What are soaps?
Soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids, for example sodium stearate, C₁₇H₃₅COO⁻Na⁺. They are made by saponification: heating an oil or fat (an ester) with sodium hydroxide gives soap and glycerol.
A soap molecule has two ends:
- Ionic head (–COO⁻Na⁺): hydrophilic, it dissolves in water.
- Long hydrocarbon tail (C₁₇H₃₅–): hydrophobic, it dissolves in oil and grease.
Micelle formation and cleansing action of soap
Most dirt is oily and does not dissolve in water. When soap is added:
- At the surface, soap molecules line up with heads in the water and tails sticking out.
- Inside the water, they gather in clusters with tails inside and heads outside. Such a cluster is a micelle.
- Around oily dirt, the tails dissolve into the oil and the heads face the water, so the oil ends up at the centre of the micelle.
- The micelle's surface is charged and water-friendly, so micelles stay spread through water (they repel each other and do not clump). Rubbing or agitating helps pull them off the cloth, and rinsing washes them away.
Soap solution looks cloudy because micelles are big enough to scatter light (a colloid).
Hard water and scum
Hard water contains calcium and magnesium salts. Soap reacts with Ca²⁺ and Mg²⁺ ions to form an insoluble white precipitate called scum, for example calcium stearate. A lot of soap is wasted before any lather forms, and the scum sticks to clothes.
Test: shake equal amounts of soap in soft (distilled) water and hard water. Soft water gives lots of foam; hard water gives little foam and a curdy scum.
Detergents and soap vs detergent
Detergents are usually sodium salts of long-chain sulphonic acids or ammonium salts with chloride or bromide ions. They have the same head-and-tail structure and form micelles, but their calcium and magnesium salts dissolve in water, so they do not form scum.
| Soap | Detergent |
|---|---|
| Na/K salt of fatty acid (–COO⁻) | Sulphonate (–SO₃⁻) or ammonium salt |
| Forms scum in hard water | Works in hard water |
| Made from natural oils; biodegradable | Made from petroleum products; some are non-biodegradable and pollute water |
| Bathing soap, cake soap | Washing powder, shampoo, dish liquid |
Key formulas and definitions
- Soap: RCOO⁻Na⁺ (R = long hydrocarbon chain, e.g. C₁₇H₃₅)
- Detergent: R–SO₃⁻Na⁺ or R–N⁺(CH₃)₃Br⁻
- Saponification: fat/oil (ester) + NaOH → soap + glycerol
- Scum: 2RCOO⁻Na⁺ + Ca²⁺ → (RCOO)₂Ca↓ + 2Na⁺
- Micelle: tails (hydrophobic) inside with oil, heads (hydrophilic) outside
Worked examples
1. Why can't water alone remove an oil stain?
Oil is non-polar and water is polar, so they do not mix. Water has nothing to hold the oil with, so it slides over the stain.
2. In a micelle, where is the oily dirt and why?
At the centre. The hydrophobic tails dissolve in the oil and point inward, while the hydrophilic heads point outward toward the water.
3. A student's shirt washed with soap in borewell water stays dull with white patches. Explain and suggest a fix.
The water is hard; Ca²⁺/Mg²⁺ ions formed scum with the soap, which stuck to the shirt. Use a detergent, or soften the water first.
4. Why do micelles not come together to form a big lump?
The outside of every micelle is covered with negatively charged heads. Like charges repel, so the micelles stay apart in water.
5. Samples A and B of water are shaken with equal amounts of soap. A gives a lot of foam; B gives little foam and white curd. Which is hard water?
B is hard water: the white curd is scum formed by Ca²⁺/Mg²⁺ ions with soap.
Common mistakes
- Writing that the tail is hydrophilic. The tail is the hydrocarbon part and is hydrophobic; the ionic head is hydrophilic.
- Drawing the oil outside the micelle. Oil is always trapped at the centre, with tails pointing into it.
- Saying detergents do not form micelles. They do; they just do not form scum in hard water.
- Thinking soap removes dirt by chemically destroying it. Soap only surrounds and lifts oily dirt so water can carry it away.