To Find The Surface Tension Of Water By Capillary Rise Method

Introduction

Surface tension is a property of a liquid surface due to which it tries to attain the minimum surface area. For example, a small drop of water has a spherical shape because for a given volume the sphere has a minimum surface area. By this property of attaining minimum surface area, the liquid surface tries to attain a stable system.

Aim

To find the surface tension of water using the capillary rise experimental method.

Materials Required for The Experiment

• A glass/plastic capillary tube
• Beaker
• Travelling microscope
• Thermometer
• Dilute nitric acid solution
• Dilute caustic soda solution
• Water
• Plumb line

Principle

When a liquid rises in the capillary tube the surface tension of the liquid is given by:

Where

ρ = density of the liquid,

T= surface tension of the liquid

h = height of the liquid column

r = inner radius of the capillary tube

Procedure

1. Clean the beaker and capillary tube with caustic soda and nitric acid and rinse it thoroughly with water.
2. Pour water into the beaker and measure its temperature.
3. Clamp the capillary tube at one end, above the beaker.
4. Move the capillary tube so that the lower end of the tube dips into the water in the beaker.
5. Now, push a pin P through a cork C, and fix it just above the water surface. Fix the pin tip such that the tip just touches the surface of the water.
6. Now focus the travelling microscope M on the meniscus of the water in capillary A, and move the microscope until the horizontal crosswire is tangential to the lowest point of the meniscus, which is seen inverted in M. Note the reading of the travelling microscope.
7. Mark the position of the meniscus on the capillary with a pen. Now carefully remove the capillary tube from the beaker, and then the beaker without disturbing the pin.
8. Note the microscope reading by focusing the microscope on the tip of the pin.
9. Cut the capillary tube carefully at the point marked on it. Fix the capillary tube horizontally on a stand. Focus the microscope on the transverse cross-section of the tube and take readings to measure the internal diameter of the tube in two mutually perpendicular directions.

Observation/Tabulation

The least count of the microscope = …mm

Determination of h

Measurement of Capillary Rise

Measurement of the diameter of the capillary tube

Mean radius r = …cm

Temperature of the water = ….°C

The density of water at 0° C = ….

Calculation

Use the value of h, r, ρ, and g in the formula for T to obtain the surface tension.

Result

The surface tension of water at ….°C = … ± …

Precaution

1. Make sure the beaker and the capillary tube are cleaned thoroughly.
2. Keep the capillary tube in a vertical position while dipping it in water.
3. Make sure the capillary tube is sufficiently wet.
4. The water level in the tube should be slightly above the edge.
5. Record the temperature before and after the experiment.
6. Measure the height of the liquid column from the lowest point of the concave meniscus.

Possible Cause of Error in Experiment

1. Inserting dry capillary tube.
2. Dirty capillary tube and beaker.
3. The capillary tube is not placed vertically while inserted into the beaker.
4. Wrong measurement of the microscope reading.

Discussion

1. In a capillary tube, the surface of the meniscus is taken to be semi-spherical and the weight of the liquid above the lowest point of the meniscus as .
2. Taking the above formula into account the surface tension can be given as . This formula gives a more precise value of the surface tension.

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