How Fat and Lye Make Soap

Chemical Reaction

Soap is made through a process called saponification. You start with a fat or oil — something like olive oil, coconut oil, or tallow. Fats are made of triglycerides, which are molecules composed of one glycerol molecule attached to three fatty acids. When you add sodium hydroxide (lye), it breaks those bonds. The fatty acids react with the sodium ions to form soap molecules, and the glycerol molecule becomes glycerin. 

Fat + Lye → Glycerin + Soap 

Glycerin Explained

Glycerin is a humectant, which means it attracts and retains moisture. This helps maintain the outer layer of the skin’s barrier and prevents transepidermal water loss. Commercial soapmakers often remove glycerin during processing to repurpose for use in lotions or cosmetics, which is why handmade bars where the glycerin stays tend to feel more moisturizing.

Soap Explained

The soap molecules are what do the cleansing. Soap molecules have two ends: a hydrophobic tail that binds to oils and grease, and a hydrophilic head that binds to water. When you wash, the tails attach to dirt and oils on your skin, forming tiny droplets called micelles that lift the grime away. When rinsed, the water-loving heads pull those micelles off your skin and down the drain. The same structure lets soap disrupt lipid membranes of many bacteria and viruses effectively breaking them apart and inactivating them.

This simple chemical reaction is what allows us to turn a few basic ingredients into something so useful.