experiment to determine surface tension of water

Collection of Physics Experiments

Measurement of surface tension of liquid, experiment number : 2125, goal of experiment.

The goal of this experiment is to show how to determine a surface tension of a liquid by dripping.

Surface tension

Surface of a liquid behaves like a thin flexible membrane that tries to achieve an equilibrium state (state with minimum energy). With increasing surface, the energy of the surface layer increases. The surface of the liquid hence tries to take the smallest and smoothest area.

The value of surface tension decreases with increasing temperature.

Surface tension measurement

One of the methods to measure surface tension of a liquid is to let the liquid drip.

Liquid in a capillary tube forms drops at the bottom of the tube. The drops will gradually grow and, when they reach a sufficient size, they will separate from the liquid.

The gravitational force acts upon a drop with mass m :

The drop is held at the end of the capillary tube by surface forces. The net force of these forces acts upwards and its magnitude is calculated from relationship

where σ is the surface tension of the liquid, l is the circumference of the drop dripping from the capillary and d its diameter.

The drop will separate from the tube when the gravitational force is greater than the net force of the surface forces. Therefore:

After substituting from the equations above:

The surface tension is then calculated from the relationship

Using this formula, it would be possible to determine the surface tension. The accuracy of this measurement is negatively affected by the fact that only a part of the drop is separated from the tube when the drop drips; part of it stays on the end of the capillary. Before separating, the drop is narrowed and its diameter is smaller than d . Therefore, the dripping method is used mainly as a reference method. If we work with the same capillary, the diameter of the narrowed drop is approximately the same with all wetting liquids.

We use two different liquids in this experiment. We measure the surface tension σ 1 of one liquid; we must know the surface tension σ 2 of the second liquid (which can be looked up). We then determine the ratio of the surface tensions of these liquids

First we determine the mass of one drop of each liquid (weigh more drops and divide the result by the number of drops). We substitute these masses m 1 and m 2 into the relationship

You can find the solved task here:   Oil Pipetting .

liquid with unknown surface tension we want to measure

liquid with surface tension we know (e.g. water)

laboratory stand

cork with a hole (of a size to fit the capillary and the funnel firmly into it)

container into which we drop the measured liquid

laboratory scales

thermometer

Note: If you do not have these tools, you can use a capillary or pipette to drip the liquid (see Technical notes).

Fig 1: Tools for dripping

Insert the funnel in one side of the hole in the cork and the capillary in the other. Fasten the cork in the laboratory stand.

Weigh the empty container which you later use to catch the drops.

Measure the temperature of both liquids.

Pour the liquid in the funnel.

Drip 100 drops (if we want to measure more accurately, we can drip more drops).

Weigh the container with 100 drops.

Determine the mass of 100 drops and then the mass of one drop.

Use the same procedure to determine the mass of one drop of water (second liquid).

Calculate the surface tension of the liquid from the relationship

Sample result

We made the measurements with water, alcohol and kerosene. We weighed the container and let all the liquids drip into it.

The temperature of both liquids was 20 °C.

We used water as the reference liquid with known surface tension ( σ 1  = 72.75·10 −3  N/m at temperature 20 °C).

Table 1 summarizes the mass of 100 drops of each liquid m 100 , the mass of one drop of each liquid m 1 and the surface tension σ .

For comparison, the table values for alcohol and kerosene at 20 °C (Czech source: Converter ):

Technical notes

We can use a pipette or a capillary to perform this experiment. Draw the measured liquid into the pipette or the capillary, block the upper end with your finger and let the liquid drip. We can influence the drip rate by pushing the top of the pipette.

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Physics practicals class xi, objective of the experiment :, to determine the surface tension of a liquid by capillary rise method..

How do you define surface tension?

Surface tension is the property of a liquid, by virtue of which its free surface at rest behaves like an elastic skin or a stretched rubber membrane, with a tendency to contract so as to occupy minimum surface area. This property is caused by cohesion of molecules and is responsible for much of the behaviors of liquids.

The property of surface tension is revealed, for example, by the ability of some objects to float on the surface of water, even though they are denser than water.  Surface tension is also seen in the ability of some insects, such as water striders, and even reptiles like basilisk, to run on the water’s surface.

The Theory Behind Surface Tension

Surface tension has been well- explained by the molecular theory of matter. According to this theory, cohesive forces among liquid molecules are responsible for the phenomenon of surface tension. The molecules well inside the liquid are attracted equally in all directions by the other molecules. The molecules on the surface experience an inward pull.

So, a network is formed against the inward pull, in order to move a molecule to the liquid surface.  It results in a greater potential energy on surface molecules. In order to attain minimum potential energy and hence stable equilibrium, the free surface of the liquid tends to have the minimum surface area and thereby it behaves like a stretched membrane.

Surface tension is measured as the force acting normally per unit length on an imaginary line drawn on the free liquid surface at rest. It is represented by the symbol T (or S). It's S.I. The unit is Nm -1  and dimensional formula is M 1 L 0 T -2.

Capillarity in Liquids

When a capillary tube is dipped in a liquid, the liquid level either rises or falls in the capillary tube.  The phenomena of rise or fall of a liquid level in a capillary tube is called capillarity or capillary action.

How do we define the surface tension of a liquid through the capillary rise method?

When a liquid rises in a capillary tube, the weight of the column of the liquid of density ρ inside the tube is supported by the upward force of surface tension acting around the circumference of the points of contact.

Thus, surface tension;

Where,  h - height of the liquid column above the liquid meniscus ρ - Density of the liquid r - Inner radius of the capillary tube θ - Angle of contact

Learning Outcomes

• Students understand the theory of the surface tension of liquids.
• Students correlate the property of surface tension with different natural phenomena.
• Students understand the concept of capillarity in liquids.
• They are able to relate surface tension and capillarity.

Materials required

• A clean and dry capillary tube
• A tipped pointer
• A beaker containing water
• A travelling microscope
• Adjustable wooden stand
• Clamps and stand

To set up the apparatus :

• Place the adjustable height stand on the table and make its base horizontal by leveling the screws.
• Fix the capillary tube and the pointer in a cork, and clamp it in a rigid stand so that the capillary tubes and the pointer become vertical.
• Adjust the height of the vertical stand, so that the capillary tubes dip in the water in an open beaker.
• Adjust the position of the pointer, such that its tip just touches the water surface.

To find the capillary rise :

• Find the least count of the travelling microscope for the horizontal and the vertical scale.
• Make the axis of the microscope horizontal.
• Adjust the height of the microscope using the height adjusting screw.
• Bring the microscope in front of the capillary tube and clamp it when the capillary rise becomes visible.
• Make the horizontal cross wire just touch the central part of the concave meniscus.
• Note the reading of the position of the microscope on the vertical scale.
• Now, carefully remove the beaker containing water
• Move the microscope horizontally and bring it in front of the pointer.
• Lower the microscope and make the horizontal cross wire touch the tip of the pointer.
• Corresponding vertical scale readings are noted.
• The difference in the two readings (i.e., height of water meniscus and height of the tip of pointer) will give the capillary rise of the given liquid.
• We can repeat the experiment by changing the height of the wooden stand.

To find the internal diameter of the capillary tube :

• Place the capillary tube horizontally on the adjustable stand.
• Focus the microscope on the end dipped in water.
• Make the horizontal cross- wire touch the inner circle at A (fig i). Note microscope reading on the vertical scale.
• Raise the microscope to make the horizontal cross wire touch the circle at B (fig ii). Note the vertical scale reading.
• The difference between the two readings will give the vertical internal diameter (AB) of the tube.
• Move the microscope on the horizontal scale and make the vertical cross wire touch the inner circle at C (fig iii). Note microscope reading on the horizontal scale.
• Move the microscope to the right to make the vertical cross wire touch the circle at D  (fig iv). Note the horizontal scale reading.
• The difference between the two readings will give the horizontal internal diameter (CD) of the tube.

• We can calculate the diameter of the tube by calculating the mean of the vertical and horizontal internal diameters. Half of the diameter will give the radius of the capillary tube.

Observations

Least count of the travelling microscope = Value of one MSD / Number of divisions on the Vernier

Mean h = ……….......... cm

= ............. ×10 -2  m

Calculations

Density of water at observed temperature, ρ = ............ kgm -3

Angle of contact of water in glass, θ   = 8 o

Note the values of  h in the first table and r in the second table for each capillary tube separately  and find the value of T in each case.

Surface tension ,

= .................. N/m

Surface tension of water  is ………………… N/m

Viva-Voce [Surface Tension]

Q.1: Define surface tension?

Ans. “The tangential cohesive force acting along the unit length of the surface of a liquid”

Where F = total force along a line

L = length of the line

In this experiment:

F = m g = weight of the pan + weight in the pan.

L = 2 (length of the slide) + 2 (breadth of the slide).

Sometime we neglect the breadth since it is very small.

Q.2: What are units of surface tension in C.G.S. and S.I. (M.K.S.) system?

Ans. Dynes / cm (C.G.S.)

Newton / meter (M.K.S.)

  Q.3: What are cohesion and adhesion force?

Ans. Cohesion force is the attractive force between like molecules, whereas, the adhesion is the

attractive force between unlike molecules, e.g. attraction between glass slide and the liquid.

Q.4: What are the factors affecting the surface tension?

Ans. (a) Nature of liquid (b) Nature of the surface in contact (c) Temperature

 Q.5: What is the effect of temperature on the surface tension?

Ans. Surface tension decreases with the rise of temperature.

Q.6: Define critical temperature.

Ans. The temperature at which the surface tension is zero.

Q.7: Why the free surface of water is concave but that of mercury is convex?

Ans. The free surface of water is concave because:

Cohesion force between water molecules >> adhesion force between water and gas molecules

Because the free surface of mercury is convex because Cohesion force >> adhesion force

Q.8: What is the shape of free surface at critical temperature?

Ans. At critical temperature the surface tension because zero hence the free surface is flat.

Q.9: Why the surface of the slide should not be oily?

Ans. The surface tension will decrease.

Q.10: Define angle of contact ?

Ans. Angle of contact, for a pair of solid and liquid, id define as

“the angle between tangent to the liquid surface drawn at the point of contact and the solid

surface inside the liquid.”

Q.11: Give some practical applications of surface tension.

Ans. (a) A drop of falling liquid is always in spherical shape.

(b) We use oily substances to set out hairs.

(c) We use soaps and detergent for cleaning clothes.

(d) A thin layer of water over the umbrella protects us against light rain.

(e) Capillary action e.g. rising of oil in the wick of a lamp.

(f) Flying insects can walk on water surface without getting their feel wet.

Physics is one of the most important subjects in Class 12. As the CBSE exam approaches, students get busy preparing for different subjects. But an essential part of the  CBSE exam  is the practical exams which consist of 30 marks.

Students must know all the experiments along with theorems, laws, and numerical to understand all the concepts of 12th standard physics in a detailed way. Two experiments (8 + 8 marks) are asked from each section in the practical exam. The experiment records and activities consist of 6 marks, the project has 3 marks and viva on the experiment consist of 5 marks.

• Physics Article
• To Find The Surface Tension Of Water By Capillary Rise Method

To Determine the Surface Tension of Water by Capillary Rise Method

Surface tension is the tendency of a liquid surface to shrink into the minimum surface area possible. The capillary rise method is one of the techniques to determine the surface tension of a liquid such as water. While experimenting with a capillary tube, it is observed that when a liquid rises in the tube, the weight of the liquid column is supported by the upward force of surface tension acting along the circumference of the points of contact. Let us learn about the experiment and the observations in detail.

To find the surface tension of water by capillary rise method.

Apparatus/ Materials Required

• Three capillary tubes of different radii
• A tipped pointer clamped in a metallic plate with a handle
• Travelling microscope
• Adjustable height stand
• A flat bottom open dish
• Thermometer
• Clean water in a beaker
• Clamp and a stand

The surface tension of water is given by the formula

where, r is the radius of cross-section, g is the acceleration due to gravity, ρ is the density of the liquid, h is the capillary rise, θ is the contact angle.

(a) Arranging the apparatus

• Place the adjustable height stand on the table and make its base horizontal by levelling the screws.
• Take dirt and grease-free water in an open dish with a flat bottom and put it on top of the stand.
• Take three capillary tubes of different radii.
• Clean the tubes and dry them and then clamp them to a metallic plate to increase the radius. Clamp a pointer after the third capillary tube.
• Clamp the horizontal handle of the metallic plate in a vertical stand so that the capillary tube and the pointer become vertical.
• Adjust the height of the metallic plate that the capillary tubes dip in the water in the open dish.
• Adjust the position of the pointer such that the tip touches the water surface.

(b) Measurement of capillary rise

• Calculate the least count of the travelling microscope for vertical and horizontal scales.
• Raise the microscope to a suitable height pointed towards the capillary tube with a horizontal axis.
• Focus the microscope on the first capillary tube.
• Make the horizontal crosswire touch the central part of the concave meniscus seen convex through the microscope
• Note the reading of the microscope on the vertical scale.
• Move the microscope horizontally and bring it in front of the second capillary tube.
• Lower the microscope and repeat steps 4 and 5
• Likewise, repeat steps 4 and 4 for the third capillary tube
• Lower the stand for the pointer tip to be visible.
• Move the microscope horizontally and bring it in front of the pointer.
• Lower the microscope and make the horizontal crosswire touch the tip of the pointer.

(c) Measurement of the internal diameter of the capillary tube

• Place the first capillary tube horizontally on the adjustable stand.
• Focus the microscope on the end dipped in water. A white circle with a green strip will be visible.
• Make the horizontal cross-wire touch the inner circle at point A.

Observation

The least count of the travelling microscope (L.C) = ….. cm.

Height of liquid rise

The internal diameter of the capillary

Calculation

Put the value h and r for each capillary tube separately and find the values of T using the following formula:

Find the mean value of the obtained T values as follows:

The surface tension of water at t °C is _____ dynes cm –1 .

1. Explain the relationship between surface tension and surface energy.

Answer: The relationship between surface tension and surface energy is given as follows:

Surface Energy = Surface tension × Change in area

2. Which side of the liquid surface has more pressure?

Answer: The pressure is more on the concave side of the free liquid surface.

3. What is capillary?

Answer : An open-ended tube with a fine bore is known as capillary.

4. Why should the liquid be free from grease?

Answer: Grease reduces the surface tension of the liquid.

5. What is the surface tension of water?

Answer: The surface tension of water is 7.275 × 10 –2 N-m –1 at 20 °C.

6. Why do you measure the internal diameter of the capillary tube in two mutually perpendicular directions?

Answer: It is done to take the mean to eliminate the error if the bore is not circular.

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