Separation by distillation

Introduction

Do you like cooking? If you have helped in the kitchen at home, you have probably heated up lots of liquids, like water, milk, and soup. Did you notice that once the solution boils, a lot of steam develops; and have you ever wondered what the steam is made of? 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 are actually mixtures of many different substances. But what if you want to separate the individual components from a liquid mixture? 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 in the food industry.

Distillation exploits the differences in the volatility of the mixture’s components, which means that every compound has a different boiling point and starts to vaporize (change from its liquid to gaseous phase) at a different temperature. When distilling, you heat up the mixture so that the component with the lowest boiling point evaporates first, leaving the other solutes behind. The vaporized component in the gas phase 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 mixture that includes a non-volatile solute, then 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 up? Make your own distillation device and find out!

Materials

Preparation

Procedure

Observations and Results

Juices are usually very sweet. This is because fruits contain a lot of fruit sugar, called fructose. However, more than 80% of most fruits 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, which is pretty hot. 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 apple 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 at all 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 non-volatile compound, which means that when you boil any sugary liquid, the sugar will stay behind in the solution and will not be transferred into the gas phase. The water component of the mixture, however, starts to evaporate at about 100ºC (212ºF), resulting in a steam consisting of pure water. Salt is also a non-volatile substance and, if you repeated the experiment 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 the boiling point of water by adding solutes; the higher the amount of solutes, the higher the boiling point will be.

Vinegar, on the other hand—or a mixture of 4-6% acetic acid and water—is not easily separable by distillation. This is because the boiling points of water (100ºC) and vinegar (about 100.6ºC) 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.

Cleanup

More to Explore

• Distillation, at Youtube
• How Oil Refining Works, from Science, How stuff works
• Science Activity for All Ages!, from Science Buddies

Credits