Soap Boat

Argh, ye landlubbers. Sail the soapy seas with this activity full of high and low surface tension.

Australian Curriculum links

  • Science > Chemical Sciences > Year 6 > ACSSU095

You'll need

  • clean plastic container or bowl
  • water
  • plastic bread tie
  • detergent
  • matchstick

Try this

  1. Make sure your bowl is clean and free of all traces of soap or detergent then fill it with about 3 cm of water.
  2. Gently lower the plastic bread tie so that it floats on the water.
  3. Using the matchstick, place a small drop of detergent in the hole at the back of the plastic bread tie.
  4. Stand back and break out your yacht shoes.

Note: This activity will only work if the container is clean of all detergent. It will not work if the water starts off soapy.

What's happening?

Water has high surface tension allowing the plastic bread tie to float on top of it. Detergent reduces the surface tension of the water behind the tie, causing the bread tie to be pulled around the bowl by the high surface tension of the rest of the water.

In a bowl of water there are lots and lots of tiny water molecules and all these water molecules are strongly attracted to each other. Water molecules close to the surface are more attracted to other water molecules than the air above them and this is what causes high surface tension. The high surface tension of the water makes the surface strong enough to hold up small objects like the bread tie.

Detergent reduces the high surface tension of water by giving the water molecules something else to be attracted to. A molecule of detergent is made up of two parts – a head that is attracted to water and a tail that is repelled by water. When detergent is added to water it normally forms a layer on top as the water-loving (hydrophilic) heads face down in the water with the water-hating (hydrophobic) tails sticking up into the air. The water molecules on the surface are now just as attracted to the detergent molecules above them as the other water molecules and the high surface tension breaks down.

When you place a drop of detergent in the hole of the bread tie it reduces the surface tension of the water, but only in a very small area. The rest of the surface of the water still has very high surface tension and the water molecules are still really attracted to each other. The difference in surface tension in different parts of the bowl makes the surface of the water move away from areas of low surface tension towards areas of high surface tension. This creates a force that pulls the bread tie around the bowl towards areas of high surface tension.

Real world links

While acting in a similar ways, soap and detergent are different things. As legend has it, soap was first discovered in Rome around 1000 BC Animal fat and ash from animal sacrifice mixed and made its way to the river Tiber where women discovered it made their clothes clean. Synthetic detergent was first developed by Germany in World War One. By using synthetic chemicals, this allowed animal fats to be used for other purposes such as manufacturing glycerine for explosives.