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Question Video: Identifying the Effect Anionic Detergent Molecules Have When Added to Water Chemistry

As well as removing stains, what other effect can anionic detergent molecules, such as those of alkylbenzene sulfonates, have when added to stained items in water? [A] Precipitating out metal cations. [B] Repairing any structural damage on the surface of the stained item. [C] Softening clothes and other fabrics. [D] Reducing the surface tension of water. [E] Penetrating and breaking down soils.

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Video Transcript

As well as removing stains, what other effect can anionic detergent molecules, such as those of alkylbenzene sulfonates, have when added to stained items in water? (A) Precipitating out metal cations. (B) Repairing any structural damage on the surface of the stained item. (C) Softening clothes and other fabrics. (D) Reducing the surface tension of water. Or (E) penetrating and breaking down soils.

Detergents are a class of substances with cleaning properties. The cleaning properties of a detergent are a result of their unique structure. Detergent molecules consist of a polar, hydrophilic head region, which can interact with water molecules, and a nonpolar, hydrophobic tail region, which can interact with nonpolar fats and oils.

There are several types of detergent molecules that are classified based on the structure of the head region. The head region of an anionic detergent is negatively charged and is often accompanied by a positively charged counterion. Examples of anionic detergents include alkylbenzene sulfonates, like sodium 4-dodecylbenzenesulfonate.

To determine what other effect apart from removing stains anionic detergent molecules can have when added to stained items in water, let’s clear some space and consider what occurs when a generic anionic detergent is first added to water. When an anionic detergent is first added to water, the counterion will dissolve and disperse throughout the water. The head of the detergent is hydrophilic or water-loving, so the heads will be attracted to the water molecules at the surface. But the tails are hydrophobic or water-fearing, so they will stick up into the air.

Already, the detergent is having an effect on the water. Water molecules are held together by strong hydrogen bonding. The introduction of a detergent interferes with the hydrogen bonding between the water molecules at the surface. This decreases the surface tension of water. The decreased surface tension allows water to wet clothing more easily and is the reason why water mixed with a detergent can foam or produce lots of semistable bubbles.

From this discussion, we can see that the other effect an anionic detergent can have when added to stained items in water is answer choice (D), reducing the surface tension of water.

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