Video Transcript
Which of the following is the most
correct statement about anionic detergents? (A) The detergent molecule is
composed of a hydrophilic head, which is a long carbon chain, and an ionized
hydrophobic tail. (B) The detergent molecule is
composed of a hydrophobic head, which is a long carbon chain, and an ionized
hydrophilic tail. (C) The detergent molecule is
composed of a hydrophilic tail, which is a long carbon chain, and an ionized
hydrophobic head. (D) The detergent molecule is
composed of a hydrophobic tail, which is a long carbon chain, and an ionized
hydrophilic head.
Detergents are a class of
substances with cleaning properties that have a head and tail region. We can represent a generic
detergent with a figure like this, where the orange circle represents the head and
the wavy blue line represents the tail. To answer this question, we need to
determine the properties of the head and tail.
Looking at the answer choices, we
can see that the words “hydrophilic” and “hydrophobic” are used to describe the head
and tail. Hydrophobic literally translates to
water fearing, and hydrophilic literally translates to water loving. To determine which portion of a
detergent molecule is which, let’s consider what happens when a detergent is added
to water.
First, the detergent molecules
accumulate at the surface of the water with the heads in the water and the tails in
the air. If the concentration of the
detergent is increased or the detergent is mixed with the water, the detergent
molecules will come together to form spheres called micelles. The heads form the outside of the
sphere, and the tails are located within the sphere. As we can see from the diagram, the
head of a detergent is often in contact with water, while the tail is often kept
outside of the water or shielded from the water. This indicates that the tail is
water fearing or hydrophobic and the head is water loving or hydrophilic. Based on this information, we can
eliminate answer choices (B) and (C).
Now, we need to determine which
portion of a detergent is a long carbon chain and which portion may be ionized. Detergents are classified by the
composition of the head region. In this question, we are told that
the detergent is anionic. This means that the head of this
detergent is a negatively charged anion.
Here is an example of an anionic
detergent. We can see that the head region
shown in orange is a negatively charged anion and the tail region shown in blue is a
long carbon chain. Looking at the remaining answer
choices, we can see that the most correct statement about anionic detergents is
answer choice (D), the detergent molecule is composed of a hydrophobic tail, which
is a long carbon chain, and an ionized hydrophilic head.
