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Chemistry: Demo: Recrystallization


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About this Lesson

  • Type: Video Tutorial
  • Length: 7:09
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 77 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Laboratory Techniques (10 lessons, $12.87)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Ever wonder how they make rock candy? Rock candy is sucrose, cane sugar that has been crystalized into great big gigantic crystals. The technique that they use is something called a crystallization. I'm going to demonstrate the technique of crystallization for you using acetyl salicylic acid, which is the active ingredient in aspirin. The crystallization is a way to purify a compound because the crystals form in a very pure form.
What I have here is 5 grams of acetyl salicylic acid. It's a white solid and it's micro crystals. They're little tiny crystals. The technique involves dissolving this acetyl salicylic acid in water, but what we're going to do is we're going to dissolve it in water at its insolubility limit at high temperature. So in other words, we're going to put it into a beaker and add some water, and we're going to heat some water up, and we're going to add just enough water until it's saturated at the high temperature at 100C. What does saturated mean? That means that the absolute maximum amount of acetyl salicylic acid has been put into the water and there's no solid present, okay? So I could take this and warm it up on the hot plate here until the water is boiling and keep adding little bits of water until I get exactly to the point where I've added just enough water to dissolve everything and no more.
Now to make it more interesting what I'm going to do is add some food coloring to this solution. Think of the food coloring as some sort of impurity that might be present in the aspirin. It doesn't have to be food coloring. But the point is, I can do this. I can add food coloring to this mixture. So now, not only do I have a mixture of acetyl salicylic acid and water, but now I've got food coloring as well. Again, I'm going to add enough water until it is fully dissolved but we're right at the solubility limit.
I've done that already here. This is a beaker in which I have dissolved the acetyl salicylic acid. You can actually see there are little bits on the outside so I might have to add just a little bit more water but this was again 5 grams of acetyl salicylic acid with just enough water to get to its solubility limit. Now what do you do? Well it is saturated at the high temperature but what happens when we take it off the heat and allow it to cool - cool all the way down to room temperature? Well it's saturated at the high temperature but as we cool it down it becomes super saturated. Its solubility increases as you heat it up so as you cool it down, its solubility goes back down. When the solubility goes back down, what happens? Well what happens is the material crystallizes, and that's what's gone on here. This is, again, a beaker that I prepared before and it's sort of this sludgy mess now that has the food coloring and the water and the crystalline, the acetyl salicylic acid. Now the crystals are not as nice as these crystals but they're still nice crystals. The point that I want to make is that if we have been able to affect purification here, then when we collect the crystals, the food coloring ought to go with the water and so the crystals ought to come out perfectly colorless again. Just as the crystals of the sugar are perfectly colorless. So let's take a look.
The way you collect crystals, after you do a crystallization, is by a vacuum filtration. So here I have a vacuum filtration apparatus. Again, it's a vacuum filter flask with a Büchner funnel and a piece of filter paper that's place in there to cover the holes, and we collect by pulling a vacuum on it. So you turn on the vacuum and then you wet the filter paper and then you start collecting the crystals. Now what I'm going to do is wash the crystals with a little bit of water. Let me stop it right here and show you what it looks like. At this point what it looks like crystals that are pale blue. And why they are pale blue is not because the blue is incorporated into the crystal. The crystals are fairly pure. The blue is coating the crystals, and what we're going to do is we're going to take a little bit of water and we're going to rinse these crystals and we're going to lose a little bit of product and obviously I could have done a better job of getting the rest of this out of here but I'm just trying to illustrate the point here. Now we're going to add a little bit of water and wash the crystals, and this ideally would be very cold water because again the crystals are not soluble in cold water, but let's do that. And when I'm done I'll show you that the crystals are colorless or nearly colorless.
So all I did was wash the crystals in some cold water and ideally I would have used very cold water, but now you can see that the crystals are almost perfectly white. Now if I had done this more carefully they would have been even more white, and if they weren't as white as I wanted them to be I could have repeated this process again, recrystallized it, I wouldn't add more food coloring because remember, that's the impurity, but if I repeat this process of crystallization, the crystal product that comes out the second time is even going to be more purer than the first time.
So what have we done? Well I've demonstrated the concept of a crystallization where you take a product that isn't pure. In this case the impurity was food coloring, bring it to its solubility limit at high temperature and then cool it down slowly and the material will crystallize and the crystals will be very pure. Then to collect the crystals what you do is a vacuum filtration. You could do a gravity filtration but its faster to do a vacuum filtration. The product, once it's washed with a little bit of cold solvent, is typically very clean and very clean. That's a technique, another technique, for effecting the purification of a compound.
Lab Instructional Demonstrations
Lab Demos
CIA Demonstration: Recrystallization Page [1 of 2]

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