Soap Basics: What's So Special About Soap?
Everyone knows what soap is, right? After all, we're surrounded by it... handsoap, barsoap, shampoo, dishsoap, not to mention all the detergents we use on our dishes, clothes, rugs... we use soap every day (hopefully!) - and have rinsed away tons of it by the time we're old enough to start picking out our favorites from the grocery store.
But, have you ever wondered - what is soap? What's it made of, and how does it work?
Chemically, soap is considered a SALT. When an acid and base combine, they neutralize each other and, by definition, produce what chemists call a salt. Not every salt is a soap; table salt, while it may have excellent scouring properties, is not much of a soap! Soaps are special types of salts known as "salts of fatty acids".
The "fatty acids" are the oils that we use in making soaps. These used to be animal fats, but often today are olive oil, coconut oil, or other natural plant oils. Salts of fatty acids (soap) have some unique properties that are incredibly useful for us...
How Soap Works
Soap works by pulling the dirt off us, and allowing it to be rinsed away. It sounds easy, but... have you ever tried to rinse olive oil off a measuring spoon, without soap? It just doesn't work - the oil sticks.
This is how dirt, especially greasy dirt, sticks to us. We could hop in the shower, and maybe loosen up some of it, but for the most part, the greasy dirt sticks. This is because the "grease" doesn't dissolve in water (it's not "water-soluble"). Just like an oil and vinegar salad dressing, if you put oil and water together in a jar, they'll soon go their merry ways - one straight to the top, one to the bottom. Even after vigorous shaking... after a minute or so, the two will manage to separate again. The oil doesn't dissolve in water, it pulls away from it.
So when we try to rinse off our measuring spoon, or rinse ourselves off in the shower, we get nowhere.
Soap has a special property though. Because of the way the soap molecule is constructed, it can grab greasy dirt AND dissolve in water, allowing us to wash the soap, with dirt attached, off easily.
Soap Experiment: Behold the Magic!
You'll need two containers (with lids, please!), some food coloring, water, and any cheap cooking oil that you don't mind sacrificing for the experiment.
Fill each jar halfway with water. Add a few drops of food coloring, then fill the remainder of the jar with cooking oil. In both jars, you'll see the oil immediately floating to the top, giving you two very clear layers.
Next, add a few drops of liquid soap to one jar. This way, we can see exactly what the soap does, compared to the jar with no soap.
So far, the jars should look the same. Put the lids on tight... and start shaking! Shake each jar vigorously, just like when you shake up salad dressing to try to get the ingredients to mix. Then set them down next to each other.
Initially, they both look pretty much the same. But the jar with no soap will quickly start to separate again, until you can see the layer of oil on top of the water. But look at the jar with the soap! The soap makes the water and oil more "mixable" - and you will see more of a true mixture instead of the layers.
How does it do that?!
The real answer to how soap works is that soap is made up of a long chain of molecules, like a rope, but with different things on each end. Imagine a rope with a magnet tied to one end, and Velcro tied to the other - both ends will stick to things, but not the same things! The Velcro end isn't going to stick to your refrigerator, and the magnet end won't stick to your fuzzy socks. But if you want the fuzzy socks to stick to the refrigerator - the rope makes a perfect middleman!
On one end of our soap-rope are molecules that LOVE grease and dirt. Grease and dirt stick to this end even better than they stick to our skin, so when the soap molecules glide over us, they pick up the grease and dirt with ease.
The other end of the soap-rope loves water. If there's water passing over, it jumps right in and goes along for the ride.
So, the dirt that would normally stay stuck on us becomes stuck to the dirt-loving side of the soap, then the water-loving side connects with the stream of water in the shower, to be rinsed down the drain, pulling the attached dirt with it. And we come out clean and happy!