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Chromatography, Distillation and Filtration: Methods of Separating Mixtures

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  • 0:06 Types of Mixtures
  • 2:16 Separating Mixtures
  • 4:34 Chromatography
  • 7:34 Summary
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Lesson Transcript
Instructor: Heather Higinbotham
What are some ways that mixtures can be separated? Watch this video to explore several examples of ways you can separate a mixture into its individual components.

Think back to the last time you ate some jelly beans. Were you selective about which colors you ate? Whenever I'm eating jelly beans I go for the green ones first. Just like the mixture of jelly beans, most substances are mixtures of things. Remember that there are two types of matter that exist: pure substances and mixtures. A mixture is a physical combination of two or more substances that are mixed but not chemically combined. The components of a mixture maintain their own physical properties. An example of a mixture is salt water. If you were to drink salt water, it would taste like water with salt in it.

Types of Mixtures

Mixtures come in two main types: homogeneous and heterogeneous mixtures. A homogeneous mixture is a mixture that is uniform throughout, meaning that one part of it has the same distribution of ingredients as another part. A heterogeneous mixture is a mixture that is not uniform throughout, meaning that there is an unequal distribution of the ingredients of the mixture.

Air is a homogeneous mixture of many different gases, including oxygen, nitrogen, carbon dioxide, and water vapor. I know it is homogeneous because each breath I take will contain nearly the same ratio of ingredients. Homogeneous mixtures are sometimes called solutions; especially when it is a mixture of a solid dissolved in a liquid.

An example of a heterogeneous mixture is a chocolate chip cookie. It contains sugar, chocolate chips, butter, eggs, and flour. Each bite I take is likely to contain a different amount of chocolate. Heterogeneous mixtures are easily distinguished because their different components can be seen as individual substances whereas a homogeneous mixture all looks the same.

Separating Mixtures

The rest of this lesson is going to go into detail on a few of the many ways a mixture can be separated into more individual ingredients. The first, and most obvious, way to separate a mixture is to manually separate it. This is probably what you did when you had a bag of jelly beans and picked out which color you wanted to eat.

My next example involves a mixture of salt, sand, and iron filings. All of the particles in this mixture would be about the same size, so how would you separate them? Would you take tweezers and separate them all out? This may get a little time-consuming, so one thing you may want to do is use a magnet. This uses magnetism to separate out the iron.

So now the iron is out; how would you separate the salt and sand? To figure this one out, we need to look at some of the physical properties of salt and sand. You may have noticed that salt has the ability to dissolve in water, whereas sand does not. So, I would add some water to this mixture and try to dissolve all of the salt, leaving the sand to sink at the bottom.

Our next technique is filtration, and it's one of the most common methods for separating a mixture in a chemistry classroom. When you brew coffee you may rely on a coffee filter to keep the grounds from getting in your drink. If our salty water and sand mixture is poured through a filter, the salty water would go through, leaving the sand behind. This is because the molecules of salt are broken up enough and the molecules of water are small enough to go through the filter, leaving the large crystals of insoluble sand behind.

The process of chromatography or color writing
Chromatography Process

Last on our list is separating the salt from the water. There are two ways we can do this. The first is evaporation. It may take some time, but eventually the water will evaporate, leaving the salt behind. This is sometimes called crystallization because the solid salt will form crystals as the water evaporates. If you're short on time, you may want to take advantage of the boiling point of water, which is much lower than the boiling point of salt. By heating the water to its boiling point, you are allowing it to change from a liquid to a gas, eventually leaving all of the salt behind. This process is known as distillation, and it's used in the purification of all kinds of things from water to crude oil.

Chromatography

The green dye from the jelly beans would look like the middle dot on the super-absorbent paper
Jelly beans and chromatograph

For our last example, we are going to go back to the jelly beans. Did you know that most dyes to make candy are actually mixtures of different pigments? The same is true for dyes that are used in markers and ink pens. Is it possible to separate the pigments of jellybeans or markers? You may know the answer to this if you've ever spilled water on a document and watched the ink bleed into different colors. What you are seeing is a separation of the pigments of ink because each pigment has a different attraction to the water based on subtle differences in each pigment's polarity. Chemists use these small differences to separate some mixtures using chromatography, which means color writing.

In paper chromatography, a small dot of the ink or dye to be separated is placed near the bottom on a piece of super-absorbent paper. Let's look at my green jelly beans. If I were to dissolve a little of the outer green layer of the jelly bean in water and put a little dot of that green dye on a piece of chromatography paper, it may look something like the middle dot in this picture.

The ink-marked paper draws up the water through capillary action
Chromatography Capillary Action

The paper is then put in a container with a small amount of solvent, maybe vinegar or ethanol. Can you picture what happens to one end of a piece of paper when it's touched to some water? The paper draws up the water in a process called capillary action. As the paper is absorbing the water, some of the water mixes with the dot of dye and forms a solution with the water traveling up the paper. The green has now separated into two different colors: yellow and blue.

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