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Chemistry: Demo: Natural Acid-Base Indicators


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About this Lesson

  • Type: Video Tutorial
  • Length: 4:40
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 49 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Acids and Bases (10 lessons, $14.85)
Chemistry: Acid and Base Strengths (6 lessons, $10.89)

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.

About this Author

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It turns out nature has her very own universal indicators, compounds that are different colors in different pH's. And one example of that is the hydrangea, which grows blue or pink, depending on the pH of the soil that it's growing in. And you can, in principle, affect the color of the flower by adding lime to the soil to make it more alkaline, or pine needles or something like that to make it more acidic. And a really convenient example of a universal indicator in nature comes from purple cabbage. The next time Mom brings home a cabbage and says, "Look, Johnny, what we're having for dinner tonight", you can say, "Sorry, Mom, can't eat that. I need it for my science project."
What you do is you take this cabbage and chop it up, and then just soak it in water for an hour or two, and what comes out is a purple liquid. And I've taken some of that purple liquid and I've put it inside to a series of test tubes. And what we're going to do is we're going to take a look at, at least semi-quantitatively, the pH of a series of different household ingredients. And we're going to do them, going from a strong base, to a weak base, to nothing, to a weak acid, to a strong acid. And for our strong base we're going to look at Drain-O, so let's take a look at Drain-O.
Remember, Drain-O is sodium hydroxide. And so I'm just going to pour a little bit of Drain-O into the test tube, and we're just sort of pouring some in to see the effect. We're not really concerned about the concentrations exactly. Obviously, the pH's are going to depend on exactly how much Drain-O I put in, but you can see that that's turned from purple to a fairly nice greenish-yellow. So that's the strong base.
And now, let's look at a weak base. Bicarbonate of soda, baking soda, is a good weak base. So let's put some baking soda in the test tube number 2. Let's give this one a little more of a swirl. So that's the weak base. So now, we've gone from yellowish-green to blue, and purple is the neutral.
For our weak acid let's take a look at a shot of vinegar. Remember vinegar is acetic acid solution. So we'll pour in a little bit of vinegar. And now, we have this scarlet color, so that's pretty nice.
And finally, I couldn't think of a strong acid that you might have in your house readily available, so what I have here is some 1 molar hydrochloric acid. Roughly speaking, 1 molar hydrochloric acid is the same pH as your stomach. So this is what happens when you eat cabbage. When it hits your stomach, it turns a very deep red. So the cabbage juice, once it hits your stomach, turns a very deep red.
So you can see now that we have a series of different colors. This is one compound. My guess is that the purple color comes from a kind of compound called anthocyanine, related to chlorophyll. If you've ever noticed in the fall, when the leaves turn colors, what happens is the chlorophyll is being catabolized. The plant is sort of getting all its chlorophyll back and storing it up, or using it and storing it up for next year. And what's left over is the anthocyanine, which is why the leaves turn sort of a purplish-red color.
Anyway, so again, nature's universal indicators. You can get different colors - and now, you can use this as a scale for measuring a bunch of other things, for instance, pop has carbonic acid and, if it's cola, it's got some phosphoric acid. Soap is the salt of a carboxylic acid, so it's going to be a weak base. Milk of Magnesia is magnesium hydroxide, so that's going to be a strong base. Sodium chloride, remember, is not very acidic or basic at all. It's essentially neutral. And ammonia, you can mop your floors with this. Ammonia is a weak base, so we'd anticipate that, if we put it in the tube, it would come out to be roughly yellow.
So that's about all I have to say today. Go home, play with them. Be careful with them. Obviously, some of these things, like the ammonia or like the Drain-O, are very dangerous. But, if you work with these and you're careful and you wear the proper eye protection and that sort of thing, I think you can have a lot of fun for essentially no cost.
Acids and Bases
Acid and Base Strengths
CIA Demonstration: Natural Acid-base Indicators Page [1 of 1]

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