Sciencemadness Discussion Board

chemistry riddle

Doktor Klawonn - 17-1-2015 at 12:28

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.

vlcsnap-2015-01-17-20h12m26s14.png - 470kB vlcsnap-2015-01-17-20h12m50s38.png - 449kB vlcsnap-2015-01-17-20h12m57s106.png - 389kB

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]

Zombie - 17-1-2015 at 15:28

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.

macckone - 17-1-2015 at 19:45

Looks similar to the blue bottle reaction.

Doktor Klawonn - 18-1-2015 at 02:15

Quote: Originally posted by macckone  
Looks similar to the blue bottle reaction.


The blue bottle is a nice experiment. However, this reaction has nothing to do with it.

[Edited on 18-1-2015 by Doktor Klawonn]

unionised - 18-1-2015 at 02:24

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]

Brain&Force - 18-1-2015 at 12:26

I would agree with unionised that the reaction involves iodine, but I think it might be going iodide > triiodide-starch complex > iodate.

HgDinis25 - 18-1-2015 at 12:49

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...

Doktor Klawonn - 18-1-2015 at 14:41

No iodine or starch are included. The liquids are of room temperature.

Zombie - 18-1-2015 at 14:55

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?:D

phlogiston - 18-1-2015 at 15:16

No, the heat control is off. Notice the left switch (heater) is not lit. The right switch (stirrer) is on.

Zombie - 18-1-2015 at 15:32

Well then color me blind. I'm out.

The Volatile Chemist - 18-1-2015 at 16:16

Sugar, potassium hydroxide, and a blue indicator similar in qualities to phenolphthalein?
Just an 'uneducated guess' :)

Zombie - 18-1-2015 at 23:06

One more guess... Copper Sulfate / Sulfuric Acid on the stir'er. and Potassium Hydroxide is the addition.
Copper hydroxide is the blue precipitate.

Doktor Klawonn - 19-1-2015 at 03:22

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.

CuReUS - 19-1-2015 at 03:29

clock reaction or complex formation
http://en.wikipedia.org/wiki/Chloropentamminecobalt_chloride

gdflp - 19-1-2015 at 07:40

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.

Doktor Klawonn - 19-1-2015 at 08:22

@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.

Hegi - 19-1-2015 at 09:30

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 -,- :D We were not expecting it! :D

CuReUS - 20-1-2015 at 03:04

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 -,- :D We were not expecting it! :D


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...


but the iodine disproportionation idea was brilliant,really really smart to think of such a thing:o:o
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.



[Edited on 20-1-2015 by CuReUS]

Give That man a Medal.jpg - 17kB

Etaoin Shrdlu - 20-1-2015 at 04:19

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.

phlogiston - 20-1-2015 at 05:12

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.

gdflp - 20-1-2015 at 10:53

Quote: Originally posted by Etaoin Shrdlu  
I figured the blue was thymolpthalein


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.

Pickardjr - 1-2-2015 at 18:18

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.

Molecular Manipulations - 1-2-2015 at 18:46

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]

woelen - 2-2-2015 at 10:22

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'.

Pickardjr - 2-2-2015 at 12:25

Off subject a bit, theres another little demo that the teacher did. It was an ammonia fountain, I pretty sure he told me had luminol in the mixture. When the light were turned off it glowed as the liquid would fountain up through a tube into inverted r/b flask.
I cant remember what the other demo was cause it was kinda boring compared to a lot of other things, teachers aren't allowed to do much anymore for demo.