Fat saponification and how soap micelles clean
ChemistryBiomolecules & PolymersAges 17–18
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Biomolecules & Polymers
Saponification mode: a fat molecule (a glycerol backbone plus three fatty-acid residues) is cut by OH⁻ ions at its three ester bonds, splitting into three soap salt molecules and one glycerol molecule; the rate rises clearly with temperature, alongside the equation and the mass of soap obtained from 1 mol of fat. Washing mode: an oil drop clings to fabric, and soap molecules turn their hydrophobic tails into the oil and their hydrophilic heads outward, forming a micelle that pulls the oil drop off the fabric.
Lesson: Fats and saponification; soap, detergents, and micellar cleaning
What it shows
Fats and oils are triesters of glycerol and long-chain fatty acids. Heating them with a strong alkali hydrolyses all three ester bonds, giving glycerol and three molecules of a fatty-acid salt, which is soap; unlike hydrolysis in acid, this reaction goes to completion. Sodium hydroxide gives hard soap and potassium hydroxide soft soap. A soap molecule has a long hydrophobic hydrocarbon tail and an ionic hydrophilic head, so soap molecules gather around grease as micelles and disperse it in water instead of dissolving it.
How to use
With Content set to Saponification reaction, choose a Fat and an Alkali solution, set the Temperature and press Run; compare how quickly the ester bonds break at 40 °C and at 90 °C, and note the mass of soap made from 1 mol of fat. Then switch Content to Soap micelles cleaning grease and press Run to follow how soap molecules surround the oil drop and lift it off the fabric.
Parameters you can change
- Content Saponification reaction, Soap micelles cleaning grease
- Fat Tristearin (C₁₇H₃₅COO)₃C₃H₅, Triolein (C₁₇H₃₃COO)₃C₃H₅, Tripalmitin (C₁₅H₃₁COO)₃C₃H₅
- Alkali solution NaOH (hard soap), KOH (soft soap)
- Temperature 30–100 °C
Questions to explore
- How many moles of NaOH are needed to saponify 1 mol of tristearin, and how many moles of glycerol form?
- Why can one soap molecule interact with both water and oil?
- How does soap made with KOH differ from soap made with NaOH?
