1. Surface tension of water

How can a water strider move on the surface of a pond without sinking?

Why are drops of water coming from a tap spherical?

Why does water trickle on to leaves and other surfaces?

These phenomena can be explained with a special property of water - surface tension; you will examine them in the following experiments.


1.     Fill the plastic cup with water to just below the rim.

2.     Put the paper clip carefully, with the help of tweezers, on the surface of the water.

more (see below)   

3.     Add a few drops of washing-up liquid to the edge of the water surface.

4.     Take a syringe needle and blunt the tip of the needle with scissors.

5.     Fill the syringe with water and spread a few drops of it on a plastic surface.

6.     Repeat step 5 with water to which you have previously added some washing-up liquid.

7.     Fill an ampoule completely with water; label it W.

8.     Put a drop of washing up-liquid into a second ampoule and fill it carefully with water; label it WS.

9.     Turn the ampoule (W) upside down above the experimental tray.

10.  Repeat step 9 with the ampoule (WS).



1.     Write down your observations and explain them.

2.     Explain in your own words, using the particle model, what causes the surface tension of water; illustrate your explanation with a drawing.

3.     Explain the effect of the washing-up liquid on the surface tension of water using the particle model; illustrate your explanation with a drawing.

4.     How do your experiments help to explain the three phenomena mentioned in the introduction.                                                                                  Video clip 1, Video clip 2