Uploaded October 2016 | Updated September 2026, 2 weeks ago
A copper coin is placed in an acidified HgCl2 solution. Although the video is 6x faster it immediately forms a dark layer of Mercury on the surface. After some minutes the coin can be polished to yield a shiny silver Mercury coating.
The redox potential of Hg(II) to elemental Mercury is close to the Ag(I) to Silver potential so like in the video on Silver coins it is a redox reaction with the difference to the silver coin that the coating can be polished without removing it.
This is because it forms an amalgamate with the copper which sticks to the surface and stays quite long. After some months the coating will be gone especially if kept at elevated temperatures
Note: At this point we leave the explanation as it is. It would be an interesting discussion about alloys and amalgams but to really talk about those we need to introduce solid state chemistry first to set the basics. This takes far too much space and will probably be a whole lecture video series in the future. .
A copper coin is placed in an acidified HgCl2 solution. Although the video is 6x faster it immediately forms a dark layer of Mercury on the surface. After some minutes the coin can be polished to yield a shiny silver Mercury coating.
The redox potential of Hg(II) to elemental Mercury is close to the Ag(I) to Silver potential so like in the video on Silver coins it is a redox reaction with the difference to the silver coin that the coating can be polished without removing it.
This is because it forms an amalgamate with the copper which sticks to the surface and stays quite long. After some months the coating will be gone especially if kept at elevated temperatures
Note: At this point we leave the explanation as it is. It would be an interesting discussion about alloys and amalgams but to really talk about those we need to introduce solid state chemistry first to set the basics. This takes far too much space and will probably be a whole lecture video series in the future. .
![Copper Chemistry: Cu(II)/As(III) chameleon [no sound, 2x speed]](https://i.ytimg.com/vi/xHic2pew9bI/mqdefault.jpg "Copper Chemistry: Cu(II)/As(III) chameleon [no sound, 2x speed]
In this video, we show another way to make a beautiful spectrum of colors using Copper chemistry. The reaction is the following: Arsenic(III) is oxidized to Arsenic(V), while Copper(II) is reduced to Copper(I), all happening at a high pH value. But of course, we cannot just add cations and so different precipitates form and dissolve in this process creating many different colors.
We start off with a CuSO4 solution, which is light blue. To this, we add some NaAsO2. We already showed you Copperarsenite in another video. The blue-green precipitate also leaves a greenish aqueous layer on top. When we add KOH the remaining CuSO4 forms the dark blue Cu(OH)2. You can see the surface of the KOH turn dark blue. There is also an equilibrium between Arsenite and
Hydroxide and so the overall color turns more blueish in this process. Now the actual reaction also requires some heat. The KOH dissolving in so little water already causes it to become quite hot. This is why you suddenly see red-brown spots appearing everywhere. When we finally heat it all the Cu(II) is reduced to Cu(I) forming Cu2O.
Much like with our silver video, this is another case where some redox-reactions are just much easier accessible when high pH values are used.
For Arsenic(III to V) the potentials are 0,56 and -0,71 for pH=0 and 14 and for Copper(II to I) the potentials are 0,159 and -0,08.
To reduce Copper(II) to Copper(I) the redox potential of the reducing agent, here Arsenic(III) needs to be lower, which is only fulfilled at higher pH values.")
![Sulfur Chemistry: [Fe(CN)5NOS] from Sodium Polysulfide](https://i.ytimg.com/vi/xm7IYOrwQfA/mqdefault.jpg "Sulfur Chemistry: [Fe(CN)5NOS] from Sodium Polysulfide
There is an interesting reagent called Sodium Nitroprusside. It is sometimes used to identify several ions and is capable of distinguishing between Sulfite and Sulfide for example as well as forming many colored complexes. As we showed in our Cadmium Sulfide video, Sodium Polysulfide, made from elemental Sulfur and Sodium Hydroxide seems to be an alternative for the rather unstable Sodium Sulfide. So we tried to make the famous Sulfide test using Nitroprusside and a selfmade Polysulfide solution.
Here is the result. Note, as we didnt have any other source of H2S or Sulfide we could not compare this to the real test. So we assume the compound formed should have the composition as given in the title.")
