The surface surrounding a body of liquid water is more than just made of water, it acquires certain properties that differ from the rest of the liquid.

One particle property is its surface tension, that is, its tendency to reduce its surface area as much as possible. This feature allows for some very strange properties, such as some insects being able to walking on water and the surface to curve downwards at its edges.

This surface is formed out of the individual tension forces on each molecule in the liquid. Beneath the surface each molecule is pushed equally in every direction, while for a thin layer under the sur-face molecules essentially only get pushed up.

Now, when a body that floats is placed on the water surface it experiences an equal push from every direction, which keeps it in the same place. This experiment is all about changing this condition.

The first step involves making our boat. For this we can use normal cardboard from an old box or anything light that achieves a similar end. The boat can have any shape you want but at its back end it should have a small hole with a narrow channel similar to the one shown in the figure (above).

After that place the boat model in the water and drop some liquid soap into the hole we cut out. That’s it, the boat should glide forward.

What’s going on? Well, liquid soap is a special kind of chemical that reduces surface tension in liquids. It does this by expanding outwards and pushing itself over the water surface. In this way the water tries to make up this loss to keep all the forces even and balanced. In doing this the water actually pushes the boat forward.

More information can be found on www.hometrainingtools.com/a/soap-boat-science-project.