Topic: Materials

Surface Tension – Pepper Magic

Difficulty

Easy

Objective

To see the effects of surface tension in action!

Outline

Materials have properties which cause them to interact uniquely with other substances. Pepper is a “hydrophobic” compound – let’s break that up: “hydro” meaning water, and “phobic” meaning fear or dislike of. So water does not interact with pepper! This is different to salt which dissolves in water. The pepper is left to float above the water, not breaking the chain of water molecules on the surface.

Materials

Water tap and sink
A white or light-coloured deep plate or shallow bowl, with higher edges to stop water from spilling out
3x Toothpicks
A few teaspoons of pepper
Cooking oil
Toothpaste
Liquid dishwashing or hand soap

Method

Pour water into the plate or bowl, until it is about two thirds full or covers most of the surface of the inside without any chance of spilling.

Sprinkle a small amount (no more than one teaspoon) of pepper evenly onto the water in the plate or bowl.
Does the pepper break the surface of the water, sink to the bottom, or stay floating? Do the pepper particles move apart or closer together? This is hydrophobicity at work!

Taking one of the toothpicks, dip one end in the cooking oil before lowering that same oil-coated end into the peppered surface of the plate. What happens to the pepper particles after the oil contacts the surface?
Once you have observed this, tip the water and rinse out your plate in the sink.

Prepare the same plate of water with pepper on top by repeating Steps 1 and 2.

Taking another toothpick, dip one end in the liquid soap before lowering that same soap-coated end into the peppered surface of the plate. What happens to the pepper particles after the soap contacts the surface?

Once you have observed this, tip the water and rinse out your plate in the sink.

Prepare the same plate of water with pepper on top again by repeating Steps 1 and 2.

This time, with a third toothpick, dip one end into a tube of toothpaste before lowering that toothpaste-coated end into the peppered surface of the plate. What happens to the pepper particles after the toothpaste contacts the surface?

You can continue this test with various other materials around the house, and remember to record your observations!

What did you observe? The oil should not have caused much change or movement of the pepper particles. This is because oil is also hydrophobic, you may already know that oil and water do not mix. So the strong surface bonds of the top layer of water are not disturbed.

With the soap and toothpaste, you may have seen how the pepper particles immediately moved away the moment the toothpick touched the surface. Both products are used for cleaning, whether dishes, hands, or teeth! Cleaning products have the property of breaking through those surface bonds of water – and the water molecules run away from the soap or toothpaste, carrying the peppered surface with them!

Want to find out more about how soap works? See here:

Source: https://www.scientificamerican.com/article/use-surface-tension-to-make-pepper-dance/