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Doktor Klawonn
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chemistry riddle
Greetings,
inspired by a post from a user on versuchschemie.de I tried this experiment. It's a chemical riddle.
A colorless liquid is poored into another colorless liquid in a beaker. On addition the liquid turns blue. Eventually on further addition the liquid
turns colorless again.
  
Do you know how it is done? Have fun finding it out!
Dr. K.
P.S.
I uploaded a video of this experiment to my youtube-channel: https://www.youtube.com/watch?v=CILOKdjAvk4
[Edited on 17-1-2015 by Doktor Klawonn]
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Zombie
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I would venture the guess that it is a chemical reaction in the beginning, and a thermal reaction at the end.
Just an uneducated guess.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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macckone
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Looks similar to the blue bottle reaction.
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Doktor Klawonn
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The blue bottle is a nice experiment. However, this reaction has nothing to do with it.
[Edited on 18-1-2015 by Doktor Klawonn]
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unionised
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A way to do it would be to put starch /KI in dilute acid in the beaker and add a solution of an oxidant in dilute alkali.
Initially the oxidant converts the iodide to iodine which gives a blue colour.
Further addition neutralises the acid and then makes the solution alkaline causing the iodine to disproportionate so the colour is removed.
There are probably other possibilities.
[Edited on 18-1-15 by unionised]
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Brain&Force
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I would agree with unionised that the reaction involves iodine, but I think it might be going iodide > triiodide-starch complex > iodate.
At the end of the day, simulating atoms doesn't beat working with the real things...
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HgDinis25
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Maybe a pH indicator. It startes out colorless (either in an acidic or basic medium, depending on the indicator). You add an acid or a base and it
turns blue. And when the medium becames too acidic/basic it goes colorless again. I don't know any indicator that does this, what so ever... A Redox
indicator could work in the same lines too...
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Doktor Klawonn
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No iodine or starch are included. The liquids are of room temperature.
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Zombie
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I beg to differ... I see the heat control on the hot plate is active. Thus my initial answer stands.
There is a chemical reaction that depends on a specific temp., and as the addition continues (cooler compound), that temp fell to negate the reaction.
What is this "room temp" witchery of which you speak?
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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phlogiston
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No, the heat control is off. Notice the left switch (heater) is not lit. The right switch (stirrer) is on.
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Zombie
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Well then color me blind. I'm out.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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The Volatile Chemist
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Sugar, potassium hydroxide, and a blue indicator similar in qualities to phenolphthalein?
Just an 'uneducated guess' 
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Zombie
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One more guess... Copper Sulfate / Sulfuric Acid on the stir'er. and Potassium Hydroxide is the addition.
Copper hydroxide is the blue precipitate.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Doktor Klawonn
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Wow, from your guesses alone one could make a bunch of interesting experiments. I am impressed.
No sugar, no copper sulfate. Two hints:
1. The temperature actually is rising a little (2-5 °C).
2. You can perform the experiment in colors like yellow or pink as well.
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CuReUS
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clock reaction or complex formation
http://en.wikipedia.org/wiki/Chloropentamminecobalt_chloride
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gdflp
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Okay, my guess. The beaker being stirred contains a base such as sodium hydroxide. The liquid you are adding is a solution of an excess of dilute
acid and some thymolphthalein. When a small amount is added, the solution is still basic and it turns a bluish color. When more solution is added,
the pH turns acidic and the thymolphthalein becomes colorless. Substituting the thymolphthalein for phenolphthalein will be the same experiment, but
it will be colorless-pink-colorless instead. I'm not sure what would provide a yellow color.
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Doktor Klawonn
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@gdflp: Yes, you are right. This is the perfect explanation. As for the yellow color, one could use nitrophenole. Mixing thymolphthaleine and
nitrophenole would give a green variant of this experiment.
regards
Dr. K.
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Hegi
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What is funny about this is the fact that everyone came up with those much more difficult possibilities than acid-base reaction with the indicator -,-
We were not expecting it!
Our webpage has been shut down forever cause nobody was willing to contribute. Shame on you all!!!
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CuReUS
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Quote: Originally posted by Hegi  | | What is funny about this is the fact that everyone came up with those much more difficult possibilities than acid-base reaction with the indicator -,-
We were not expecting it! |
someone already guessed that it was a pH indicator
| Quote: Originally posted by HgDinis25 | | Maybe a pH indicator. It startes out colorless (either in an acidic or basic medium, depending on the indicator). You add an acid or a base and it
turns blue. And when the medium becames too acidic/basic it goes colorless again. I don't know any indicator that does this, what so ever...
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but the iodine disproportionation idea was brilliant,really really smart to think of such a thing
| Quote: Originally posted by unionised | A way to do it would be to put starch /KI in dilute acid in the beaker and add a solution of an oxidant in dilute alkali.
Initially the oxidant converts the iodide to iodine which gives a blue colour.
Further addition neutralises the acid and then makes the solution alkaline causing the iodine to disproportionate so the colour is removed.
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[Edited on 20-1-2015 by CuReUS]
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Etaoin Shrdlu
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I figured the blue was thymolpthalein but was assuming it was in the stirred beaker and couldn't think of a mechanism. Just goes to show.
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phlogiston
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There is one thing that does not seem to fit with this explanation (pH indicator+dilute acid)
If you observe the video, you can see that the first additions are poured into the vortex and give rise to a faint blue color in the vortex which
dissapears upon mixing. Only when a critical amount of liquid has been added does the color stay (as is commonly observed in titrations).
If it was an indicator+acid added to an alkaline solution, you would expect to see initially a relatively strong color in the vortex which is diluted
but persists upon mixing, and not a critical treshold.
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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gdflp
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I did too! It was bothering me how that was possible until I had that "duh!" moment.
| Quote: Originally posted by phlogiston | There is one thing that does not seem to fit with this explanation (pH indicator+dilute acid)
If you observe the video, you can see that the first additions are poured into the vortex and give rise to a faint blue color in the vortex which
disappears upon mixing. Only when a critical amount of liquid has been added does the color stay (as is commonly observed in titrations).
If it was an indicator+acid added to an alkaline solution, you would expect to see initially a relatively strong color in the vortex which is diluted
but persists upon mixing, and not a critical threshold. |
If you watch carefully, it never really loses the blue color entirely, I think it appears this way because the thymolphthalein is so dilute in the
acid, it is not noticeable when only a small amount of the solution is poured in. As more of the acid is poured in, the concentration of
thymolphthalein starts rising to a point where it becomes noticeable. The appearance of a critical point may just be due to the solution being poured
in quickly and not being thoroughly mixed quick enough.
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Pickardjr
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The Chem professer did this one, cant remember what he called it, something like a chemical time clock. The reaction would change colors and then back
again just like clock work at specific time intervals. Don't know if thats what it is.
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Molecular Manipulations
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A clock reaction usually goes in one direction, like the iodine clock reaction. It can be calibrated to change colors at the time the chemist wants, by adjusting the quantity of the limiting reactant,
like a clock.
An oscillating reaction also goes in one direction, but when one reactant (generally the one or one of the ones responsible for the color) gets to a
high enough concentration, it reacts again and goes back to the original color or lack thereof. This happens until the limiting reactant runs out,
which allows quite a few "oscillations". It appears to oscillate, but is in fact not really going backwards, otherwise, what would keep it from going
forever? An actual oscillating reaction cannot happen.
[Edited on 2-2-2015 by Molecular Manipulations]
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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woelen
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Indeed, true oscillation is not possible in chemistry. One of the reactants is the 'driving force' and as soon as that is depleted, the
oscillation-like behavior stops and a steady state situation is reached.
It can be compared with a relaxation oscillator from electronics, as long as the source of charge is working (e.g. a power supply) the relaxation
oscillator still works, but when the source of charge becomes depleted, the 'oscillations' become slow and finally come to an end.
http://woelen.homescience.net/science/physics/exps/relaxatio...
The principle behind the 'oscillators' mentioned on this webpage are the same as the ones, driving chemical 'oscillators'.
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