WBBSE Solutions For Class 9 Physical Science Chapter 3 Matter Structure And Properties Topic B Surface Tension

Chapter 3 Topic B Surface Tension Synopsis

  1. Tangential force per unit length on a liquid surface that acts perpendicularly with a line imagined on the free surface of a liquid is called surface tension of the liquid.
  2. If length of the line = l and force acting on the line = F, then surface tension, S = \(\frac{F}{l}\)
  3. Units of surface tension in CGS system and SI are dyn/cm and N/m, respectively.
  4. Dimensional formula of surface tension is MT-2
  5. If temperature increases, surface tension of a liquid decreases. The particular temperature, at which surface tension of the liquid becomes zero is called critical temperature.
  6. If at any point of the flow line during the flow of a fluid, magnitude and direction of the flow remains unchanged, then that flow is called streamline flow or laminar flow.
  7. In case of a streamline motion, the path followed by a particle in the fluid is called streamline.
  8. A tangent drawn at any point of a streamline indicates the direction of velocity of the fluid at that point.
  9. If at any point of a flow line during the flow of a fluid, magnitude and direction of the flow changes in a haphazard way, then that flow is called turbulent flow. If the velocity of the fluid exceeds the critical velocity, then the flow becomes turbulent. In a turbulent motion, whirls are created at different places of the fluid.

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Chapter 3 Topic B Surface Tension Short And Long Answer Type Questions

Question 1. What do you mean by surface tension? Which natural phenomena gives us ideas about surface tension?

Answer:

A tangential force per unit length on a liquid surface that acts perpendicularly with a line imagined on the free surface of a liquid is called surface tension of the liquid.

If length of the line = l and force acting on the line = F, then surface tension, S = \(\frac{F}{l}\).

Water droplets, raindrops, soap bubble, air bubbles inside water, etc. are spherical in shape due to surface tension.

Question 2. What do you mean by cohesive force and adhesive force?

Answer:

Cohesive Force:

Molecules always attract each other. The force of attraction between like molecules of the same element is called cohesive force.

Adhesive Force:

The force of attraction between unlike molecules of a different type is called adhesive force.

Question 3. What do you understand by molecular range?

Answer:

Theoretically, cohesive and adhesive forces are supposed to act up to an infinite distance but in reality, it is seen that this force is felt up to a definite distance (r ≈10-7 cm) and afterwards, its influence is insignificant.

The measure of this distance is called molecular range. So, molecular range is defined as the maximum distance up to which mutual cohesive and adhesive forces between the molecules of a material remains effective.

Question 4. With the help of a simple experiment, how can you prove that surface tension is working on the free surface of a liquid?

Answer:

If a needle or a blade is floated in water, it sinks.

But if the same needle or blade is kept on a piece of newspaper or on a blotting paper and then floated, it is seen after some time that the paper has became heavier by soaking water. At one point of time, the piece of paper sinks in water but the needle or the blade is found floating on the surface of water.

WBBSE Solutions For Class 9 Physical Science Chapter 3 Topic B Surface Tension Surface Tension Is Working On the Free Surface Of A Liquid

If a small amount of pressure is given by the finger on a tightly held rubber membrane, it is found that the membrane is slightly depressed around the finger. In the same way, water surface is slightly depressed along the side adjacent to the needle or the blade.

From this experiment, it is proved that surface tension is working on the free surface of a liquid.

Question 5. Write down the influence of different factors on surface tension.

Answer:

Surface tension of a liquid depends on the following factors:

Temperature:

If temperature increases, surface tension of the liquid decreases. At a particular temperature, surface tension of a liquid becomes zero. This is called the critical temperature.

Pollution:

If the water surface is polluted by some material, there is a change of surface tension. For example, surface tension decreases if oil or fat is spread on water.

Presence Of A Solute:

The surface tension of a liquid changes, if any other material is dissolved in the liquid. For example, surface tension increases if any inorganic substance is dissolved in water and surface tension decreases, if any organic substance is dissolved in water.

Medium Above The Surface Of Liquid:

Surface tension depends on the medium present above the free surface of the liquid. At ordinary temperature, if dry air is present above water, surface tension of water is 72 dyn/cm. At the same temperature, if water vapour is present above water, surface tension is reduced to 70 dyn/cm.

Question 6. A circular ring made of thin wire is dipped in soap water and then withdrawn, a small water-soaked knot of thread is on the ring and finally, a hole is made in the knot by means of a needle. Explain what happens.

Answer:

Experiment:

A ring made of thin wire is dipped into soap water. It is found that a thin film of soap water has been formed in the ring. This is the free surface of the liquid. A small knot made of thread is soaked in soap water and kept on the film formed in the ring. It is seen that the knot is lying on the film in a haphazard way.

Now a pointed pin is taken to pierce the film inside the knot. It is seen that the knot is stretched to take a spherical shape. In this condition also, the film of soap formed in between the ring and the knot of thread remains intact.

WBBSE Solutions For Class 9 Physical Science Chapter 3 Topic B Surface Tension A Circular Ring Made Of Thin Wire Is Dipped In Soap Water And Then Withdrawn

Conclusion:

  1. At first, equal and opposite forces act in the inside and outside of every point of the knot of thread due to surface tension. Therefore, at every point of the thread at that time, total force due to surface tension is zero. For this reason, thread is lying on the film in a haphazard and static manner.
  2. Afterwards, when the film inside the knot is pierced, equal forces act in a tangential way to the free surface at every point outside the thread of the knot. Due to this reason, the thread takes a spherical shape.

Question 7. Why is a small water drop or a mercury drop spherical in shape?

Answer:

Due to cohesive forces between the molecules of a liquid, there is always a tension acting on the free surface of a liquid for which a liquid wants to reduce the free surface of its area. This tendency of contraction of the area of free surface is called surface tension.

WBBSE Solutions For Class 9 Physical Science Chapter 3 Topic B Surface Tension A Small Water Drop Or A Mercury Drop Spherical In Shape

Of all the spherical bodies that may be constructed with a given specific volume, a sphere has the minimum area. A small water drop or a mercury drop takes the shape of a sphere only due to surface tension.

As a water drop or mercury drop always wants to reduce the area of its free surface, it takes a spherical shape. But as there is some influence of gravity on them, a large sized water drop or mercury drop is slightly distorted.

Question 8. If a painting brush is soaked in water and then taken out, the hairs stick to each other. why?

Answer:

If a painting brush is soaked in water, it is found that the hairs stick out from each other. Now if the brush is taken out of the water, it is observed that the hairs stick together.

After it is brought out of water, a coating of water is formed in the hairs of the brush. This coating of water tries to reduce the area of the free surface due to the property of surface tension. As a result, the hairs stick to each other.

WBBSE Solutions For Class 9 Physical Science Chapter 3 Topic B Surface Tension A Circular Ring Made Of Thin Wire Is Dipped In Soap Water Then Withdrawn

Question 9. If oil is poured on a rough sea, it calms down—explain.

Answer:

If oil is poured on water, surface tension of water reduces. If some amount of oil is poured on a rough sea, oil spreads in the direction of the movement of air. As a result, surface tension of oily water reduces.

Pure water having greater surface tension which remains just behind the oily water drags water backwards with less surface tension. As a result, waves of lesser heights are produced than before and as a result, the rough sea calms down.

Question 10. If A piece of camphor is thrown on water, the piece keeps moving on water in a haphazard way. why?

Answer:

Camphor is soluble in water. The place where water mixes with camphor gets a reduced surface tension. Now the surrounding area of water, having higher surface tension drags the surface having low surface tension towards itself.

The camphor also moves along the surface of low surface tension to the surface of high surface tension, where it again gets soluble with water and reduces the surface tension of that region.

Thus in this way, a non-uniform force acts on camphor due to this non-uniform surface tension of water, causing the camphor to move in a haphazard manner on water.

Chapter 3 Topic B Surface Tension Very Short Answer Type Questions Choose The Correct Answer

Question 1. At critical temperature, the surface tension of a liquid is

  1. Zero
  2. Maximum
  3. 100 dyn/cm
  4. 200 dyn/cm

Answer: 1.  Zero

Question 2. Dimensional formula of surface tension is

  1. MT-2
  2. MT-1
  3. M-1T-2
  4. MLT-2

Answer: 1. MT-2

Question 3. Unit of surface tension in SI is

  1. N
  2. N•m
  3. N•m-1
  4. N•m-2

Answer: 3. N•m-1

Question 4. If temperature is increased, surface tension of a liquid

  1. Increases
  2. Remains unchanged
  3. Decreases
  4. Increases at first, and then decreases

Answer: 3. Decreases

Question 5. If water vapour is present above water instead of dry air, surface tension of water

  1. Increases
  2. Decreases
  3. Remains unchanged
  4. Increases or decreases depending on the amount of water vapour

Answer: 2. Decreases

Question 6. A blade is floating on water. This phenomenon is mainly due to

  1. Viscosity
  2. Gravity
  3. Buoyancy
  4. Surface tension

Answer: 4. Surface tension

Question 7. If oil is poured on a rough sea, then the sea becomes calm. This is due to

  1. Viscosity
  2. Surface tension
  3. Buoyancy
  4. None of these

Answer: 2. Surface tension

Question 8. Surface tension is a general property of

  1. Solids
  2. Liquids
  3. Gases
  4. All of these

Answer: 2. Liquids

Question 9. The upper surface of mercury is convex due to

  1. Surface tension
  2. Viscosity
  3. Density
  4. Pressure

Answer: 1. Surface tension

Question 10. Which of the following shapes (of equal volume) has minimum surface area?

  1. Sphere
  2. Cone
  3. Cylinder
  4. Cuboid

Answer: 1. Sphere

Question 11. When a piece of camphor is put on water, it darts in different direction. This is due to

  1. Viscosity
  2. Vapour density
  3. Humidity
  4. Surface tension

Answer: 4. Surface tension

Chapter 3 Topic B Surface Tension Answer in Brief

Question 1. What do we call the force of attraction between two molecules of the same material?

Answer: Force of attraction between two molecules of the same material is called cohesion.

Question 2. What do we call the force of attraction between two molecules of different materials?

Answer: Force of attraction between two molecules of different materials is called adhesion.

Question 3. What do we call the tendency of the surface area of the free surface of a liquid to contract?

Answer: The tendency of the surface area of the free surface of a liquid to contract is called surface tension.

Question 4. What is the dimensional formula of surface tension?

Answer: Dimensional formula of surface tension is MT-2.

Question 5. With rise of temperature, does the surface tension of a liquid increase or decrease?

Answer: Surface tension decreases with rise of temperature.

Question 6. At what temperature, does the surface tension of a liquid become zero?

Answer: At critical temperature, surface tension of a liquid becomes zero.

Question 7. If some inorganic material is dissolved in water, does the surface tension increase or decrease?

Answer: Surface tension increases if some inorganic material is dissolved in water.

Question 8. If some organic material is dissolved in water, does the surface tension increase or decrease?

Answer: Surface tension decreases if some organic material is dissolved in water.

Question 9. In place of dry air, if water vapour is present over a water surface, then does the surface tension increase or decrease?

Answer: In place of dry air, if water vapour is present over a water surface, then surface tension decreases.

Question 10. Does surface tension work inside a liquid?

Answer: No, surface tension does not work inside a liquid.

Question 11. What is the power of’ L’ in the dimensional formula of surface tension?

Answer: Power of ‘L’ in the dimensional formula of surface tension is zero.

Chapter 3 Topic B Surface Tension Fill In the Blanks

Question 1. If inorganic material is dissolved in water, its surface tension _________

Answer: Increases

Question 2. Surface tension becomes zero at ________ temperature.

Answer: Critical

Question 3. Due to _________ equal force acts in every direction along the surface of a liquid at every point.

Answer: Surface tension

Question 4. When a laminar flow of liquid takes place over a firmly held horizontal plane, velocity of the liquid layer in contact with the bottom surface becomes _________

Answer: Zero

Question 5. Due to the property of ______, raindrops are spherical in shape.

Answer: Surface tension

Question 6. A blade can float on water due to the property of ______ of water.

Answer:  Surface tension

Question 7. The tendency of a liquid surface to ________ its area is called surface tension.

Answer: Scalar

Chapter 3 Topic B Surface Tension Stae Whether True Or False

Question 1. At critical temperature, surface tension of a liquid becomes zero.

Answer: True

Question 2. When temperature increases, tension of a liquid decreases.

Answer: True

Question 3. Surface tension of liquids is a molecular phenomena.

Answer: True

Question 4. The force applied by the liquid tangentially on the surface adjacent to the liquid is known as surface tension of the liquid.

Answer: False

Question 5. The tendency of the surface area of the free surface of a liquid to expand.

Answer: False

Question 6. Due to surface tension capillary action is observed in liquids.

Answer: True

Question 7. Surface tension decreases if some inorganic material is dissolved in water.

Answer: False

Chapter 3 Topic B Surface Tension Numerical Examples

  1. If we imagine a line of length l on the surface of a liquid and F force acts perpendicular to the line, then surface tension of the liquid, S = \(\frac{F}{l}\)
  2. Units of surface tension in CGS system and in SI are dyn • cm-1 and N • m-1 respectively.
  3. 1N • m-1 = 1000 dyn • cm-1

Question 1. Surface tension of water is 72 dyn/cm. Find the total force acting on a 12 cm line imagined on surface of the liquid.

Answer:

Surface tension of water (S) = 72 dyn/cm

∴ Total force (F) = s x d = 72 x 12 = 744 dyn

Question 2. A beaker of radius 15 cm is filled with a liquid of surface tension 0.075 N/m. Find the total force acting on a diameter imagined on the surface of the liquid.

Answer:

Diameter of the beaker,

(d) = 2 x 15 cm s 0.30 m

Surface tension of the liquid, (s) = 0.075 N/m

∴ Total force

(F) = s x d = 0.075 x 0.30 N = 2.25 x 10-2 N

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