Paper 1

CBSE Science Question Paper (Set 31/2/1) 

1. The correct balanced chemical equation showing exothermic reaction in which natural gas burns in air is:  

   (A) CH₄ + O₂ → CO₂ + 2H₂O  

   (B) CH₄ + 2O₂ → 2CO₂ + 2H₂O + Energy  

   (C) CH₄ + 2O₂ → CO₂ + 2H₂O  

   (D) CH₄ + 2O₂ → CO₂ + 2H₂O + Energy  

   **Answer: (D)**  

   Natural gas (CH₄) burns to form CO₂ and 2H₂O. The equation must be balanced, show it as exothermic (+ Energy), and have correct stoichiometry (1 CO₂, not 2).

2. The warning sign shown in the given figure must invariably be displayed/pasted on the containers which contain hydroxide of:  

   (A) Aluminium  

   (B) Calcium  

   (C) Sodium  

   (D) Magnesium  

   **Answer: (C)**  

   Sodium hydroxide (NaOH) is highly corrosive and requires a corrosive warning sign. Others are less aggressive or not typically labeled as such.

3. The body of human beings works within the pH range of:  

   (A) 6.1 to 6.8  

   (B) 6.5 to 7.3  

   (C) 7.0 to 7.8  

   (D) 7.5 to 8.1  

   **Answer: (C)**  

   Human body fluids (e.g., blood) maintain a pH around 7.0–7.8 for optimal functioning.

4. Aluminium powder is used in thermit welding because:  

   (A) Its reaction with iron is highly exothermic.  

   (B) When it is heated with iron (III) oxide, molten iron is obtained.  

   (C) When it is heated with iron (III) oxide, molten aluminium oxide is obtained to join railway tracks.  

   (D) Its melting point is low as compared to iron and a molten alloy of iron and aluminium is formed on heating which is used to join railway tracks.  

   **Answer: (B)**  

   Thermit reaction: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe (molten Fe joins tracks). It's with Fe₂O₃, not Fe directly.

5. Two metals zinc and tin are dissolved separately in definite proportions in molten copper (the primary metal) to obtain two different alloys respectively known as:  

   (A) Bronze and Brass  

   (B) Brass and Solder  

   (C) Brass and Bronze  

   (D) Solder and Bronze  

   **Answer: (C)**  

   Brass = Cu + Zn; Bronze = Cu + Sn (tin). "Respectively" matches zinc → Brass, tin → Bronze.

6. The opening and closing of stomata is regulated by:  

   (A) CO₂ concentration in stomata  

   (B) Temperature in guard cells  

   (C) O₂ concentration in stomata  

   (D) Amount of water in guard cells  

   **Answer: (D)**  

   Guard cells open/close stomata based on turgor pressure from water intake.

7. One-cell thick blood vessels are known as:  

   (A) Alveoli  

   (B) Capillaries  

   (C) Arteries  

   (D) Veins  

   **Answer: (B)**  

   Capillaries are single-cell thick for gas/nutrient exchange.

8. Observe the given figures A and B. When chhui-mui (sensitive) plant is touched, its leaves fold. This is due to:  

   (A) Hormonal effect  

   (B) Thermal effect  

   (C) Change in amount of water in cells  

   (D) Electromagnetic effect  

   **Answer: (C)**  

   Thigmonasty in Mimosa pudica (touch-me-not) is due to loss of turgor (water) in pulvini cells.

9. Bryophyllum produces new plant through:  

   (A) Apical buds formed on the tip of the plant  

   (B) Vegetative buds produced in the notches of the leaf  

   (C) Flowers produced in the notches of the branches  

   (D) Fruits formed on the branches of the plant  

   **Answer: (B)**  

   Bryophyllum reproduces vegetatively via adventitious buds on leaf margins/notches.

10. The number of chromosomes in a cell division is halved. This kind of cell division is observed in:  

    (A) Only testis  

    (B) Only ovary  

    (C) Ovary and testis both  

    (D) All cells of the body  

    **Answer: (C)**  

    Meiosis (reduction division) occurs in gonads (ovary and testis) to form gametes.

11. If the absolute refractive indices of two media X and Y are 6/5 and 4/3 respectively, then the refractive index of Y with respect to X will be:  

    (A) 10/9  

    (B) 9/10  

    (C) 9/8  

    (D) 8/9  

    **Answer: (A)**  

    n_{YX} = n_Y / n_X = (4/3) / (6/5) = (4/3) × (5/6) = 20/18 = 10/9.

12. An object is placed at a distance of 30 cm from the pole of a concave mirror. If its real and inverted image is formed at 60 cm in front of the mirror, the focal length of the mirror is:  

    (A) -15 cm  

    (B) -20 cm  

    (C) +20 cm  

    (D) +15 cm  

    **Answer: (B)**  

    Using mirror formula: 1/f = 1/v + 1/u (u = -30 cm, v = -60 cm for concave). 1/f = -1/60 - 1/30 = -1/20; f = -20 cm.

13. In the given figure the angle of incidence and the angle of deviation respectively are:  

    (A) 1 and 5  

    (B) 7 and 6  

    (C) 7 and 4  

    (D) 1 and 6  

    **Answer: (D)**  

    In standard prism diagrams, angle 1 is incidence (i), angle 6 is deviation (d).

14. An electric bulb is connected to a power supply of 220 V. If the current drawn by the bulb from the supply is 500 mA, the power of the bulb is:  

    (A) 11 W  

    (B) 110 W  

    (C) 220 W  

    (D) 1100 W  

    **Answer: (B)**  

    P = V × I = 220 × 0.5 = 110 W.

15. Four identical resistors of 12 Ω each are connected in series to form a square ABCD as shown in the figure. The resistance of the network between the two points 1 and 2 is:  

    (A) 48 Ω  

    (B) 36 Ω  

    (C) 9 Ω  

    (D) 6 Ω  

    **Answer: (C)**  

    Assuming points 1 and 2 are adjacent vertices: Direct path = 12 Ω; Other path = 36 Ω (3 × 12). Equivalent = (12 × 36)/48 = 9 Ω.

16. Identify from the following a group containing all non-biodegradable substances.  

    (A) Leather, Glass, Plastic  

    (B) Cotton, Wood, Nylon  

    (C) DDT, Polyester, Glass  

    (D) Leather, Silk, Wool  

    **Answer: (C)**  

    DDT (pesticide), Polyester (synthetic fiber), Glass (inorganic) are all non-biodegradable.

17. Assertion (A): The metals high up in the reactivity series cannot be obtained from their compounds by heating with carbon.  

    Reason (R): Displacement reactions can also be used to obtain metal.  

    **Answer: (B)**  

    Both true, but R does not explain A (highly reactive metals need electrolysis, not carbon reduction).

18. Assertion (A): A mango seed will germinate to form a mango tree.  

    Reason (R): Heredity determines the process by which traits and characteristics are reliably inherited from parents to offspring.  

    **Answer: (A)**  

    Both true, and R explains A (germination follows inherited traits).

19. Assertion (A): Nichrome is an alloy which is commonly used in electrical heating devices such as electric irons, toasters, etc.  

    Reason (R): The resistivity of nichrome is high and its resistance decreases with increase in temperature.  

    **Answer: (C)**  

    A true (high resistivity, stable), but R false (resistance increases with temperature for nichrome, though minimally).

20. Assertion (A): Animals will not get energy if they eat (consume) coal as food.  

    Reason (R): Specific enzymes are needed for the breakdown of a particular food.  

    **Answer: (A)**  

    Both true, and R explains A (animals lack enzymes to digest coal).