Water does not easily remove grease from dishes or hands because grease is nonpolar and water is polar. The addition of soap to water, however, allows the grease to dissolve. Study the structure of sodium stearate (a soap) and describe how it works.

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Identify the structure of sodium stearate, which consists of a long hydrocarbon chain (nonpolar) and a carboxylate group (polar).

Understand that the nonpolar hydrocarbon chain interacts with nonpolar grease molecules through London dispersion forces.

Recognize that the polar carboxylate group interacts with water molecules through ion-dipole interactions.

Explain that when soap is added to water, the nonpolar tails of sodium stearate embed into the grease, while the polar heads remain in the water, forming micelles.

Describe how micelles trap grease in their nonpolar centers, allowing the grease to be emulsified and washed away with water.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Polarity of Molecules

Polarity refers to the distribution of electrical charge over the atoms in a molecule. Water (H2O) is a polar molecule, meaning it has a partial positive charge on one side (hydrogens) and a partial negative charge on the other (oxygen). This polarity allows water to interact with other polar substances but makes it ineffective at dissolving nonpolar substances like grease.

Structure of Soap

Soap molecules, such as sodium stearate, have a unique structure with a long nonpolar hydrocarbon tail and a polar head. The nonpolar tail interacts with grease, while the polar head interacts with water. This dual nature allows soap to act as an emulsifier, enabling the nonpolar grease to be surrounded and suspended in water, facilitating its removal.

Emulsification

Emulsification is the process of mixing two immiscible liquids, such as oil and water, into a stable mixture. Soap aids in emulsification by reducing the surface tension between the two phases, allowing grease to be dispersed in water. This is crucial for cleaning, as it allows nonpolar substances to be washed away with polar solvents like water.

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