Assertion and Reason Questions for Class 11 Physics Chapter 10 Mechanical Properties of Fluids

Assertion and Reason Questions for Class 11 Physics Chapter 10 Mechanical Properties of Fluids

Directions:
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.

Q.1. Assertion : It is easier to spray water in which some soap is dissolved.
Reason : Soap is easier to spread.

Answer

Answer: (c) When a liquid is sprayed, the surface area of the liquid increases. Therefore, work has to be done in spraying the liquid, which is directly proportional to the surface tension.
Because on adding soap, surface tension of water decreases, the spraying of water becomes easy.

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Q.2. Assertion : The angle of contact of a liquid decrease with increase in temperature.
Reason : With increase in temperature, the surface tension of liquid increase.

Answer

Answer: (c) With increase in temperature surface tension of the liquid decreases and angle of contact also decreases.

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Q.3. Assertion : When height of a tube is less than liquid rise in the capillary tube, the liquid does not overflow.
Reason : Product of radius of meniscus and height of liquid in capillary tube always remains constant.

Answer

Answer: (a) As h decreases, R increases and the liquid meniscus becomes more and more flat, but the liquid does not overflow.

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Q.4. Assertion : A needle placed carefully on the surface of water may float, whereas a ball of the same material will always sink.
Reason : The buoyancy of an object depends both on the material and shape of the object.

Answer

Answer: (c)

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Q.5. Assertion : A large force is required to draw apart normally two glass plates enclosing a thin water film.
Reason : Water works as glue and sticks two glass plates.

Answer

Answer: (c) The two glass plates stick together due to surface tension.

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Q.6. Assertion : The impurities always decrease the surface tension of a liquid.
Reason : The change in surface tension of the liquid do not depend upon the degree of contamination of the impurity.

Answer

Answer: (d) The presence of impurities either on the liquid surface or dissolved in it, considerably affect the force of surface tension, depending upon the degree of contamination. A highly soluble substance like sodium chloride when dissolved in water increase the surface tension. But the sparing soluble or substance like phenol when dissolved in water reduces the surface tension of water.

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Q.7. Assertion : The concept of surface tension is held only for liquids.
Reason : Surface tension does not hold for gases.

Answer

Answer: (b) We know that the intermolecular distance between the gas molecules is large as compared to that of liquid. Due to it the forces of cohesion in the gas molecules are very small and these are quite large for liquids. Therefore, the concept of surface tension is applicable to liquid but not to gases.

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Q.8. Assertion : At critical temperature, surface tension of a liquid becomes zero.
Reason : At this temperature, intermolecular forces for liquids and gases become equal. Liquid can expand without any restriction.

Answer

Answer: (a) Zero surface tension means no opposition to expansion.

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Q.9. Assertion : A large soap bubble expands while a small bubble shrinks, when they are connected to each other by a capillary tube.
Reason : The excess pressure inside bubble (or drop) is inversely proportional to the radius.

Answer

Answer: (a) Since the excess pressure due to surface tension is inversely proportional to its radius, it follows that smaller the bubble, greater is the excess pressure. Thus, when the larger and the smaller bubbles are put in communication, air starts passing from the smaller into the large bubble because excess pressure inside the former is greater than inside the latter. As a result, the smaller bubble shrinks and the larger one swells.

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Q.10. Assertion : It is better to wash the clothes in cold soap solution.
Reason : The surface tension of cold solution is less than the surface tension of hot solution.

Answer

Answer: (d) The soap solution, has less surface tension as compared to ordinary water and its surface tension decreases further on heating. The hot soap solution can, therefore spread over large surface area and also it has more wetting power. It is on account of this property that hot soap solution can penetrate and clean the clothes better than the ordinary water.

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Q.11. Assertion : Tiny drops of liquid resist deforming forces better than bigger drops.
Reason : Excess pressure inside a drop is directly proportional to surface tension.

Answer

Answer: (b) When a drop of liquid is poured on a glass plate, the shape of the drop is governed by two forces, the force of gravity. For very small drops, the potential energy due to gravity is insignificant compared to that due to surface tension. Hence, in this case the shape of the drop is determined by surface tension alone and drop becomes spherical.

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Q.12. Assertion : The shape of a liquid drop is spherical.
Reason : The pressure inside the drop is greater than that of outside.

Answer

Answer: (b) The free surface of liquid tries to acquire a minimum area due to surface tension, hence liquid drop is spherical because sphere has minimum area than other shape.

Q.13. Assertion : The water rises higher in a capillary tube of small diameter than in the capillary tube of large diameter.
Reason : Height through which liquid rises in a capillary tube is inversely proportional to the diameter of the capillary tube.

Answer

Answer: (a) For smaller r the value of h is higher.

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Q.14. Assertion : Hot soup tastes better than the cold soup.
Reason : Hot soup has high surface tension and it does not spread properly on our tongue.

Answer

Answer: (c) With increase in temperature of liquid its surface tension decreases so that it tends to acquire larger area. Hence hot soup having low value of surface tension spread properly on our tongue & provides better taste than cold soup.

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