Measuring the Surface Tension Measuring the Surface Tension Constant of Liquid Constant of Liquid

by the Use of Capillaryby the Use of Capillary

Aim

To determine the surface tension value of water against air by the method of capillary rise.

To learn the use of reading microscope.

This unequal attraction causes the water at the air-water interface to act as a cohesive surface and the liquid to shrink to the smallest possible surface area.

A water molecule deep within the liquid experience forces of attraction by all its neighbors

net force = zero

A water molecule at the surface lacks neighboring water molecules toward the interface

net force is not zero

(directed downwards )

Introduction

defined as:

Surface tension - Scientificaly

where is the surface tension of the liquid. As defined here the dimensions of surface tension are force per length, and its units are .

The force along a line of unit length, where the force is parallel to the surface but perpendicular to the line, tending to minish the area of the surface

1mN

Lf /

Theory Surface tension produces several phenomena. The rise of a liquid in a capillary is the result of surface tension. Consider the simple situation in Fig. 1, in which the end of a capillary tube is immersed in a liquid, one observes a substantial rise of liquid in the capillary due to surface tension. The balance point can be used as a means to measure the surface tension:

)6

(4

1)

3(

2

1 dhgd

rhgr

where d is the diameter of the capillary, h is the capillary rise, ρ is the liquid density, g is the acceleration due to gravity. Thus, after determine the value of h and d in this experiment, the surface tension of water can be calculated.

Fig. 1

2r

h

Apparatus

Reading microscope Beaker Capillary Thermometer Light source Support of capillary

Reading microscope

Eyepiece

Objective lens

Focus handwheel

Micrometrical drum

Reading scale

Reading dial

Fig.2

For example, reading is 29.723mm in Fig.3. （ Note: The recorded data should include the last estimated figure. ）

1－main scale

2－ reading line of main scale

3－micrometrical drum

4－ reading line of micrometrical dru

m

Fig.3

Procedure

1 Illuminate the object by light source, adjust the direction of light and make the field of view be bright.

2 Adjust eyepiece until the cross-hair is clear.

3 Get rid of the water in capillary. Put capillary into the hole of support and make the end of capillary dip into the beaker filled with water. Raise and lower the capillary several times until it is wetted. When the water wets the walls of the capillary, the water is “pulled” up by the surface tension force. Pull capillary upward from lower slowly until its liquid level achieves the highest.

Procedure

4 Put the beaker on object stage ， find the clear image of liquid levels of capillary and beaker separately by adjusting the distance from lens to object.

Procedure

h

h

5 Use the reading microscope to determine the capillary rise of water.

times1 2 3

1H

2H1 2h H H

h

Procedure

Fill the table with experiment data ：

h

mm

6 Take out the capillary and get rid of water in it. Put the capillary on the support and making the end of it toward objective lens. Adjust the reading microscope until find the ring image of capillary and the color of the ring is green (as below).

Procedure

d

d

7 Determine the internal diameter of the capillary.

Procedure

Fill the table with experiment data ：

8 Record the temperature t of the water.

mm

Data processing

Calculate the surface tension of water.

Compare the result with the standard value and calculate the relative error.