Two drops of glycerol make potassium permanganate change its color!
Put on protective gloves and eyewear.
Conduct the experiment on the tray.
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- Keep young children, animals and those not wearing eye protection away from the experimental area.
- Store this experimental set out of reach of children under 12 years of age.
- Clean all equipment after use.
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- Ensure that all empty containers are disposed of properly.
- Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
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- In case of injury always seek medical advice.
- The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
- This experimental set is for use only by children over 12 years.
- Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
- The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
- The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
- Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.
Pour out the entire contents of KMnO4 bottle (containing 200 mg of sodium sulfate Na2SO4 and KMnO4 mixture, 1% mass fraction of the latter) into a plastic vial.
Add all the saturated calcium hydroxide solution from the Ca(OH)2 (5 mL) bottle.
Close the vial securely and shake vigorously. Ensure the resulting solution is transparent.
Add 2 drops of 10% aqueous glycerol solution to the resulting violet solution.
Close the vial securely and shake vigorously.
Keep watching the solution in the vial! Observe the chameleon color change.
Two drops of glycerol turn violet KMnO4 solution green and then orange.
Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.
Why will potassium permanganate solution change its color?
First of all, let’s understand why it has such a rich, violet color. What happens with potassium permanganate (KMnO4) when you dissolve it in water?
In water, KMnO4 dissociates into two charged particles: K+ and MnO4-. These charged particles are called ions.
The MnO4- ion is responsible for the violet color of the solution.
Now, what happens to this ion when we add some glycerol to the solution? Why do we see a rapid change of color? Why does our vial turn into a chameleon?!
The MnO4- particle reacts with the glycerol.
- First, it gives one electron to glycerol and becomes a green MnO42-.
- Then it turns into brown MnO2.
An atom of manganese Mn in KMnO4 has a considerable positive charge (+7). Mn+7 is a potent oxidant, which means it really wants to get some electrons (e-) into its possession. Usually, Mn+7 takes several electrons at a time – it’s so hungry to “treat itself.” However, the conditions of our reaction are chosen in such a way that a “hungry” manganese cannot hurry and will saturate itself with electrons gradually. And glycerol acts like a caring nurse, feeding the manganese with electrons portion by portion:
MnO4- + e- → MnO42-;
MnO42 + 2e- → MnO2;
MnO2 + 2e- → Mn2+.
A violet permanganate MnO4- (manganese charge +7, Mn7+) receives an electron and becomes a green manganate MnO42- (manganese charge +6, Mn6+). Glycerol “feeds” it with two electrons and the manganate turns into a brown manganese dioxide MnO2 (Mn4+). The manganese dioxide could take two more electrons and turn into a colorless Mn2+. However, in our reaction conditions MnO2 precipitates, so this does not happen.
The reactions in which substances exchange electrons are called the oxidation-reduction reactions. The atoms hungry for electrons are called the oxidants, and the atoms that give away their electrons (“feed” oxidants) are called the reducers. In our case, manganese is the oxidant and glycerol is the reducer.
By the way, we chemists call the atom charges (e.g.: +1 from K+, +7 from Mn+7 or -1 from Cl-) “oxidation levels.” You can read more about them in the description of the experiment "Disappearing Iodine".
Why does the solution turn blue at the beginning?
If you watch this chameleon reaction closely, you will see that a few seconds after the glycerol is added to the solution, the solution turns blue. The blue color appears when the violet (from permanganate MnO4-) and the green (from manganate MnO42-) solutions are mixed. However, it becomes green pretty fast – the solution has less and less of MnO4- and more and more of MnO42-.
Scientists have discovered the form in which manganese can color the solution blue. This happens when it forms a hypomanganate-ion MnO43-. Here manganese has the oxidation level +5 (Mn+5). However, MnO43- is very unstable, and special conditions to acquire it are needed. Therefore, we will not see it in our experiment.
What happens to the glycerol in our experiment?
Glycerol reacts with potassium permanganate, giving away its electrons. We have a huge surplus of glycerol in our reaction (its quantity is approximately 10 times greater than the potassium permanganate KMnO4). So the small amount of glycerol in our reaction turns into glyceraldehyde, and then into glyceric acid.
As we have already found out, glycerol C3H5(OH)3 is oxidized by potassium permanganate. Glycerol is quite a complex organic molecule, and therefore, a reaction that includes it is often not that simple. Glycerol oxidization is a complex reaction, during which a lot of different substances are formed. Many of them exist only for short periods of time before turning into something else, while some can be found in the solution even after the reaction is over. This situation is typical for organic chemistry in general. Usually the substances that are formed as a result of a chemical reaction in the greatest quantity are called the main products of the reaction, and all the others are called the byproducts of the reaction.
In our case, the main product of glycerol oxidization by potassium permanganate is glyceric acid.
Why do we add calcium hydroxide Ca(OH)2 to our solution?
In a water solution, calcium hydroxide Ca(OH)2 dissociates into three charged particles (ions):
Ca(OH)2 → Ca2+(solution) + 2OH-.
In the subway, a shop, a café, or school – in all of these places we are surrounded by different people. And we behave ourselves differently in different places. Even if we are doing the same thing, for instance, reading a book, while surrounded by different people, we do it in a slightly different way: in some places slower, in some places faster, sometimes remembering what we have read, other times unable to remember even a line of what we have read the next day. Potassium permanganate behaves in a special way when surrounded by OH- ions. It captures the electrons from glycerol more “gently,” with no hurry. Therefore, we can see the chameleon change of color.
What will happen if we don’t add to the solution Ca(OH)2?
When there is a surplus of OH- ions in the solution, the solution is called alkaline (or it is said to have an alkaline reaction). If, on the contrary, the solution has a surplus of H+ ions, the solution is called acidic. Why “on the contrary”? Because together the ions OH- and H+ form a molecule of water H2O. If the ions H+ and OH- are present in equal quantities (which means we have water, in fact), the solution is called neutral.
In an acidic solution, the main oxidant KMnO4 becomes really ill-mannered, even rude. It very quickly takes the electrons from glycerol (five electrons at a time!) and manganese turns from Mn^+7 (in permanganate MnO4-) to Mn2+:
MnO4- + 5e- → Mn2+
The last (Mn2+) does not color the water at all. Therefore, in an acidic solution potassium permanganate will become colorless very quickly and the chameleon will not appear.
A similar process happens in the case of a neutral solution of potassium permanganate. We will “loose” not all of the chameleon colors, as in the acidic solution, but two – the green manganate MnO42- will not be formed, which means the blue color will disappear as well.
Can we create a chameleon using something other than KMnO4?
We can! A chameleon from chrome (Cr) will have the following coloring:
orange (bichromate Cr2O72-) → green (Cr3+) → blue (Cr2+).
One more chameleon from vanadium(V):
yellow (VO3+) → blue (VO2+) → green (V3+) → purple (V2+).
However, it is much more complicated to make the solutions of chrome or vanadium compounds change their color as prettily as they do with potassium permanganate. Besides, we would have to continuously add other substances to the mixture. Therefore, a true chameleon, one that can change its color “on its own,” can be made only from potassium permanganate.
Manganese Mn, as well as chrome Cr and vanadium V, are transition metals – a big group of chemical elements that possess a number of interesting properties. One of the peculiarities of transition metals is the rich and diverse coloring of their compounds and solutions.
For example, one can easily get a chemical rainbow from the solutions of transition metals compounds:
Richard Of York Gave Battle In Vain:
Red (iron thiocyanate (III) Fe(SCN)3), iron Fe;
Orange (bichromate Cr2O72-), chrome Cr;
Yellow (VO3+), vanadium V;
Green (Nickel nitrate, Ni(NO3)2), nickel Ni;
Blue (copper sulphate, CuSO4), copper Cu;
Indigo (tetrachlorcobalt, [CoCl4]2-), cobalt Co;
Violet (permanganate MnO4-), manganese Mn.