With a few simple observations, you can classify a mixture as a solution, suspension or colloid. Learn how we use properties, such as visibility of particles, how light is affected and the ability of particles to settle out to classify mixtures.

## Mixtures

Here we have three common kitchen ingredients: sugar, oil, and gelatin powder. We’re going to mix these ingredients into three separate glasses of water to perform a kitchen-style science project. Before we create our mixtures, can you guess which ingredients will mix well and which will not? Will the oil mix with the water or separate into layers? Will the mixtures be clear or cloudy? Our observations about these mixtures will allow us to classify them as solutions, suspensions, or colloids. Did you make your predictions? Great, let’s go ahead and start mixing.

## Solution

One of our three mixtures is a solution, which is a homogeneous mixture of two or more substances. We describe a solution as homogeneous because the added components mix completely and are uniformly distributed throughout the mixture. With a homogenous mixture, like a solution, the particles are very small. This small size prevents them from being filtered out or separated. The small size also means that the individual particles cannot be seen, which is one of the properties that all solutions have in common. Another property that we can attribute to solutions is that the particles will not settle out, regardless of how long the solution sits.

With these tidbits of knowledge, we can eliminate one of the three mixtures from our kitchen experiment. Oil and water do not mix well. With that mixture, the added components remain separate and do not mix completely, so we call it a heterogeneous mixture.

That leaves us with the sugar and the gelatin mixtures. Which one of these is a solution? Well, to answer that, we need to know one more property of solutions, which is that solutions do not scatter light. If we were to place a flashlight behind the glass of sugar water, the light would pass through easily, but we cannot say that for the glass of gelatin and water. That mixture is cloudy, and if we were to shine the same flashlight through it, we would see that the light would get dispersed and not easily shine through. Congratulations, you just identified sugar water as a solution!

## Suspension

Now, we have to decide which of our two remaining mixtures is a suspension. A suspension is defined as a heterogeneous mixture that contains large particles. I bet you can already guess that the oil and water mixture matches this description. When we stir the oil and water, the globs of oil get smaller, but they never completely mix. The particles are visible, and when we stop stirring, the particles settle out to form a separate layer.

With a suspension, it’s easy to separate the particles using a filter, which is not something we can do with the other two types of mixtures. To complete our look at suspensions, we can try our flashlight test. When the light is shown through a suspension, it may scatter light, but sometimes suspensions are opaque, so the light might not be able to penetrate the mixture at all. For example, if we were to put mud in our water glass, we would create a suspension that would be so dark and murky that light would not be able to pass through.

## Colloid

The only remaining mixture in our kitchen experiment is the gelatin mixture. So, by process of elimination, we see that this is a colloid. A colloid has properties that make it fall in between a solution and a suspension. For one thing, a colloid is a mixture of intermediate sized particles. These particles are not quite as big as those found in a suspension and not quite as small as those found in a solution. We also see that colloids are typically thought of as heterogeneous mixtures but share some of the same qualities as homogeneous mixtures. Colloids also take the middle road with particle visibility. Their intermediate size means the particles are not easily seen, but we cannot rule out seeing them if we have a strong enough microscope.

With all of this middle ground, you might think it’s hard to tell a colloid from the other two mixtures; but fortunately, there are a couple of properties shared by colloids that make them easily distinguishable from the other mixtures. One property that strongly differentiates a colloid from a suspension is the fact that with colloids the particles will not settle out. One property that strongly differentiates a colloid from a solution is that colloids do scatter light. Do you remember our first flashlight test? When light tries to pass through a colloid, it hits the intermediate sized particles and gets dispersed.

## Lesson Summary

Let’s review.

In this lesson, we learned that mixtures can be homogeneous, which means that components mix completely and are uniformly distributed throughout the mixture, or heterogeneous, which means components remain separate and do not mix completely.

A solution is a homogeneous mixture of two or more substances. Solutions have small particles, which means that the individual particles cannot be seen and the particles will not settle out. We also see that solutions do not scatter light.

A suspension is a heterogeneous mixture that contains large particles. The large particle size means that the particles are visible and the particles settle out. Suspensions may scatter light if conditions are right.

A colloid is a mixture with intermediate sized particles. With colloids, the particles are not easily seen, and the particles will not settle out. Colloids do scatter light.