Video Transcript
Which of the following diagrams shows a piece of dirt successfully trapped by detergent molecules prior to being washed away with water?
A detergent is a class of substances with cleaning properties that has a hydrophilic head and a hydrophobic tail. A cartoon drawing like this one is used to represent a detergent molecule in each of the answer choices. The orange circle represents the hydrophilic head, and the wavy blue line represents the hydrophobic tail. “Hydrophilic” literally translates to water loving. So we should expect that when a detergent is added to water that the heads will be attracted to the water. “Hydrophobic,” on the other hand, literally translates to water fearing. So we should expect that the tails of detergent molecules will not form strong interactions with water.
When a detergent is highly concentrated in water or mixed with the water, the detergent molecules will group together to form spheres called micelles. In this arrangement, the hydrophilic heads are in contact with the water, while the hydrophobic tails are insulated from the water by the heads. We need to know what happens to this arrangement of molecules in order to successfully trap a piece of dirt.
To understand what happens, we need to take another look at the tail of a detergent molecule. The tail of a detergent molecule is water fearing, or hydrophobic, because water is polar but the tail of a detergent molecule is nonpolar. When a micelle or free-floating detergent molecules come close to nonpolar dirts, fats, oils, or stains, the nonpolar hydrophobic tails will be attracted to the nonpolar substance. Now that the tails are interacting with the dirt, the micelle can reform with the dirt encapsulated inside. Now, the successfully trapped dirt can easily be washed away with water.
Looking at the answer choices, we can see that the diagram which best represents a micelle with an encapsulated piece of dirt is answer choice (C). Therefore, the diagram which shows a piece of dirt successfully trapped by detergent molecules prior to being washed away with water is the diagram shown in answer choice (C).
