Soap motion

Flee by water!

Difficulty:
Danger:
Duration:
10 minutes
Soap motion

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

My boat isn’t moving. Why not?

Try to spread the sides of the boat so that there is a gap between them. This should help the boat move. And, of course, make sure the water in the bowl is clean.

My boat stopped moving, even though it was moving well before.

Don't worry, this is normal! Wash the boat and change the water in the bowl in order to continue. Or try repeating this experiment on larger bodies of water, such as a bathtub.

Can I repeat this experiment with the liquid soap that I have at home?

Your kit contains enough liquid soap to perform this experiment several times. We don't recommend that you use your own liquid soap — it might prove difficult to measure the amount you need, and the result probably won’t be quite as impressive.

If you want to repeat the experiment, be sure to rinse the soap off the boat and the bowl completely.

Step-by-step instructions

Put the boat on the surface of water.

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Make the boat move using nothing but soap!

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See how far the boat can travel on just one drop of soap.

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Wash the boat and change the water to repeat. Run the boat on larger bodies of water, like a bathtub or a pond. Organize a race!

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On a simple, uninterrupted water surface, such as that of a pond or a bathtub, soap will spread evenly. But interesting things can happen if the surface has a complex shape, like in a labyrinth!

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Chlorophyllin acts pretty much like soap, but is clearly visible. Try dripping a drop at the start of your labyrinth.

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Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink and wash with an excess of water.

Scientific description

Water molecules  are attracted to each other quite strongly . Any object placed on the water's surface, like our boat, also experiences this attraction, but this force pulls such an object equally strongly in all directions  and the object won't move.

Soap  is really good at reducing water's intermolecular forces . Simply reducing the pull on the boat from all sides won't do much, but what if we reduce this force on only the rear side of the boat? The water will still pull the boat strongly forward, but much less so in the opposite direction . In other words, the water at the front will win this "tug-o-war", causing the boat to surge forward.

This peculiar method may not be very effective for larger vessels, but some small aquatic bugs can move as fast as 17 cm per second using it.

Why do we use soap?

Soap contains surfactants, molecules with a peculiar relationship to water. Their opposite ends have opposing attitudes to water. One end of a surfactant molecule is attracted to water, while the other end is repelled. When surfactants are introduced to water, they line up along the border separating the water and the air, with the water-loving "heads" submerged in the water and the water-hating "tails" sticking out into the air. Thus, the surface of the water is rapidly covered with a thin soap film. This reduces the forces that attract the molecules of the surface layer to each other, and thus lowers the surface tension.

Why does the water have to be changed to repeat the experiment?

In order to move, the boat needs to have clean water in front of it and soapy water behind it. At the end of the experiment, however, there is no clean water left – the soap has covered the entire surface with a thin film. In this case, adding more soap won’t affect the surface tension anymore and won’t cause the boat to move.

Where else can we observe surface tension at work?

Surface tension forces reveal themselves in the spherical shape of small drops of dew, water droplets scattered on a hot plate, and small drops of water on a dusty road. Inside the liquid, the molecules’ attraction to each other is balanced in every direction. But not on the surface. The water molecules that lie deep within the drop are pulling on the molecules on the surface. Therefore, water tends to stay as compact as possible. As a spherical shape has the smallest surface area, water tends to form drops.

That's interesting!

What are surfactants?

We all use soaps, shampoos, facial cleansers, and various detergents. Do you know how they work? How do they get rid of dirt, and especially grease, so easily?

These cleaning agents usually contain long organic molecules known as surfactants . Surfactants contain both hydrophobic groups  (their "tails," which are afraid of water) and hydrophilic groups  (their "heads," which love water). When you wash your hands or dishes, the surfactant molecules form micelles, arrangements resembling tiny circles. All of the hydrophobic groups hide from the water in the center of the micelle, while the hydrophilic groups remain outside, in the aqueous medium. These micelles are able to "capture" particles of water-insoluble substances  and wash them with water off the skin or other surfaces.