The Soap Boat Experiment Teaches Alternative Energy

What do your kids know about energy? They probably have some understanding that gasoline allows your car to run, electricity powers your TV, and batteries get their toys moving, screeching, and blazing with lights.

But do they know about alternative energies? And can they imagine that something like soap can actually be fuel?

In this experiment, you will be able to make a small boat zip around without anything more than a few drops of soap. It’s a fun and easy experiment that doesn’t cost anything, and it will get your kids interested in the all-important discussion on alternative energies.

What You’ll Need

  • Material to make a boat (For best results use a foam tray from your butcher or a piece of non-corrugated cardboard)
  • A large tray, bowl, or cookie sheet full of water (the boat will zoom across for several feet if you do it right, so the bigger the better)
  • Scissors
  • Liquid dish soap (dish detergent works the best)
  • A toothpick or match

Powering It Up

To start, you’re going to have to make your boat. Simple: cut the foam tray or cardboard into the shape of a boat (around 2 inches long).

Make sure your large tray is clean, without any soap residue, or other residue and fill the tray with water (at least an inch deep).

Carefully lower your boat into the water so it is floating, and point the front of the boat toward the far end of the tray. Gather some liquid soap on the toothpick, and place one drop in the hole at the back of the boat.

The moment the soap hits the water, the boat will scoot across the water for several seconds. (To repeat the experiment, you will need to rinse out the tray to remove any soap from the previous demonstration).

For Extra Credit

Try different boat designs, and see which one works best. Or have a race with two boats and different amounts of soap. Draw a line down the middle of the tray, and see which one makes it to the end first.

Can you make the boat go faster or further? What materials work the best?

The Science behind the Magic

The boat moves because of the Marangoni Effect, which means that an object in an area of low surface tension tends to move to a place with higher surface tension.

Surface tension is what allows water to have a “skin” on top. Basically, it means that water molecules stick to each other (especially at the surface) more than they stick to the air molecules. This is what keeps your little boat floating on the water. Since it is flat and light, the surface tension of the water is the same on all sides of the boat, and keeps it on top of the water’s “skin”.

That’s not how regular boats work, though. Boats are able to stay afloat because the downward force (from gravity) is less than the upward force (from buoyancy). Basically, a boat will float if it weighs less than the water it displaces.

Soap is a surfactant, which means it breaks down the surface tension of water. The soap attaches to the water, and gives it something else to stick to, other than other water molecules.

Detergent is made up of two parts – a part that is attracted to water and a part that is repelled by water. When detergent is added to water it forms a layer on the water’s skin. The water-loving half (hydrophilic) faces into the water, while the water-repelling half (hydrophobic) sticks up into the air. The water molecules on the surface are more attracted to the detergent molecules than to each other, so the surface tension is broken.

That’s why we use soap to wash our clothes, it sticks to the water on one side, and it sticks to dirt and grime on the other. The reason we use hot water to wash our clothes is because when it is heated, water has a lower surface tension, allowing it to go into all the nooks and crannies in our clothes.

When you place a drop of detergent in the hole in the back of your boat, it reduces the surface tension of the water in that region, but the rest of the water molecules in the tray are still attracted to each other. So, the force of the surface tension pulling on the front becomes greater than the force pulling on the back of the boat – and the boat moves forward.

As Newton’s third law of motion states, “For every action, there is an equal and opposite reaction.”

This is why you have to clean the water after every time you try the experiment, otherwise, the soap will lower the surface tension throughout the water, and there will be no more pull.

How Does It Apply to Me?

While no one is going to start pouring soap into their gas tanks, this experiment gets kids thinking about how the laws of physics might lend themselves well to exploring alternative forms of energy. We’re all keenly aware of how our current resources are affecting our environment, and an early interest in exploring energy and physics will only help our children’s generation.

Have you tried any experiments in alternative energies? Which ones did your kids love most?

Tags : education   science   experiments   activities   

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