Soap is the metallic salt of a fatty acid. What in the world does that mean? Well, in chemistry, salt isn’t just the white stuff that always hangs with the pepper, it’s any product that comes from neutralizing an acid and a base. Let’s take kitchen salt (sodium chloride) as an example. When you combine hydrochloric acid (HCl) with sodium hydroxide (NaOH), you get sodium chloride (NaCl) and water.
HCl + NaOH –> NaCl + H2O
This is particularly cool because hydrochloric acid is crazy acidic (it’s the stuff that your stomach uses to break down food) and sodium hydroxide is suuuuper alkaline (remember the scene in Fight Club when Brad Pitt burns that ugly hole in Edward Norton’s hand? That’s lye AKA sodium hydroxide) which means achtung, baby! Put them together, though, and you get two completely safe and consumable substances.
Making soap – or saponification, if you’re savvy – is pretty much the same thing. Add lye to an oil and you get a salt that we call soap! More specifically, you combine a trigylceride with lye and end up with soap and gylcerine.
C3H5 COOR COOR COOR + 3 NaOH + (H2O) –> 3NaCOOR + C3H3 OH OH OH
You already know what lye is, but what’s about triglycerides? As it turns out, all the oils we’re familiar with are different types of triglycerides. If you look at the formula for triglyceride, you’ll notice there are some R’s up in there. Those R’s stand for fatty acid radicals, three of which are combined to create a single type of oil. Olive oil, for example, is made up of two oleic acid radicals and one palmitic acid radical. Because of their chemical makeup, different oils and fats have specific properties that can be helpful or harmful in soap making. We’ll talk more about this when we get to the recipe.
The glycerin that’s a byproduct of the reaction is one of the best reasons to make your own soap. Glycerin is a humectant (a thing that keeps moist things moist) and makes your skin soft and moisturized. Often, store-bought soaps have been stripped of most of their glycerin, which is then sold to be used in more expensive bath products.