Sciencemadness Discussion Board

Sequence of 'color reactions'

jan1234 - 12-12-2022 at 13:08

Hi!

When i had my first chemistry lesson in school (in 8th grade, at the age of 14), my teacher chose a pupil to 'assist' her:
She had several beakers in a row on her desk, only the first one containing a visible amount of clear liquid. The other beakers of course weren't empty, but contained only a small amount of something, which wasn't quite visible. She told her 'assistant' to pour the liquid from the first one in the second, then from the second in the third one and so on.
In each step, the color of the liquid changed and everyone was excited.

Now, (20 years, a master's degree in chemistry and an uncompleted PhD later), i remembered this and wanted to recreate this (or a similar) experiment for the 'show effect'. Not to teach any chemical background, my 'audience' doesn't know anything about chemistry.

This gets quite tricky, since the reactions shouldn't disturb each other and furthermore, i can't use potentially dangerous chemicals - 'dangerous' in terms of the german laws for use of chemicals in schools. (On one side those laws are quite restrictive and on the other side most chemstry teachers i knew/know don't 'dare' to use other substances than metal salts, common pH-indicators or some organic dyes. thus, i'm sure my teacher didn't use anything 'exotic'.)

So first, i'm collecting colorful reactions which fit in my restrictions. I will look into the interaction of the reagents from the different reactions later, since a list of possible reactions will be useful otherwise for me. If i run into too much disturbances, i just will perform them seperately i guess.

For now i have:
- Turnbull's Blue (a solution of K3[Fe(CN)6] poured into a beaker with a little bit of a Fe(II)-salt at the bottom as the 'invisible' reagent)
- Pouring water on dehydrated copper(II)sulfate (as the 'invisible' reagent on the bottom of the beaker), afterwards poured into a beaker with a little aqueous ammonia to form the tetraammine-complex
- EDTA or salicylate complexes (i don't think this will work in a sequence. with various metal ions in the solution)
- precipitating chloride with AgNO3, then dissolving it with aqueous ammonia

Not very much until now. Of course, i could use a row of different pH-values and just go up (or down) the pH scale on an indicator, but somehow i think that's too easy.

Allthough i spent a lot of time synthesizing colorful metal-organic complexes, i'm a bit rusty on the topic 'simple colorful inorganic reactions' (maybe i'm kind of blind for some obvious reactions, since having mainly researched on Os/Ir compounds might make me forget the simple things from analytical chemistry in the first semester 15 years ago).

Are there any ideas/hints you can give me?


[Edited on 12-12-2022 by jan1234]

DraconicAcid - 12-12-2022 at 13:40

I do a version of this where I pour one solution into several beakers, getting different colours. Not a sequence, but a dilute solution of iron(III) into beakers containing tea, potassium iodide, sodium salicylate, potassium thocyanate, and potassium ferrocyanide (you could use ferricyanide and a bit of ascorbic acid, I think, as a reductant).

(I'm in a similar situation- I teach simple stuff after years of Rh/Ir chemistry.)

Deathunter88 - 13-12-2022 at 02:37

Maybe this is what you're looking for:
https://www.flinnsci.ca/api/library/Download/fbe399585eab47d...

BTW anhydrous CuSO4 takes a while to fully dissolve so might not give the effect you are going for.

Bedlasky - 13-12-2022 at 03:35

You can try reduction of permanganate. Pour permanganate solution in to strongly alkaline thiosulfate solution --> green manganate. And than pour this solution in to some acid --> colorless Mn2+. Make sure that you have acid strong enough to make manganate solution strongly acidic.

Another nice colorfull sequence is displacing (pseudo)halides in Mo(V) complexes. Brown [MoOBr5]2- + conc. HCl --> green [MoOCl5]2- + KSCN --> very dark red [MoO(NCS)5]2- + NH4F --> red [MoOF5]2-. I didn't try last step, but fluoride form usually very strong complexes, so it should work. [MoOBr5]2- can be prepared by heating ammonium heptamolybdate in conc. HBr (do it in fumehood because of Br2 and HBr vapors) or by mixing Mo(VI) in conc. HBr with Mo(III) (which can be prepared by reduction of molybdate with zinc powder in 1+2 HBr).

jan1234 - 13-12-2022 at 06:49

Quote: Originally posted by DraconicAcid  
(I'm in a similar situation- I teach simple stuff after years of Rh/Ir chemistry.)


yeah, while my job only requires me to know basic stuff (mainly concerning additives in the food industry), chemistry is still my passion. i'm teaching at a university for agriculture, so the students are... rather not-so-good in chemistry and physics. they are only required to get over the basics in their first semester and won't have to know 99% of the stuff they're told later on anymore.
so their motivation isn't very high to really understand things; and those who try to still have a hard time - my boss (professor in chemistry) is happy if most of them are able to calculate a pH value from a given acid concentration in the final exam :(
Quote: Originally posted by Deathunter88  
Maybe this is what you're looking for:
https://www.flinnsci.ca/api/library/Download/fbe399585eab47d...

BTW anhydrous CuSO4 takes a while to fully dissolve so might not give the effect you are going for.


Thank you! The described effects would match the lesson i remembered, i will try this as soon as i can.
Quote: Originally posted by Bedlasky  
You can try reduction of permanganate. Pour permanganate solution in to strongly alkaline thiosulfate solution --> green manganate. And than pour this solution in to some acid --> colorless Mn2+. Make sure that you have acid strong enough to make manganate solution strongly acidic.

Another nice colorfull sequence is displacing (pseudo)halides in Mo(V) complexes. Brown [MoOBr5]2- + conc. HCl --> green [MoOCl5]2- + KSCN --> very dark red [MoO(NCS)5]2- + NH4F --> red [MoOF5]2-. I didn't try last step, but fluoride form usually very strong complexes, so it should work. [MoOBr5]2- can be prepared by heating ammonium heptamolybdate in conc. HBr (do it in fumehood because of Br2 and HBr vapors) or by mixing Mo(VI) in conc. HBr with Mo(III) (which can be prepared by reduction of molybdate with zinc powder in 1+2 HBr).


Thank you too!
While i think i can't get my hands on Mo compounds, i will look at those out of sheer personal interest.

As you mentioned Mn, i remember other experiments that go through the oxidation states of Mn. A teacher of mine did an experiment, where he modified the 'chemical chameleon' experiment (look here (wikipedia) or here (german pdf on the procedure with a nice picture)):
he took a large petri dish (about 20cm diameter) and put an icecube in the middle of it. he then slowly poured the reagents on the icecube and the reaction (slowed down by the decreased temperature) proceeded from the middle of the petri dish to the edge. he also put the petri dish on an overhead projector, projecting the colorful image on the wall.

[Edited on 13-12-2022 by jan1234]

pantone159 - 13-12-2022 at 07:27

The book Chemical Magic, by Leonard A. Ford, has the following demonstration:

Jug of Mystery.

Action: 'Water' is poured from a jug into a series of 'empty' water glasses. The glasses become become filled with liquids colored: red, white, blue, black, green, amber.

You need: In the jug, 5g of ferric ammonium sulfate in 500 ml of water; in each of the glasses about a half gram of the following solids dissolved in a few ml of water: 1) potassium thiocyanate, 2) barium chloride, 3) potassium ferrocyanide, 4) tannic acid, 5) tartaric acid, 6) sodium hydrogen sulfite.

Pumukli - 13-12-2022 at 11:32

For the sequence of pourings:

1st glass of diluted (pH 10 or so) NaOH - colourless

2nd glass a few drops of neutral (alcohol?) solution of phenolphthaleine : turns pink after mixing in glass 1

3rd glass some acid in the bottom - turns colourless again after mixing with glass 2

4th glass a few drops of a different acid/base indicator (say bromothymolblue or something like that) in acid - turns green or blue or anything but pink or colourless after mixing with glass 3, depending on the acid color of the dye

5th glass small amount of NaOH in the bottom - turns some colour but pink after mixing in glass 4, depending on the high pH colour of the dye from glass 4 plus the pink colour from the phenolphthaleine

Of course, you have to put the proper amount of base and acid in each glass to ensure the required pH swing, otherwise the "magic" would fizzle! :D

Edit: I was thinking about the white color. It could be fairly easily achieved at the beginning by mixing some finely powdered CaCO3 into glass 1. So the colour sequence would be: colourless - white - pink - colourless - blue/green/whatever - pink plus green/blue/whatever

Using CaCO3 however may restrict the usable acids in the later stages to HCl and HNO3 to avoid precipitate (hence whitish/milky appearance) formation. It may or may not be a real problem though.


[Edited on 13-12-2022 by Pumukli]

Bedlasky - 14-12-2022 at 01:59

Jan1234: That chemical chameleon with temperature gradient sounds nice!

Molybdenum chemistry is really complex, this maybe experiment for students interested in chemistry than for ordinary students.