Learn about hydrophobicity, density, and surface tension with the soap and pepper experiment!
Key Words:
Hydrophobic
Density
Surface tension
MATERIALS:
- Pepper
- A bowl with water
- Soap ( only about the size of a pea)
- Optional: Q-tip
Step 1:
First, you need to fill your bowl about halfway with water
Next, you’re going to sprinkle some pepper into the bowl of water. You can add a few sprinkles of pepper (just don’t overdo it!). The pepper should float at the top.
Step 2:
Take some soap, about the size of a pea, and put it on your finger or a q-tip
Step 3: (Results)
You’re going to put either your finger with soap, or the Q-tip, into the bowl of floating pepper. If done right, the pepper should dart quickly to the edge of the bowl (some will/may sink to the bottom) as soon the soap touches the water.
You can repeat this process however many times you’d like. It’s very cool to see the way the pepper seems to be running away from the soap.
Now we can stop and ask ourselves why.
- Why does the pepper float and not dissolve in water?
- Why does the pepper move as soon as the soap touches the water?
- To answer the first question we are going to learn the term hydrophobic.
Hydrophobic – tending to repel water and/or something that fails to mix with water
The pepper is hydrophobic because the water molecules cling to one another and are not attracted to the pepper therefore, the pepper can not mix into the water.
The pepper floats because of its density.
Density is how much space something takes up. (how big, thick, small, thin) If something is big, it has a higher density than something small. The pepper has a lower density than water, so it floats.
- Now to answer the big question, Why does the pepper dart to the sides of the bowl, as soon as the soap touches the water?
Because of the fact that the pepper floated at the top, it created surface tension.
Surface tension is what causes things to float to the top, and stops them from sinking.
The soap breaks the surface tension. (this is also what makes it good for cleaning!)
The surface tension breaks when the soap touches the water so the pepper can’t float at the top anymore. The molecules from water still want to keep the surface tension, so in the process it repels from the soap, and with it goes the pepper.
Experiment with different types of liquids and track results! Tells us how it goes in the comments below