All that sticks is not soap

D. Indumathi, IMSc, Chennai

1. Try out this experiment. Place two match sticks parallel to each other in a plate of water. Wait until the water is no longer disturbed and the matches float next to each other without moving. 

Now, dip a piece of soap into the water in the space between the two matches and remove it at once. What do you observe?

The matches fly apart. This happens because of the effect of soap on the surface tension of water. Water is made up of many molecules. Each molecule attracts every molecule around it and that is how water holds together as a liquid.

Now, you can see from the figure that there is a difference between a water molecule deep inside a glass of water and one at the surface. The one inside is being pulled equally on all sides. The one on the surface is only being pulled downwards by the molecules below it, since there are no molecules above it. Of course, it is being pulled equally on all sides by other molecules on the surface. This leads to a net force on the surface that holds it taut like the surface of a drum.

This force is called surface tension and is what allows some water insects to walk on water. So when you float the matches on water, the force due to surface tension pulls the matches equally on all sides, as shown by the arrows in the figure.

Soap reduces surface tension. We have discussed this elsewhere in this issue of JM. So when you dip soap in the gap between the matches, the surface tension at that point decreases. This is shown by the decrease in the number of arrows in the region between the matches. So there is a net force pulling each match away from the other and so they fly apart.

2. Try out another experiment.


Place a glass on a plate on a flat table. Fill it with water to the brim. Keep it in a place without any breeze or other disturbance. The question is, how many coins can you put into this glass without the water spilling out? It may surprise you to know that you can put in 15 or even 20 coins!! How is this possible?

You may have learned that water has a concave meniscus while a liquid such as mercury has a convex meniscus.  This just means that when you look at the surface of water in a glass, it is rather flat, with a slight upward tilt at the edges. If you float a small piece of matches on it, it will move to the edge since the water level is highest there.

Now, drop the coins carefully, one by one. Make sure that the coin is held vertically and submerged half-way into the water before you let it go. This disturbs the surface the least. Because of surface tension, the surface of the water holds together like a taut cloth and the water inside the glass is not able to flow out even when many coins are put in. After a while, you can see that the surface of the water has become convex, with the piece of match floating in the middle of the glass.

If you look from the side you can see that the water level is more than 1 mm above the edge of the glass!!