Soaps bridge oil and water
Water alone leaves a greasy plate greasy. One squeeze of soap changes that.
- One soap molecule: a water-liking head, an oil-liking tail.
- Many molecules ring an oil drop — tails in, heads out.
Slide in soap molecules. Watch them ring the grease.
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The rule
A soap molecule has an oil-liking tail and a water-liking head. Tails ring an oil drop. Heads face the water. The wrapped drop rinses away.
Soaps are sodium or potassium salts of long-chain carboxylic acids. The tail is a long hydrocarbon chain. The head is a small charged end.
Many molecules gather round a grease drop. This ring is a micelle. It stays in the water and the rinse carries it off. The grease is moved, not destroyed.
Hard water has calcium and magnesium salts dissolved in it. Soft water has few such salts.
| Case | What happens |
|---|---|
| Soft water | Soap lathers and wraps grease. |
| Hard water | Soap reacts with calcium and magnesium salts. A scum forms. |
| Detergent | It wraps grease the same way. No scum forms in hard water. |
Detergents also have a long tail and a charged head. Their heads do not form scum with calcium or magnesium.