Lab 5.1: Fractional Distillation
In Chapter 3, we learned that boiling point is a characteristic property of substances. In Lab 3.4 you compared the boiling temperature of water to the boiling temperature of isopropyl alcohol. It was discovered that no matter how much of each liquid you had, the water would always boil around 100 degrees celsius, and the alcohol would always boil around 80 degrees celsius. What would happen if we mixed them together and tried to boil them?
Well, I can tell you because it's
not a secret: the two liquids still boil at their own boiling points even though
they are mixed together! That means that the alcohol in the mixture will begin
to boil as soon as the mixture reaches 80 degrees.
|I'm sure you remember that when a liquid boils, it turns to vapor. If we collect those vapors (in a rubber hose), we can cool them (in a test tube in a beaker of cold water), and condense the substance back into a liquid. So, at 80 degrees the alcohol evaporates from the mixture leaving the water behind. Then we can continue to heat the remaining water until it is 100 degrees and collect that the same way we collected the alcohol (by evaporating then condensing). This process of removing a pure liquid by evaporating it is known as "distillation." This same process can be used to separate pure water from salty or muddy water. It is also used to remove ethanol from wine to make stronger drinks ("distilled spirits"... don't try this at home because it's illegal - and it's not healthy!).|
When distillation is used to separate a mixture of liquids that have different boiling points, this is called "fractional distillation." This same process is used to separate the different components of crude oil into such products as kerosene, gasoline, propane, etc. For more information, see the article at How Stuff Works.
The equipment you will use is similar to what you used in Lab 3.4. You will boil the mixture of liquids in a test tube with boiling chips over a burner. You will measure the vapor temperature with a thermometer, and collect the vapor in a rubber hose that leads to a beaker of cold water. (See page 91 in your text). This lab is spearated into 6 sections (A through F). In Part A you will test the characteristic properties of the mixture. In Part B you will boil the mixture and record the temperature of the vapors every 30 seconds until it has all boiled. Your data should give you a clear idea of what the two boiling points were (remember that temperature does not rise while a substance is boiling). In Part C you will swicth the collecting test tubes after the first boiling point is reached so that you separate the liquids. In Parts D& E you will test each "fraction" (or purefied liquid) for characteristic properties. Finally, in Part F you will summarize all of your findings.
This is potentially one of the more dangerous labs we do in this course. Isopropyl alcohol is flammable, and you will be using it over a burner flame. Be very careful as you are working around the burner. Check your boiling test tube for any cracks or defects before you begin heating. Be sure that you do not have any dangling clothing or hair. Safety goggles and aprons are a must.
a) Summarize what you did in this lab. Be sure to include how you used the different characteristic properties of the two liquids to separate the mixture.
b) Use your data to discuss whether you were able to separate the mixture entirely. What could have been done to further purify each fraction?
c) Identify what you had in each fraction (the test tubes of liquid you collected) using your data as evidence.
d) Discuss how distillation can be used in real-life.
e) Discuss any lab error or other factors that may have affected your results.
Your Lab Report Should Include:
Pre-lab materials (purpose, materials, procedure)
4 graphs (temperature vs. time for the mixture and each of three fractions)
1 discussion and conclusion
And now, a graphic from an advanced distillation web site to put on your computer screen so everyone will think you're very smart:
back to Lab
back to Physical Science Lab Resources