Video Transcript
In the illustration shown, which part of a common anionic detergent molecule does the yellow circle represent? (A) Hydrophobic polar head, (B) hydrophilic nonpolar head, (C) hydrophilic polar head, or (D) hydrophobic nonpolar head.
Represented in the illustration is a detergent molecule. Detergents are a class of substances with cleaning properties that has a head and tail region. As all of the answer choices include the word head, we know that the yellow circle represents the head region. Therefore, the wavy blue line must represent the tail. Detergents are effective cleaning agents because the detergent molecule has both a polar and nonpolar region. The polar region of a detergent molecule interacts with water molecules, which are also polar, while the nonpolar region interacts with nonpolar fats and oils.
We are told in the question that the illustration shown represents an anionic detergent molecule. Sodium lauryl sulfate is an example of anionic detergent. The head region is a negatively charged anion, and the tail region is a long hydrocarbon chain. The sodium ion is the counter ion,which balances out the negative charge of the head region. As the tail of this detergent molecule is a long hydrocarbon chain, this region of the molecule is nonpolar. Therefore, the head region must be the polar region of a detergent molecule. As we have determined that the head region of a detergent molecule is polar, we can eliminate answer choices (B) and (D).
We now need to determine if the head is hydrophobic or hydrophilic. Let’s break down each of these words to uncover their meaning. Hydro- means water. -phobic comes from the Greek phobia, which means fear of. So hydrophobic can be translated as water fearing. -philic comes from the Greek philos, which means loving. So hydrophilic can be translated as water loving. As the polar region of a detergent interacts with water, we wouldn’t expect this region to be classified as water fearing, but rather as water loving. Therefore, the head region of a detergent molecules, which is polar, can be considered hydrophilic because it interacts with water.
On the other hand, the tail region, which is nonpolar, can be considered hydrophobic because the tail region of a detergent molecule doesn’t tend to have strong interactions with water. With this in mind, we can eliminate answer choice (A), leaving us with answer choice (C). So in the illustration of a common anionic detergent molecule, the yellow circle represents the hydrophilic polar head, or answer choice (C).
