The Magic of Surface Tension: A Pepper and Soap Experiment

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Exploring surface tension can be pretty magical: we can make a BIG change with one touch!

The Pepper and Soap Experiment demonstrates surface tension by applying a thin layer of pepper and using soap to break the surface tension of the water. When the surface tension is broken, the thin layer of pepper quickly scatters away from the dish soap.

You’ll have to watch closely because this magic happens quickly!

How to make the Magic of Surface Tension experiment

Supplies you will need

For this experiment, you’ll need:

Supplies needed for the Magic of Surface Tension experiment

Before you start

This experiment could get messy! Place a towel below the bowl to help catch the mess.

Instructions

Here is how to do this experiment with your child:

Step 1: Pour water and pepper into a bowl

Pour enough water into your shallow bowl that you have about 1-2″ of water in the bowl.

Sprinkle black pepper in the bowl, creating a layer of pepper. As long as you are using finely ground black pepper, it should be floating at the top of the water.

Pour water into the bowl, about 1-2″ deep

Sprinkle a layer of pepper on top of the water

Step 2: Add dishwashing liquid to finger or Q-tip

Place a drop of dishwashing liquid on your finger or Q-tip. It doesn’t have to be much.

Step 3: Lightly touch the surface with the dishwashing liquid

Lightly touch the layer of pepper and it should scatter away from the dishwashing soap!

The result is based on the surface tension of water and how it changes once exposed to soap. Once soap comes in contact with water, the water molecules will try to retain the surface tension and thus pull away from the soapy finger, taking the pepper with them.

Touch the surface of the pepper with some dishwashing liquid

The STEM behind the Magic of Surface Tension experiment

This experiment teaches:

Surface tension
Germs and Hygiene
Careful observation

How it works

Water molecules are polar (oppositely charged) and therefore very attracted to other water molecules. That attraction causes a high surface tension in our container of water.

Dishwashing detergent has two ends, one of which is polar and the other is hydrophobic (water-hating). That polar end is attracted to the polar water molecules, and that reduces the surface tension.

So when we touch the water with the dishwashing liquid, it reduces the surface tension in that spot and creates a ripple effect.

Surface tension

We can demonstrate surface tension by observing how the soap affects the black pepper in the water.

Surface tension is like a tight skin on top of the water. It happens because the water molecules like to cling to each other, but instead of having molecules all around like inside of the liquid, the molecules at the surface only have molecules next to and below it.

Those molecules at the surface therefore pull inwards, forming a thin sheet at the surface. That’s why water droplets are round and some bugs can walk on water!

Dish soap disrupts this film, making the pepper spread wildly.

Germs and Hygiene

Here’s an interesting concept to teach in addition to surface tension: a lesson on germs and hygiene!

In this experiment, the black pepper can represent the germs that are on our hands. When we wash our hands, the soap washes away the germs.

It’s a fun side lesson, but an important one!

Careful observation

When we add the dishwashing liquid to the black pepper, the reaction happens very quickly.

In order to really see what’s going on (how the pepper disperses from the dishwashing liquid), we have to watch very carefully to see how the pepper reacts in the experiment.