Structure of a soap molecule

The idea

Soaps are sodium or potassium salts of long-chain carboxylic acids; the ionic end of a soap molecule is hydrophilic and interacts with water, while its hydrocarbon chain is hydrophobic and interacts with oil.

Two different ends

A soap molecule has two ends. One end is attracted to water and the other end is attracted to oil.

Which end of a soap molecule do you think is the long carbon chain?

• The end that interacts with oil
• The end that interacts with water
• Both ends

Head and tail

Open every tab.

Hydrophilic and hydrophobic

One molecule, two behaviours: the ionic end likes water and the carbon chain likes oil.

Soaps are sodium or potassium salts of long-chain carboxylic acids. The ionic end of a soap molecule is hydrophilic and interacts with water, while its hydrocarbon chain is hydrophobic and interacts with oil.

• Part | Behaviour
• ionic end | hydrophilic: interacts with water
• hydrocarbon chain | hydrophobic: interacts with oil

Notes

Soaps are sodium or potassium salts of long-chain carboxylic acids. The ionic end is hydrophilic and interacts with water; the hydrocarbon chain is hydrophobic and interacts with oil.

Check yourself

Soaps are sodium or potassium salts of which compounds?

Which end of a soap molecule is hydrophilic?

Which part of a soap molecule interacts with oil?

What does hydrophobic mean?

• Short-chain alcohols. Soaps are salts of long-chain carboxylic acids, not alcohols.
• Long-chain carboxylic acids — correct. Yes!
• Mineral acids. Soaps are salts of long-chain carboxylic acids.
• The hydrocarbon chain. The hydrocarbon chain is hydrophobic. The ionic end is hydrophilic.
• Neither end. The ionic end is hydrophilic and interacts with water.
• The ionic end — correct. Yes!
• The hydrocarbon chain — correct. Yes!
• The ionic end. The ionic end interacts with water.
• The sodium ion alone. The book says the hydrocarbon chain interacts with oil, while the ionic end interacts with water.
• Interacting with water. That is hydrophilic.
• Interacting with oil rather than water — correct. Yes!
• Dissolving in water completely. Hydrophilic ends interact with water. A hydrophobic end interacts with oil.