Soaps

What is soap?

• a salt of a long-chain fatty acid (a type of carboxylic acid)

• made by reacting fats or oils with a strong base (like NaOH)

What is the general structure of a soap molecule?

• A long hydrocarbon tail (nonpolar, hydrophobic)

• a polar head group (carboxylate salt, hydrophilic)

What is the process of making soap called?

• Saponification

  • the base-catalyzed hydrolysis of esters (triglycerides) into glycerol and soap (fatty acid salts)

What are triglycerides?

• Esters formed from glycerol and three fatty acids

• found in fats and oils

What base is commonly used in soap-making?

• NaOH → hard soaps (solid)

• KOH → soft soaps (liquid/gel-like)

What is the saponification reaction?

Fat or oil (triglyceride) + NaOH → Glycerol + Soap (R–COO⁻ Na⁺).

What are the products of saponification?

Glycerol (glycerin) and Soap (a salt of a fatty acid).

Why is soap considered a carboxylic acid derivative?

• It is the salt of a carboxylic acid

  • specifically, the ionized form of a fatty acid

How does soap clean?

• The hydrophobic tail binds to oils and grease,

• the hydrophilic head interacts with water

• forming micelles that lift dirt away and allow it to be rinsed off

What is a micelle?

• A spherical structure formed when soap molecules organize in water

  • Tails inside (interact with grease)

  • Heads outside (interact with water)

Why doesn’t soap work well in hard water?

• Ca²⁺ and Mg²⁺ ions in hard water react with soap to form insoluble scum (soap scum)

  • reducing effectiveness

How is soap different from detergent?

• Soap: made from natural fats/oils + base; biodegradable

• Detergent: made from synthetic surfactants; often work better in hard water

What kind of fatty acids are used in soaps?

Long-chain saturated or unsaturated carboxylic acids, like: Stearic acid (C₁₇H₃₅COOH)

What is glycerol used for after saponification?

In cosmetics, food, pharmaceuticals, and as a moisturizer.

Are all soaps the same?

No — differences:

• Fatty acid length

• Saturation level

• Base used (NaOH vs. KOH)

These affect soap’s: hardness, lather, and moisturizing ability.