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Have you ever tried to clean a greasy tupperware with just water? It probably didn't go all too well. This is again down to the polarity of molecules. Grease, and other types of oils are non-polar molecules, which don't have the necessary split in charge required by water molecules (polar) to break them apart, and thus dissolve them.

However, add some soap and the grease comes right off. So the logical conclusion to reach might be that soap is made up of non-polar molecules, but this too isn't strictly true.

Soap, like fats, is a hydrocarbon chain. i.e. a large chain consisting of interlinking carbons and hydrogens. The difference between most fats and soap is that soap molecules have two different ends. One end of the soap molecule is a saturated hydrocarbon end, i.e. it is non-polar. Whilst the other end the polar, i.e. hydrophilic. Thus when you have something that is non-polar the non-polar end of the soap molecule interacts with that allowing it to be dissolved, whilst polar substances interact with the hydrophilic portion of the molecule.

Quite the versatile substance I would have to say.

If you've ever had a chilli before you're probably quite familiar with the not entirely pleasant feeling of having your tongue lit on fire. On more than one occasion, having been a bit too flamboyant with tabasco bottle I almost thought I could breath fire.

Now you're mom, or someone else, will probably have told you to drink milk. Water doesn't help they say, and I have no doubt you've experience it for yourself. There's actually a very logical chemical explanation behind this.

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Now substances can be classified as one of two things. Either polar, or non-polar. This essentially states whether or not there is a greater electron density in one area of the molecule versus another. To determine whether or not a molecule is polar you first look at the bonds. If two substituent atoms are different molecules, they will have a polar bond. The amount of polarization will depend on their respective electronegativities (the pull of a nucleus on a shared pair of electrons), the greater the difference in electronegativtiy the more polar the bond. But just having polar bonds doesn't necessarily mean that a molecule is polar, to determine this you need to look at the symmetry of the molecule. If it is symmetrical, i.e. the shape of it as well as the substituent atoms attached, the molecule is said to be polar. If this isn't the case, it is a non-polar molecule.

Now when such substances are dissolved in solution, there is a helpful phrase that helps characterize their properties: "like dissolves like". In other words, a polar molecule will be dissolved by a polar solvent, and a non-polar molecule will be dissolved in a non-polar solvent.

Capsaicin, the molecule largely responsible for "spicyness" is a non-polar molecule. Water, on the other hand, with highly polarized bonds and two lone pairs (making it non-symmetrical), is highly polar. Thus drinking water doesn't really do much. In contrast, milk contains some amounts of fat, i.e. triglycerides, which are long chains of saturated hydrocarbons. These fat molecules are non-polar, allowing the capsaicin to dissolve into the milk and thus is removed from the mouth. And voila, your mouth no longer feels like a dragon breathed into it.

By the way, ice cream does the same trick.