Soap and Pepper Repulsion and Intermolecular Forces

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Intermolecular forces are responsible for the solubility, viscosity, and phase changes of substances.

London Dispersion IMF’s are exerted by any atom or molecule with electrons, this attraction pushes electrons to one side of an atom creating a temporary dipole.

Dipole-dipole IMF’s is where the positive end of an atom or molecule lines up with the negative end of another atom or molecule, forming a permanent dipole.

Hydrogen bonding is formed when hydrogen forms with highly electronegative atoms which are Nitrogen, Oxygen, and Fluorine.

Forces are listed in order of increasing IMF strength.

Water experiences two IMF’s hydrogen bonding and London dispersion (as it contains electrons). Water is polar as electrons are unevenly distributed through the molecule mostly being concentrated around the the oxygen.

Soap experiences London dispersion forces, and soap also contains a non-polar end and a polar end. The polar ends are hydrophilic(attracted to water) while the non-polar ends are hydrophobic(not attracted to water). The polar ends of soap are attracted to each other while the non-polar ends face out and that’s how bubbles are formed.

So why does pepper repel as soon as soap is added to the water pepper mixture. First off soap does not repel pepper. The pepper floats on top of water due to surface tension. Surface tension in water is high due to strong IMF’s with two hydrogen bonds per molecule of water. When soap is added to the water it breaks this surface tension of water causing the water molecules to pull away from the center taking the pepper with it. This is like if you were in a tug-of-war contest and your opponent lets go of the rope you pull away from this person. This is the same idea with the water and pepper. This experiment will also work with any material that will float on top of water not breaking the surface tension.

http://socratic.org/questions/which-liquid-water-or-alcohol-has-stronger-intermolecular-forces

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Sources: http://drholly.typepad.com/ask_me_a_chemistry_questi/2006/01/pepper_and_soap.html

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About chriscatanach

I enjoy long walks on the beach and watching the sun set. I am in a long term relationship with math and science. The movies are what I consider a man-made heaven. Most of all I think my self out to be the fictional reincarnation of Jay Gatsby.
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