Idea Bank 2020: Dancing Pepper

Section 1: Materials

1. Three Shallow Bowls

2. Three Toothpicks

3. Dishwashing Soap

4. Toothpaste

5. Oil

6. Pepper

7. Masking Tape and Marker

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Section 2: Procedure

Initial Preparation

1. Label the three bowls "oil", "soap", and "toothpaste" respectively using a masking tape and marker

2. Fill each of the three bowls with water, until roughly ⅔ of the bowl is filled.

3. Sprinkle pepper over each of the bowls: notice that the pepper floats on the water surface

Conduct Experiment and Record Observations

1. Dip a toothpick in oil and touch the surface of the water in the bowl labelled "oil" with the toothpick

2. Watch how the pepper reacts and record down/draw your observations

   1. These observations can include how the pepper moves in the water

   2. Example: When in contact with oil, pepper does not move

3. Repeat steps 1 and 2, with dishwashing soap, and toothpaste instead of oil for the bowls labelled "soap" and "toothpaste" respectively.

Section 3: Explanation

• Adhesion & Cohesion of Water Molecules

• Surface Tension

Cohesion is the property that allows water molecules to stick to each other. You can think of water molecules as small balls of water.

Adhesion is the property that allows these water balls to stick to other surfaces.

To understand this concept better, we need to look into the structure of water molecules.

Water molecules are made up of both a +(positive) side and a -(negative) side. Just like how opposite poles of a magnet attract each other, these + and - sides of water allow them to attract each other.

If you have ever noticed water droplets hanging onto a leaf, the water droplets are able to cling onto the leaf without falling to the ground.

This is where we use adhesion to explain how water droplets can stay attached to the leaf, despite the force of gravity pulling the water droplets downwards.

Water Droplets Sticking to Leaf Surface

Additionally, the water droplets are also able to maintain a spherical shape, instead of becoming a flat puddle.

This is a result of cohesion, where water molecules are able to stick to each other and form a water droplet.

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Now when water is placed in a bowl, all the water molecules will stick to each other using cohesive forces.

Water molecules that are not surrounded completely by other water molecules (i.e. the molecules at the water surface) will exhibit stronger cohesive forces with the molecules beside them.

This creates a 'layer' or ‘skin’ on the surface of the water in the bowl. This is known as surface tension. This tension allows water to resist any external forces, like gravity.

Another example of surface tension occurs when we spill water on a table.

Water will form a puddle and not spread over a large area of the table. Surface tension of the puddle of water prevents the water molecules from spreading over a large area.

Now, from the observations that you have made when conducting this experiment you should have noticed that the pepper has no reaction when it is in contact with the oil.

However, the pepper moves away from the toothpick quickly when in contact with toothpaste and dishwashing soap.

Question: What is the one thing both toothpaste and dishwashing soap have in common, that oil does not have?

Answer: The ability to clean!

Oil will always leave a mess, but toothpaste and soap are good at cleaning up messes and dirty things!

Most cleaning agents have substances that break down surface tension. This means the cleaning agents will try to break the attraction between the water molecules.

Water molecules have cohesive properties, meaning they want to stick to each other.

So, when these cleaning agents are placed in water, water molecules will try to resist the break in tension and move away from the toothpick. When they move, they carry the pepper with them.

This is what we have observed as the Dancing Pepper phenomenon!

Dancing Pepper Phenomenon

Section 4: Application & Extension

Camp Einstein Introductory Experiments: Colovaria

One of Operation Einstein's introductory experiments for our STEM Camps is titled Colovaria after the color-changing spell in Harry Potter.

Materials Required:

Milk Rainbow

• 1 Packet of Milk

• 1 Bowl and Spoon

• Food Coloring (the more colors the better!)

• Hand Soap

Experimental Procedure:

1. Pour a small amount of milk into the bowl

2. Add a few drops of food coloring into the center of the milk

3. Place a drop of soap on spoon

4. Gently place spoon in middle of milk for 10 seconds. Record your observations

5. Experiment with placing spoon containing soap at different parts of milk

Explanation:

You will observe that the food coloring appears to "run away" from the soap. Actually, it is the milk and not the food coloring that is running away!

In Dancing Pepper, water molecules will try to resist the break in tension and move away from the cleaning agents. When they move, they carry the pepper with them.

Photo Credit: American Chemical Society

In Colovaria, we are using milk which is composed of water, fat, and proteins. These different constituent molecules have positive (+) and negative charges (-) in different areas.

When you first put the detergent on the milk, the negative (-) end of the soap molecules line up with the positive (+) end of the water molecules.

This causes the soap molecules to spread out over the surface of the milk and push the food coloring radially outwards.

In addition, the negative charge (-) on the detergent molecules are also attracted to the positive (+) parts of the protein molecules.

Photo Credit: American Chemical Society

You can imagine how messy this "mating dance" becomes as all the molecules in the mixture start to move around and the original surface tension of the milk is reduced. This is also why cleaning agents such as hand soap are known as surfactants because they reduce surface tension.

Like how the pepper moves together with the water, the food coloring molecules are also caught in the middle of this "dance" and start to swirl about, creating the beautiful color patterns that you observe.