Do not repeat the experiments shown in this video!
So, today I want to tell you about the hardest metal on Earth – chromium. As usual, let’s first see where chromium is in the periodic table of chemical elements. Oh, there it is, hiding behind the vanadium in the 6th group.
And yes, I did not deceive you, it is true that chromium has the highest hardness of all metals, based on the Mohs scale that shows the resistance of a mineral to scratching.
Externally, pure chromium is a lustrous white metal that does not tarnish in air. To demonstrate the main feature of chromium, namely its topmost hardness, I will scratch an ordinary silicate glass with it.
As you can see on this chart, the hardness of the glass is much smaller. As usual, you cannot fool physics, chromium easily scratched the surface of the glass, leaving a clearly discernible markings. Although, I think that the hardness of chromium is very much dependent on its purity.
Metallic chromium is obtained from the chromium ore mineral by reducing it using carbon to produce ferrochromium, an alloy of chromium and iron, which is used in production of stainless steel.
We will now turn to the chemical properties of chromium which this metal is highly enriched with. The fact is, even the name of the metal, chromium, comes from the Greek "khroma" meaning color.
This name came to pass due the fact that chromium compounds can have almost all the colors of a rainbow. In a laboratory the most common chromium compound is potassium dichromate.
It is used often in analytical chemistry, or to clean chemical dishes from the very stubborn dirt. Also, this substance is used as an intermediate compound in the production of chromium metal from ore.
In this compound chromium has an oxidation state of plus six.
By the way, if you mix the potassium dichromate with any reducing agent, for example aluminium powder, and then ignite it, you will see a very violent reaction that will produce particles of chromium metal.
Potassium dichromate is very soluble in water. If you add a little bit of alkali, i.e. sodium hydroxide, to the orange solution of potassium dichromate, in an alkaline environment the dichromate will turn into chromate, and become yellow.
In the oxidation state of plus six, chromium may have a yellow and an orange color, depending on the acidity of the environment. However, that’s not all.
If you add acid to the solution of potassium dichromate and also any reducing agent, for example potassium metabisulfite, there will be a reaction in which sulfur from metabisulfite will donate electrons to chromium, and will be restored to its trivalent state, therefore changing its color to green.
Compounds of trivalent chromium have a green color, this is possible to prove with another reaction. In order to do it, let’s take another orange chromium compound - ammonium dichromate, in which chromium also has an oxidation state of plus six.
Now I’m igniting this orange powder and what do we see? Ammonium dichromate started decomposing by itself, forming a dark-green substance – chromium oxide.