b) Both A and R are true but R is NOT the correct explanation of A. Explanation: Both statements are factually correct based on average physiological data. However, the reason why the skeleton is 12-15% is due to the density and volume of bone tissue, not directly because muscle mass is higher. R provides a comparison but does not explain the specific percentage value in A.

Q10 Assertion & Reason 1 mark · Ch 3: Tissues in Action

Assertion (A): The outer layer of human skin is composed of stratified squamous epithelium. Reason (R): This tissue type is designed for rapid diffusion of gases and nutrients between blood and tissues.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (c)

(c) A is true but R is false. Explanation: The skin's outer layer is indeed stratified squamous epithelium (A is true). However, Reason (R) is false because this tissue is designed for protection against abrasion and microbial entry, not diffusion. Simple squamous epithelium is designed for diffusion.

Q11 Assertion & Reason 1 mark · Ch 4: Describing Motion Around Us

Assertion (A): If a velocity-time graph intersects the velocity axis at a positive value, the object had an initial velocity greater than zero. Reason (R): The point where the graph intersects the velocity axis corresponds to time t = 0.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (a)

(a) Both A and R are true and R is the correct explanation of A. Explanation: The y-intercept (velocity axis) is the value of velocity when time is zero. A positive intersection means u > 0. The reason correctly identifies the physical meaning of the intercept.

Q12 Assertion & Reason 1 mark · Ch 4: Describing Motion Around Us

Assertion (A): The area under a velocity-time graph represents the displacement of the object. Reason (R): Multiplying velocity by time gives distance, which is always equal to displacement.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (c)

(c) A is true but R is false. Explanation: Area under v-t graph is indeed displacement. However, distance is not always equal to displacement; they are equal only in unidirectional motion without change in direction.

Q13 Assertion & Reason 1 mark · Ch 4: Describing Motion Around Us

Assertion (A): The area under a velocity-time graph gives the acceleration of the object. Reason (R): Acceleration is the rate of change of velocity with respect to time.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (d)

(d) A is false but R is true. Explanation: The area under a velocity-time graph gives displacement, not acceleration. The slope of the velocity-time graph gives acceleration. The reason correctly defines acceleration but does not support the false assertion.

Q14 Assertion & Reason 1 mark · Ch 5: Exploring Mixtures and Their Separation

Assertion (A): Crystallization is preferred over simple evaporation for purifying solids like sugar or copper sulfate. Reason (R): During crystallization, soluble impurities remain in the mother liquor and do not get trapped in the forming crystals.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (a)

(a) Both A and R are true and R is the correct explanation of A. Explanation: Crystallization yields purer solids because impurities stay dissolved in the remaining solution (mother liquor), whereas evaporation may leave insoluble impurities mixed with the solid.

Q15 Assertion & Reason 1 mark · Ch 5: Exploring Mixtures and Their Separation

Assertion (A): The path of a laser beam becomes visible when passed through a beaker containing milk diluted in water. Reason (R): Milk is a true solution, and the particles in a solution are large enough to scatter light.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (c)

(c) A is true but R is false. Explanation: The Tyndall effect is observed in milk because it is a colloid, not a true solution. Colloidal particles (1-1000 nm) scatter light, whereas particles in a true solution are too small (<1 nm) to scatter light.

Q16 Assertion & Reason 1 mark · Ch 5: Exploring Mixtures and Their Separation

Assertion (A): If 10 mL of ethanol is mixed with 90 mL of water, the resulting solution is 10% v/v ethanol. Reason (R): Volume by volume percentage is calculated based on the total volume of the solution being 100 mL.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (d)

(d) A is false but R is true. Explanation: The reason is true in definition. However, the assertion is false because volumes are not always additive; mixing 10 mL ethanol and 90 mL water may result in a total volume slightly less than 100 mL due to intermolecular forces. Thus, it is not exactly 10% v/v without knowing the final volume.

Q17 Assertion & Reason 1 mark · Ch 6: How Forces Affect Motion

Assertion (A): In a collision between two objects, the total momentum before the collision is equal to the total momentum after the collision. Reason (R): Newton's second law relates force to the rate of change of momentum.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (b)

(b) Both A and R are true but R is NOT the correct explanation of A. Explanation: Assertion A states the law of conservation of momentum, which is true for isolated systems. Reason R states Newton's second law, which is also true. However, conservation of momentum is derived from Newton's third law (action-reaction pairs cancel out internal forces), not directly from the definition of force in the second law.

Q18 Assertion & Reason 1 mark · Ch 6: How Forces Affect Motion

Assertion (A): A book lying on a table is at rest because the forces acting on it are unbalanced. Reason (R): The gravitational force pulling the book down is balanced by the normal force pushing the book up from the table.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (d)

(d) A is false but R is true. Explanation: The book is at rest because the forces ARE balanced, not unbalanced. The weight (gravity) and the normal force are equal in magnitude and opposite in direction, resulting in zero net force.

Q19 Assertion & Reason 1 mark · Ch 7: Work, Energy, and Simple Machines

Assertion (A): One kilowatt-hour (kWh) is equal to 3.6 million joules (3.6 x 10^6 J). Reason (R): Energy is the capacity to do work, and its SI unit is the joule.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (b)

(b) Both A and R are true but R is NOT the correct explanation of A. Explanation: The assertion is a correct conversion (1 kWh = 1000 W * 3600 s = 3,600,000 J). The reason is also true, defining energy and its SI unit. However, the definition of energy and its SI unit does not explain the specific numerical conversion factor between kWh and Joules. The conversion depends on the definition of power (Watt) and time (hour).

Q20 Assertion & Reason 1 mark · Ch 7: Work, Energy, and Simple Machines

Assertion (A): Two objects with different masses but the same momentum will have the same kinetic energy. Reason (R): Kinetic energy is given by K.E. = p² / 2m, where p is momentum and m is mass.

  1. (a) Both A and R are true and R is the correct explanation of A.
  2. (b) Both A and R are true but R is NOT the correct explanation of A.
  3. (c) A is true but R is false.
  4. (d) A is false but R is true.
Show answer

Correct option: (d)

(d) A is false but R is true. Explanation: Reason (R) correctly states the formula K.E. = p²/2m. If momentum p is constant, K.E. is inversely proportional to mass m. Therefore, objects with different masses will have different kinetic energies. The lighter object will have more K.E. Thus, Assertion (A) is false.

Assertion and reason — common questions

What is an assertion and reason question?
A question giving two statements — an Assertion (A) and a Reason (R) — where you decide whether each is true and, if both are, whether R actually explains A. It is worth one mark, and it is the one-mark question students most often lose.
What are the four options in an assertion-reason question?
(a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true. Some papers reorder (c) and (d) — read the options each time rather than answering from memory.
How do I answer assertion-reason questions correctly?
In three separate steps. Decide whether A is true, ignoring R entirely. Then decide whether R is true, ignoring A. Only if both are true do you ask the third question: does R explain A, or is it merely another true fact? Most wrong answers come from collapsing these three checks into one impression.
Why is option (b) the one students miss?
Because both statements read as correct, so the instinct is to pick (a). Option (b) is the trap where R is a true statement that does not cause A. Practising it is the only reliable fix — several questions below are exactly this case.
How many assertion-reason questions come in the exam?
CBSE builds about 20% of the Class 9 and Class 10 papers from objective questions, and assertion–reason is part of that block. Expect a small number of one-mark questions in this format in every science paper.

Exam rules and marks on this page were last checked on 23 August 2026. Boards do change them. Always confirm dates and the final syllabus against cbseacademic.nic.in.

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