Surface Tension of Water by Capillary Rise Method

The phenomenon of surface tension is observed in our daily lives. Surface tension enables small insects to float on water, to disinfect the water using light disinfectants, in the medical diagnosis of jaundice treatment, etc.

It is, therefore, essential to know about the basics of surface tension – its basic idea, principle and theory using the following simple hands-on given regarding the capillary action of water.

Table of Contents

• Aim

• Setting Up Capillary Tube

• Setting Up Reference Needle

• Measuring the Height Difference

• Result

Aim

To study surface tension of a liquid (water) using the capillary rise method.

Apparatus Required

1. A Transparent Plastic Capillary Tube

2. Water

3. Travelling Microscope

4. A Beaker

5. A Cork with Pin

6. Clamp and Stand

7. Thermometer

8. Caustic Soda Solution

9. A Plumb Line

Theory

Capillary action arises in a liquid due to the mutual attractive forces among the molecules of the liquid and between them and the walls of the container. Capillary action causes the liquid to rise gradually in a fine tube. Liquids having greater surface tension show more prominent capillary action due to stronger intermolecular forces.

When a liquid of density \[\rho \] rises in a capillary tube, the weight of the liquid column below the meniscus is supported by the upward force of surface tension acting around the circumference of the points of contact.

Therefore, if T denotes the surface tension of the liquid, h denotes the height of the liquid column, and r denotes the inner radius of the capillary tube, then

\[2\pi rT = \pi {r^2}h\rho g\] or \[T = \frac{{h\rho gr}}{2}\]

Following is a simple experiment to determine the surface tension of water using capillary rise method. The method uses a travelling microscope to determine the small height of rise of water in a capillary due to surface tension with respect to the surface of water.

Procedure

1. Clean the capillary tube properly with caustic soda and water.

2. Fill the beaker with water, measure its temperature.

3. Clamp the capillary tube vertically using a clamp stand using a plumbline. Move down the tube so that its lower end dips into the water in the beaker.

Surface Tension by capillary rise method

4. Clamp a cork C above the beaker and push a pin P through it. Lower the cork so that the tip of the pin exactly touches the surface of water (tips of pin and its reflection should coincide).

5. Wait for 5 minutes. Focus the travelling microscope M on the meniscus of the water level in the capillary tube. Move the microscope until the cross wires in the eye piece are tangential to the lowest point of the meniscus. Note the reading of the travelling microscope.

Water meniscus through the microscope

6. Mark the position of meniscus on the capillary with a pen. Carefully remove capillary tube from beaker without disturbing the pin.

7. Now focus the microscope on the tip of the pin. Note its reading.

8. Cut the capillary tube neatly at the marked point. Fix the tube horizontally on a stand. Focus the microscope on the transverse cross section of the tube and take readings of the internal diameter of the tube. Do this for two perpendicular directions.

Measurement of internal diameter of capillary tube

Observations

Least (LC) count of the microscope = .......... cm

MSR = Main scale reading

VSD = Vernier scale division

Observation Table – Measurement of Capillary Rise

Mean h = .......... cm

Observation Table – Diameter of Capillary Tube

Mean radius, r = .......... cm

Temperature of water,\[\theta  = ..........\;^\circ C\]

Density of water at\[0^\circ C,\;\rho  = ..........\;g/c{m^3}\]

Result

From the formula

\[T = \frac{{h\rho gr}}{2}\]

Value of surface tension at ..........\[^\circ C\] is, T = ..........\[N/m\]

Precautions

1. Capillary tube should be cleaned properly with caustic soda and water.

2. Tube must be kept vertical while dripping in water.

3. Temperature should be recorded before and after the experiment.

4. Height of the liquid column should be measured from the lowest point of the meniscus.

5. Ensure that the end of the capillary tube is sufficiently dipped into the water.

Lab Manual Questions

1. What would happen if the length of the capillary tube is taken to be less than the height of rise of water?

Ans: When the length of capillary tube is less than the height of rise of water, then the molecules of water upon reaching the top of the tube experience zero surface tension due to horizontal surface. Hence, the liquid ceases to rise any further.

2. What happens to the surface tension of water if soap solution is mixed with water?

Ans: Upon adding the soap solution, the surface tension of water decreases as the effective cohesive force of water molecules decrease.

3. Explain the effect of temperature on surface tension.

Ans: As temperature increases, the kinetic energy of liquid molecules increases which leads to the decrease in the intermolecular force of attraction. Hence, surface tension decreases.

4. How would the phenomena change if the capillary tube gets tilted by a bit?

Ans: If we tilt the capillary tube, the perpendicular height of rise of water from the surface would still be the same as it was before tilting. This is because in a closed tube, the pressure P at the top of the tube is 

\[P = {P_0} - \rho gh\]

Where \[{P_0}\] is the atmospheric pressure and h is the height of rise of the liquid.

Viva Questions

1. What is surface tension? Give its definition and explanation.

Ans: Surface tension of a liquid is the tension experienced on the outer surface of the liquid among its particles due to mutual attraction between them. It causes the liquid to form a shape with minimum surface area due to stretching of its surface, much like a film.

2. What is the formula and unit of surface tension?

Ans: The surface tension F is given by the formula

\[T = \frac{F}{{2L}}\]

Where,

F is the force exerted by the liquid

L is the length of the liquid column

The SI unit of surface tension is newton per meter (N/m).

3. What small insects do not get wet when lying on the surface of liquid?

Ans: Surface tension of the liquid acts like a thin stretched film, it binds the surface of the liquid and prevents small bodies such as insects from drowning inside.

4. What are cohesive and adhesive forces?

Ans: The force acting between molecules of the same type is known as cohesive force, whereas that acting between two molecules of different types is known as adhesive force.

5. Enlist the factors affecting surface tension.

Ans: The factors affecting surface tension are:

1. Nature of the liquid

2. Nature of the surface in contact

3. Temperature

6. What is the critical temperature?

Ans: The temperature at which surface tension vanishes is known as critical temperature.

7. Why is the meniscus of water concave but that of mercury is convex?

Ans: The meniscus of water is concave because the adhesive force between water and container molecules is greater than the cohesive force between any two water molecules. On the other hand, the meniscus of mercury is convex because mercury molecules have a stronger cohesive force than adhesive force.

8. What is the angle of contact?

Ans: The angle of contact is defined as the angle subtended between the tangent to the liquid surface drawn at the point of contact and the solid surface inside the liquid.

9. What is the effect of temperature on surface tension?

Ans: As temperature increases, surface tension decreases due to increase in the kinetic energy of liquid molecules.

10. Where does surface tension find practical examples?

Ans: Among practical examples,

1. A falling drop of liquid assumes a spherical shape.

2. Usage of soaps and detergents for cleaning.

3. Rise of oil in the wick of a lamp due to surface tension.

Practical Based Questions

1. The tension in the free surface of a liquid is called

1. Stress tension

2. Strain tension

3. Surface tension

4. None of these

Ans: (C) The tension in the free surface of a liquid is called surface tension.

2. A ball pen works on the principle of:

1. Viscosity

2. Boyle’s law

3. Gravitational force

4. Surface tension

Ans: (D) A ball pen works on the principle of surface tension.

3. Among the following, which is not an example of capillary action?

1. Oil reaching the end of wick

2. Roots of a plant drawing water from earth

3. Small insects walking on the surface of water

4. Water retained in a sponge

Ans: (C) Small insects walking on the surface of water is not an example of capillary action.

4. What kinds of fluids have zero surface tension?

1. Real fluids

2. Ideal fluids

3. Both A and B

4. Neither A nor B

Ans: (B) Ideal fluids have zero surface tension.

5. .......... force is responsible for the spherical shape of rain drops.

1. Air resistance

2. Surface tension

3. Viscosity

4. Atmospheric pressure

Ans: (B) Surface tension force is responsible for the spherical shape of rain drops

6. The CGS unit of surface tension is:

1. Dyne-cm

2. Dyne/cm

3. \[Dyne/c{m^2}\]

4. None of these

Ans: (B) The CGS unit of surface tension is dyne/cm.

7. Surface tension can be reduced by:

1. Adding salt

2. Decreasing temperature

3. Adding soap solution

4. All of the above

Ans: (C) Surface tension can be reduced by adding soap solution.

8. Which among them is the necessary condition for capillary effect?

1. Adhesive forces must be greater than cohesive forces

2. Fluid pressure must be zero

3. Capillary should be porous

4. Temperature should be very high

Ans: (A) Adhesive forces must be greater than cohesive forces.

9. Capillary action happens due to:

1. Surface tension

2. Density of liquid

3. Temperature

4. Length of capillary tube

Ans: (A) Capillary action happens due to surface tension.

10. The surface tension of a liquid:

1. Increases with its surface area

2. Decreases with its surface area

3. Increases with an increase in temperature

4. Decreases with an increase in temperature.

Ans: (D) The surface tension of a liquid decreases with an increase in temperature.

Conclusion

From this experiment, we can conclude that the effect of surface tension is prevalent in our surroundings – from lifting the weights of light objects to the cure of many diseases. Surface tension finds many commercial uses such as in the structural design of soaps, detergents, disinfectants, formulation of water compass and capillary action in medicines.

We hope that the reader has got some useful insight of the topic after reading this article and he is motivated to explore the topic in physics for the times to come.