Page 146 - Physics - XI

6. Note the position of the water level inside h and outside h the capillary tube on the scale. The rise of
1
2
water in the capillary is
h = h – h
2 1
7. Rinse the capillary thoroughly in running water and dry it.
8. Take some quantity with detergent and mix it with water in the beaker.

9. Repeat the experiment with detergent solution and fi nd the rise of mixture (water and detergent) in the
capillary tube again. Let it be h'. You will fi nd that the value of h' will be less than that of h.

Rubber Capillary tube
band

Scale
Clamp stand
h'
2
h'
Rubber
band h' 1

Water with detergent

Fig. 4.2: Experimental set up to study capillary rise in mixture of water and detergent

Observations

1. The height to which water rises in the capillary tube,
h = (h – h ) = _________ cm
2 1
2. The height to which the detergent solution rises in the capillary tube,

h' = (h' – h' ) = _________ cm
2 1
Result

The capillary rise of detergent solution h' is less than the capillary rise of water h. Thus, it can be concluded
that detergent reduces the surface tension of water.

Precautions

1. Capillary tube should be vertical.
2. Tip of the needle should just touch the solution surface.
3. Capillary tube should be of uniform bore.

4. Capillary tube should be washed and dried properly when it is used for the diff erent detergent solutions.

Sources of Error

Distilled water may not be free from grease.

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