1. There is a horizontal film of soap solution. On it a thread is placed in the form of a loop. The film is pierced inside the loop and the thread becomes a circular loop of radius R. If the surface tension of the loop be T, then what will be the tension in the thread
Ans: 2RT
Sol: Suppose tension in thread is F, then for small part Δl of thread
2. A large number of water drops each of radius r combine to have a drop of radius R. If the surface tension is T and the mechanical equivalent of heat is J, then the rise in temperature will be
Ans: 
Sol:
3. An air bubble in a water tank rises from the bottom to the top. Which of the following statements are true
(a) Bubble rises upwards because pressure at the bottom is less than that at the top.
(b) Bubble rises upwards because pressure at the bottom is greater than that at the top.
(c) As the bubble rises, its size increases
(d) As the bubble rises, its size decreases
Ans: b,c
Sol:
Since as the bubble rises upward, pressure decreases, then from above law volume of bubble will increase i.e. its size increases.
4. In a surface tension experiment with a capillary tube water rises upto 0.1 m. If the same experiment is repeated on an artificial satellite, which is revolving around the earth, water will rise in the capillary tube upto a height of
Ans: Full length of the capillary tube
5. A soap film of surface tension 3 × 10-2 Nm-1 formed in rectangular frame, can support a straw. The length of the film is 10 cm. Mass of the straw the film can support is
Ans: 0.6 gm
Sol: 
6. Energy required to form a soap bubble of diameter 20 cm will be (Surface tension for soap solution is 30 dynes/cm)
Ans: 24000π ergs
Sol: 
7. If the work done in blowing a bubble of volume V is W, then the work done in blowing the bubble of volume 2V from the same soap solution will be
Ans: 
Sol:
8. Surface tension of soap solution is 2 × 10-2 N/m. The work done in producing a soap bubble of radius 2 cm is
Ans: 
Sol: 
9. Excess pressure inside a soap bubble is three times that of the other bubble, then the ratio of their volumes will be
Ans: 1 : 27
Sol:
10. When a capillary tube is dipped in water it rises upto 8 cm in the tube. What happens when the tube is pushed down such that its end is only 5 cm above the outside water level
Ans: The radius of the meniscus increases and therefore water does not overflow
Sol:
11. A bubble of 8 mm diameter is formed in the air. The surface tension of soap solution is 30 dynes/cm. The excess pressure inside the bubble is
Ans: 300 dynes/cm2
Sol: 
12. The height upto which water will rise in a capillary tube will be
Ans: Minimum when water temperature is 4°C
Sol:
13. Water rises to a height of 10 cm in capillary tube and mercury falls to a depth of 3.112 cm in the same capillary tube. If the density of mercury is 13.6 and the angle of contact for mercury is 135°, the ratio of surface tension of water and mercury is
Ans: 1 : 6
Sol:
14. The angle of contact between glass and water is 0o and it rises in a capillary upto 6 cm when its surface tension is 70 dynes/cm. Another liquid of surface tension 140 dynes/cm, angle of contact 60o and relative density 2 will rise in the same capillary by
Ans: 3 cm
Sol:
15. A drop of water breaks into two droplets of equal size. In this process, which of the following statement is correct.
Ans: The sum of masses of the two droplets is equal to the original mass of the drop.
16. A soap bubble of radius R is blown. After heating the solution a second bubble of radius 2R is blown. The work required to blow the second bubble in comparison to that required for the first bubble is
Ans: Slightly less than four times
Sol: Work done to form a bubble of radius R 
Work done to form a bubble of radius 2R
If surface tension of soap solution is same then W2 = 4W1
But in the problem temperature of solution is increased so its surface tension decreases.
... W2< 4W1
17. A false statement is
Ans: If the radius of capillary is reduced to half, the rise of liquid column becomes four times
Sol: If radius of capillary is reduced to half, the rise of liquid column will be two times.
18. The diameter of rain-drop is 0.02 cm. If surface tension of water be 72 × 10-3 newton per metre, then the pressure difference of external and internal surfaces of the drop will be
Ans: 1.44 × 104 dyne - cm-2
Sol: 
19. Water rises to a height of 16.3 cm in a capillary of height 18 cm above the water level. If the tube is cut at a height of 12 cm
Ans: Water will stay at a height of 12 cm in the capillary tube
Sol: Because if the length available is less than required, then water will rise upto available height and adjust its radius of curvature.
Graphical Questions
20. The correct curve between the height or depression h of liquid in a capillary tube and its radius is
Ans:
Sol: 
. So the graph between h and r will be rectangular hyperbola.
21. A soap bubble is blown with the help of a mechanical pump at the mouth of a tube. The pump produces a certain increase per minute in the volume of the bubble, irrespective of its internal pressure. The graph between the pressure inside the soap bubble and time t will be-
Ans:
Sol: 
As radius of soap bubble increases with time 
22. Which graph represents the variation of surface tension with temperature over small temperature ranges for water
Ans:
Sol: 
i.e. surface tension decreases with increase in temperature.
Assertion & Reason
Read the assertion and reason carefully to mark the correct option out of the options given below:
(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.
(e) If assertion is false but reason is true.
23. Assertion : It is easier to spray water in which some soap is dissolved.
Reason : Soap is easier to spread.
Ans: Assertion is true but reason is false.
Sol: 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.
24. Assertion : It is better to wash the clothes in cold soap solution.
Reason : The surface tension of cold solution is more than the surface tension of hot solution.
Ans: Assertion is false but reason is true.
Sol: 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.
25. 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.
Ans: Both assertion and reason are true and the reason is the correct explanation of the assertion.
Sol:
Hence when the tube is of insufficient length, radius of curvature of the liquid meniscus increases, so as to maintain the product hR a finite constant.
i.e. as h decreases, R increases and the liquid meniscus becomes more and more flat, but the liquid does not overflow.
26. 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.
Ans: Assertion is true but reason is false.
Sol: Needle floats due to surface tension there is no role of buoyant force in its floating Buoyant force = Vσg
Where V = volume of body submerged in liquid
σ = density of liquid.
i.e. the buoyancy of an object depends on the shape of the object.
27. 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.
Ans: Assertion is true but reason is false.
Sol: The two glass plates stick together due to surface tension.
28. Assertion : The impurities always decrease the surface tension of a liquid.
Reason : The change in surface tension of the liquid depends upon the degree of contamination of the impurity.
Ans: Assertion is false but reason is true.
Sol: 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.
29. Assertion : The angle of contact of a liquid decrease with increase in temperature.
Reason : With increase in temperature, the surface tension of liquid increase.
Ans: Assertion is true but reason is false.
Sol: With increase in temperature surface tension of the liquid decreases and angle of contact also decreases.
30. Assertion : The concept of surface tension is held only for liquids.
Reason : Surface tension does not hold for gases.
Ans: Both assertion and reason are true but reason is not the correct explanation of the assertion.
Sol: 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.
31. 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.
Ans: Both assertion and reason are true and the reason is the correct explanation of the assertion.
Sol: Zero surface tension means no opposition to expansion.
32. 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.
Ans: Both assertion and reason are true and the reason is the correct explanation of the assertion.
Sol: 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 1.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.
33. Assertion : Tiny drops of liquid resist deforming forces better than bigger drops.
Reason : Excess pressure inside a drop is directly proportional to surface tension.
Ans: Both assertion and reason are true but reason is not the correct explanation of the assertion.
Sol: 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.
34. 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.
Ans: Both assertion and reason are true and the reason is the correct explanation of the assertion.
Sol: The height of capillary rise is inversely proportional to radius (or diameter) of capillary tube i.e. 
So for smaller r the value of h is higher.
35. 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.
Ans: Assertion is true but reason is false.
Sol: 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.
36. Assertion : The shape of a liquid drop is spherical.
Reason : The pressure inside the drop is greater than that of outside.
Ans: Both assertion and reason are true but reason is not the correct explanation of the assertion.
Sol: 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.
