Surface tension    Floating insects and metal    Water zip-wire

Introduction

A very simple way to demonstrate the “stickiness” of water (surface tension) is to drip one drop of water on to a hard, polished surface . You can see it forms a little mound as all the molecules try to cling to each other (Fig. 5), rather than spreading out over the surface. You will be familiar with this phenomenon when rain falls on to glass, or when you see dew drops on a leaf or a blade of grass.

Figure 5 - Water beads on polished surfaces

An interesting challenge for children is to see how many drops they can fit on the surface of a coin, before the water spills over the edge. They should be able to see that the liquid forms a mound on the coin. If they start with a one pence piece, they could try and predict how many drops a two pence would hold. To extend the activity further, they could try using other liquids e.g. milk, juice, washing up liquid, vinegar etc. Glass or plastic eyedroppers can be bought from your local chemist.

Figure 6 - Dome of water on coin caused by surface tension

Pupils can also see surface tension at work if they fill a container (plastic cup, yoghurt carton) right to the top with water, and then add coins to see how many can be slipped in before the water spills. Make sure the rim of the container is dry at the start. There should be an obvious bulge of water above the top of the container as the coins are added.

Figure 7 - How many coins can you add before it overflows?
 

Floating insects and metal

Pond skaters (common water-strider or Gerris lacustris, Fig. 8) are insects (2 cm long) which use surface tension, in combination with their sensitive water repellent hairs, to move over the surface of ponds to catch other insects which fall in the water.

Figure 8 - Pond skater

Try floating a paper clip on the water surface in a beaker of water. Most children would expect, from previous experience, that such a metal object would sink. To achieve this, lay the paper clip on a small piece of tissue or kitchen roll. Carefully lower the paper on to the surface of the water. In a short time the paper should get soggy and sink, leaving the paper clip floating (Figs. 9, 10 & 11). (You can push the paper down with a pencil if it is taking too long and your audience is getting restless!) How many paper clips can you float at once? Try adding a drop of washing up liquid or soap to the water. What happens to the surface tension and therefore the paper clip?

Figures 9,10 & 11 - Float paper and paperclip on the surface, the paper absorbs water and sinks & how many can you float at once?

Water zip-wire

Because of its sticky properties, water can be persuaded to bridge a gap by travelling along a string. Tie a piece of string round the end of a tap (Fig. 12) and slowly drip water on to the top end. Have a container at the other end of the string to prove that the water has travelled and has been collected. As you have already seen, water molecules will stick to each other (cohesion); they can also stick to other materials, like string (adhesion). You can experiment with different kinds of string, wool, threads etc. Try using two or more strands close together. It helps to wet the string first.

Figure 12 - Water hangs on to the string as it follows it down

Figure 13 - Reservoir fills up at the end of the line                                          Video - move mouse over to start

  Moving water