Pepper is the choice material for this experiment simply because it is black and shows up well. We repeated the experiment with flour and other floating materials on the surface of the water and it worked the same way. So there is nothing special about using pepper.
The pepper is not repelled by the soap. That is an illusion. I know that water molecules are highly attracted to each other. Think of the water molecules as little magnets that stick to each other.
Now imaging you have a bowl full of identical molecules that all stick pretty well to one another (that’s the water molecules, but this phenomenon also works for other molecules that attract one another readily.) When you have sticky molecules like this in a container, there is a difference between a molecule at the surface of the liquid, and one inside of it:
A molecule inside the liquid is surrounded by identical magnet-like molecules on all sides, so it is pulled in all directions at once, and it feels no net pull in any direction. However, a molecule at the surface has no identical molecules above it to attract it. It feels a net force downwards, because the only identical molecules that attract it are below it. This phenomenon at the surface of a liquid made of sticky molecules is called surface tension. Water has a high surface tension, and this creates something like a skin of water molecules on the surface of the liquid, which feel a net downward pull.
Water bugs can skate on the surface tension of water.
Now, soap destroys the surface tension of water. Soap is any sort of molecule that has one end that attracts water (usually because it has a positive or negative charge and water loves charges), and another end that is attracted to grease or oil, so it causes oil and water to mix.
Now, there is no oil in our experiment, but that is how soap works to clean oil off of things.
When you add the soap to our experiment you are adding all these great big molecules that are attracted to water, but are big and bulky and get in between water being attracted to water. It physically gets in the way of water’s ability to interact with other water molecules, which is what creates surface tension.
The surface tension remains strong wherever there is NO soap. The pepper can only stay afloat in those regions. This makes it appear as if the pepper were repelled by the soap, but really, the pepper just stays afloat on the skin of water, which “pops” like you might pop a balloon, and the skin of water scoots away from the soap, carrying the pepper with it.
Now imaging you have a bowl full of identical molecules that all stick pretty well to one another (that’s the water molecules, but this phenomenon also works for other molecules that attract one another readily.) When you have sticky molecules like this in a container, there is a difference between a molecule at the surface of the liquid, and one inside of it:
A molecule inside the liquid is surrounded by identical magnet-like molecules on all sides, so it is pulled in all directions at once, and it feels no net pull in any direction. However, a molecule at the surface has no identical molecules above it to attract it. It feels a net force downwards, because the only identical molecules that attract it are below it. This phenomenon at the surface of a liquid made of sticky molecules is called surface tension. Water has a high surface tension, and this creates something like a skin of water molecules on the surface of the liquid, which feel a net downward pull.
Water bugs can skate on the surface tension of water.
Now, soap destroys the surface tension of water. Soap is any sort of molecule that has one end that attracts water (usually because it has a positive or negative charge and water loves charges), and another end that is attracted to grease or oil, so it causes oil and water to mix.
Now, there is no oil in our experiment, but that is how soap works to clean oil off of things.
When you add the soap to our experiment you are adding all these great big molecules that are attracted to water, but are big and bulky and get in between water being attracted to water. It physically gets in the way of water’s ability to interact with other water molecules, which is what creates surface tension.
The surface tension remains strong wherever there is NO soap. The pepper can only stay afloat in those regions. This makes it appear as if the pepper were repelled by the soap, but really, the pepper just stays afloat on the skin of water, which “pops” like you might pop a balloon, and the skin of water scoots away from the soap, carrying the pepper with it.
2 comments:
nw i understand the concept nicely...thanku
Thanks for this!
My little boy did it for a class talk. You beautifully explained it to, he easily understood what was happening and he is 6.
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