3 Ingredient Surface Tension Experiment

3 Ingredient Surface Tension Experiment

I’m a sucker for an experiment that only requires staple items and quick preparation time, not to mention, NO MESS. This was a great boredom buster for a quiet afternoon. No need for a trip to the shops, no extra expenses and no wait time - just fun!

The magic pepper and soap experiment explores cause and effect, buoyancy, surface tension and physical reactions through super simple activity.

Materials

Easy surface tension experiment with pepper and detergent or dish soap

  • Pepper
  • Plate
  • Water
  • Dish soap
  • Blue food colouring (optional)
  • Cotton ear bud or cotton ball (optional)

P.S - The optional step of adding the food colouring to the water and watching it disperse was just as exciting for Ethan who is 4 as the actual experiment!

Method

Pepper evenly distributed on water before surface tension is broken by detergentPepper distributed at edge of plate on water after surface tension is broken by detergent

  1. *Optional* Add a drop of blue food colouring to a glass of water.
  2. Pour the water onto the plate, covering most of the plate.
  3. Sprinkle some pepper on top of the water.
  4. Dip one finger, cotton ear bud or cotton ball into the dish soap.
  5. Place finger (or tip of ear bud or cotton ball) in the centre of the plate.
  6. Watch the magic happen

The Science

Water has surface tension. That is the layer of water molecules that sit between the air and the water below behaves like a thin elastic barrier.

Surface tension exists because each water molecule is pulled by the other water molecules around it, but water molecules at the surface do not have molecules above them so there is no upward pull. The difference in force pulls the water molecules at the surface closer together than those inside the liquid and this produces surface tension.

Detergent molecules have one end water attracting (hydrophilic) and one water repellent (hydrophobic) end. This dual nature allows detergent to attract both water and oil, which gives it its ability to clean dishes. These properties are also effective at reducing the surface tension of water by pushing apart water molecules with the hydrophobic end of the detergent molecule.

When the detergent touches the water, the surface tension is reduced, this causes some of the pepper to sink to the bottom of the plate. The water repelling molecules of the detergent push water away towards the edges of the plate. The pepper moves to the edge with the water that it has been floating on. The process that you have seen is an example of a physical reaction taking place.


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