Separation by Distillation

Key concepts

Physics

Boiling point

Condensation

Distillation

Introduction

Do you like cooking? If you have helped in the kitchen at home or watched someone else cook, you have probably seen lots of liquids—such as water, milk and soup—heated. Did you notice that once the liquid boils, a lot of steam develops? Have you ever wondered what the steam is made of and what happens to all the substances such as sugar or salt that are dissolved in the solution you are boiling? Do they boil off, too, or do they stay behind in the solution? In this activity you will build a distillation device that allows you to sample the steam that you generate while boiling a fruit juice! How do you think it will taste?

Background

What do you need to make a solution? First, you need water or a solvent and then you need a substance such as sugar or salt to dissolve, also called the solute. The solvent and solute become one solution—a homogeneous mixture—in which you cannot see the difference between them anymore. Most solutions actually contain many different substances. But what if you want to separate the individual components from a liquid solution? There is a process called distillation that allows you to do just that. It is used in many real-world applications, such as making medicine, perfumes or some food products.

Distillation exploits the differences in the volatility of the solution's components, which means that every compound has a different boiling point and starts to vaporize (change from its liquid to gaseous state) at a different temperature. When distilling, you heat up the solution so that the component with the lowest boiling point evaporates first, leaving the other solutes behind. The vaporized component in the gaseous state can then be collected in a different container by condensation and is called distillate. This means that the vapor is cooled down so the gas becomes a liquid again. By changing the distillation temperature, you can separate many different substances according to their different volatilities. If you have a solution that includes a nonvolatile solute, however, this compound will always stay behind in the solution.

Knowing now how distillation works, what do you think will happen to the fruit juice once you heat it? Make your own distillation device and find out!

Materials

• Stove (Always work with an adult helper when using the stove.)
• Deep cooking pot with sloped lid (transparent lid, if you have one)
• Ceramic bowl
• Small ceramic plate or ceramic coffee cup
• Three glasses
• Apple or cranberry juice (about half a liter)
• Liquid measuring cup
• Ice
• Oven mitts
• Broth (optional)
• Cooking thermometer (optional)
• Vinegar (optional)

Preparation

• Make sure all of your materials are clean. (Then you will be able to sample the juice and products at the end of the activity.)
• Place the small ceramic plate in the center of the cooking pot. Depending on how deep your pot is, you can also place a ceramic coffee cup in its center.
• Place a ceramic bowl on top of the small plate or coffee cup.
• Put your pot on the stove.

Procedure

• Measure out and pour one cup of the fruit juice into a glass. Have a look at its color and take a small sip to taste it. Is it very sweet? How does the color look; is it very intense? Keep the rest of the juice for comparison at the end.
• Pour an extra cup of colored fruit juice in the bottom of the pot. (Your small ceramic plate or ceramic coffee cup will now be standing in the juice.)
• Together with your adult helper, turn on the stove to medium heat and bring the juice to a boil. It should be a moderate rather than a rolling boil. Can you see the steam developing once your juice starts boiling?
• Now place the cover on the pot, upside down, so that the tip of the sloping lid is facing toward the bowl placed inside the pot. What happens to the steam once you close the lid?
• Put ice in the cover of the pot. You might have to replace the ice in the lid as it melts. If you use a transparent lid, can you see droplets forming on the inward-facing side of the lid? Where do they come from and what happens to the droplets?
• Allow the juice to boil for 20 to 30 minutes, making sure some juice always remains in the bottom of the pot. Do you see any changes in the amount of juice inside the pot?
• After 20 to 30 minutes, turn the burner off. Allow the pot to cool for a few minutes.
• Put on oven mitts and carefully remove the cover from the pot. What do you notice about the empty bowl that you placed under the lid?
• Still wearing hot mitts, lift the bowl off the small ceramic plate or coffee cup and set it down on a heat-resistant surface.
• Remove the small plate or coffee cup. Looking at the remaining juice in the pot, is there more or less juice left than the amount you poured in?
• After it cools, pour the remaining juice from the pot into a glass. Did the juice change during boiling? What is different?
• Pour the cooled distillate (the condensed steam), which is now the liquid inside the small bowl, into a glass. How does the distillate look?
• Now take the glass from the beginning with the original juice, and place it next to the remaining juice and distillate. Compare their appearances. How do they differ? Did you expect these results? Why do you think the juice changed the way it did? How much fruit juice is left compared with what you poured into the pot?
• Let the liquids cool to room temperature. Because you used clean kitchen utensils and edible fruit juice in this experiment, go ahead and take a sip of each of the solutions. How do the three different liquids compare in taste? Which one is the sweetest? Which one is the least sweet? How does the condensed steam taste? Why do you think is there a difference?
• Finally, recombine the distillate and the remaining fruit juice by pouring the distillate into the remaining fruit juice. Do the volumes add up to what you put in at the beginning? How do the appearance and taste of this solution compare with the original fruit juice?
• Extra: Repeat this activity with a salty solution, such as broth, instead of the sweet fruit juice. Do you think the results will be similar? What happens to the salt in the broth when you are boiling it?
• Extra: Try to do this experiment again with household vinegar. Vinegar is a mixture of about 4 to 6 percent acetic acid and water. Can you separate these two liquids by distillation? How does your distillate taste in this case?
• Extra: You might know that the boiling temperature of pure water is 100 degrees Celsius (212 degrees Fahrenheit) at normal atmospheric pressure. Adding a solute such as sugar, salt or other compounds to water will change the boiling point of the resulting solution. Try heating up your three liquids (original juice, distillate and remaining juice) and measure their boiling points with a thermometer. Are they very different? How does the boiling point change with increasing solute concentration?

Observations and results

Juices are usually very sweet. This is because fruits contain a lot of fruit sugar, called fructose. More than 80 percent of most fruits, however, consist of water, so basically the apple or cranberry juice is a mixture of water and sugar. Once you reach the boiling point of the juice, it will start to evaporate and you will see steam coming out of the pot. If you close the pot with a lid, the steam rises up to the lid, and because the lid is much colder than the steam (especially after you put the ice on top), the vapor cools down rapidly and it condenses, becoming a liquid again that you can see in the form of droplets inside the lid. These droplets fall and are collected in the bowl that you have placed in the pot. As the juice boiled, you probably noticed that the amount of water in the bowl increased whereas the amount of juice in the pot decreased. This is because the steam, which was part of the juice, was collected in a separate container. If combined, the distillate and the remaining juice should add up to a similar volume of juice that you had in the beginning.

When you compared the three different solutions at the end (original juice, distillate and remaining juice), the first thing you probably saw was that the color of the remaining juice became much darker and the distillate had no color at all and looked like pure water. And it actually is pure water; it shouldn't have had any sweetness when you tasted it whereas the remaining juice should have tasted much sweeter than the original juice. The reason for this is that sugar is a nonvolatile compound, which means that when you boil any sugary liquid, the sugar will stay behind in the solution and not be transferred into a gaseous state. The water component of the mixture, however, starts to evaporate at about 100 degrees C, resulting in a steam consisting of pure water. Salt is also a nonvolatile substance and if you repeated the activity with broth, your distillate also should have been pure water. If you compared the boiling points of all three solutions at the end, you might have noticed that you can increase water’s boiling point by adding solutes—the higher the amount of solutes, the higher its boiling point will be.

Vinegar, on the other hand—or a mixture of 4 to 6 percent acetic acid and water—is not easily separable by distillation. This is because the boiling points of water (100 degrees C) and vinegar (about 100.6 degrees C) and are too close together to result in a full separation of both components. You should have noticed that the distillate still tasted like vinegar.

More to explore

Distillation, from TutorVista

How Oil Refining Works, from How Stuff Works

Science Activity for All Ages!, from Science Buddies

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Ditulis mukri — Monday 1 February 2016 — Add Comment — Modern Physics