Video Transcript
Which of the following substances or phenomena is an example of a gel: a liquid
dispersed in a solid? (A) Milk, (B) whipped cream, (C) blood, (D) vinegar, (E) agar.
Let’s start by taking a look at each of the answer choices and determining the
composition.
Every glass of milk has two primary components. There are water and water-soluble components that make up the majority of milk. The second group of components is made up of liquid-phase oils and fats. These oils and fats are typically tiny droplets that are dispersed throughout the
water layer. And because milk is composed of liquid oils and fats dispersed throughout a liquid
water layer, we can eliminate answer choice (A).
Whipped cream, like you would normally find in the grocery store, may look like a
white fluffy solid. But it is actually a colloid, which is a type of heterogeneous mixture in which
particles are dispersed through a medium, resulting in a mixture with what looks
like a homogeneous composition. However, if you’ve ever made whipping cream at home, you’ll know that it’s made from
vigorously stirring heavy cream. As you stir the heavy cream, air, which is a gas, becomes trapped in the water and
lipids, which are both liquid. This entrapment of air gives us the colloid that we know as whipped cream, a gas
dispersed through a liquid. But the question is asking for a liquid dispersed in a solid. So we can eliminate answer choice (B).
Blood is a complex mixture that is both a suspension and a colloid. Larger particles like some cells can be separated and filtered out of blood, making
it a suspension. Many other components however are too small to be filtered making it a colloid. But because suspensions and colloids are examples of a solid dispersed through a
liquid, we can eliminate answer choice (C).
Answer choice (D), vinegar, is a solution, and is made up primarily of acetic acid
that’s dissolved in water. By definition a solution is a homogeneous mixture where all components are fully
solvated. So we know that vinegar cannot be an example of a liquid dispersed in a solid.
And that leaves us with answer choice (E). Agar is a jellylike substance that’s made primarily from two components: agarose and
agaropectin, which are both solids. When water is added to the solids, it becomes dispersed through them, forming a
gel. And this gel is commonly used in biology labs as a nutrient material to grow bacteria
and other microorganisms.
Now, with this information, we can answer the question. Which of the following substances or phenomena is an example of a gel? And so, we can correctly choose answer choice (E) agar.
