Directions (Q.17 to Q. 20): In the following questions, a statement of assertion (A) is followed by of reason (R). Mark the correct choice as :
(a) If, Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of (A)
(b) If, Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of
(c) If, Assertion (A) is true but reason (R) is false.
(d) If, Assertion (A) is false but reason (R) is true.
17. Assertion (A): Sodium metal is stored in Kerosene.
Reason (R): Metallic sodium
melts when exposed to air.
18. Assertion (A): Nerve impulse is a one way conduction.
Reason (R): Nerve impulse is transmitted from dendrite to axon terminals.
19. Assertion (A): Double fertilisation is unique to angiosperms.
Reason (R): Two types of fusion takes place in double fertilisation.
20. Assertion (A): Convex mirror is used in the headlights of a car.
Reason (R): Field of view of convex mirror is larger and it forms virtual, erect and dimin
of the objects behind.
Answers
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17. Assertion (A): Sodium metal is stored in Kerosene.
Reason (R): Metallic sodium
melts when exposed to air.
c. Assertion is true but reason is false.
• Sodium is a soft metal and is highly reactive. To avoid it reacting with moisture and oxygen in the air it is stored in kerosene oil.
• But sodium metal does not melt in air.
• If there is moisture content in air sodium reacts and forms sodium hydroxide.
• If sodium comes in contact with dry air it burns.
18. Assertion (A): Nerve impulse is a one way conduction.
Reason (R): Nerve impulse is transmitted from dendrite to axon terminals.
b. Both assertion and reason are true but reason is not the correct explanation for assertion.
• Nerve impulses travel in only one direction.
• Neurons conduct nerve impulses.
• Dendrites are present at the head of the neuron. They receive the impulse from the previous neuron.
• In the neuron the impulse travels from dendrites to axon. The axon conducts the nerve impulse.
• From the axon the impulse is given to the dendrite of the next neuron.
• The reason for the one direction of nerve impulse is the synaptic conduct. Synapses are the junctions between two neurons through which the impulses are exchanged.
19. Assertion (A): Double fertilisation is unique to angiosperms.
Reason (R): Two types of fusion takes place in double fertilization.
a. Both assertion and reason are true and reason the correct explanation for assertion.
• Double fertilization is the process that occurs only in angiosperms.
• Double fertilization involves two types of fusion. Fusion between one male gamete and the ovule to form the zygote and fusion between another male gamete and the polar nuclei to form the endosperm.
• The ovary is absent in gymnosperms. So, while the male gamete fuses with the female gamete to form the zygote there is no second fusion.
20. Assertion (A): Convex mirror is used in the headlights of a car.
Reason (R): Field of view of convex mirror is larger and it forms virtual, erect and diminished of the objects behind.
d. Assertion is false but reason is true.
• Concave mirrors are used the headlights of car. This is because with concave mirrors the bulb is placed in the focus of the mirror and this allows the light to extent infinitely.
• Convex mirrors have a larger field of view compared to plain mirrors. The images in convex mirrors are virtual, erect and diminished in size. The diminished size of images increases the number of images that can fit into the mirror resulting in a greater field of view.