Why is copper sulphate blue?

Hi Laura, I can think of two possibilities:
A1. Maybe as the copper plates out of the solution, something has to fill up the holes left there. Nature abhors a vacuum. So it sucks air into it, which, as anybody knows who has looked at the sky, is blue :-)

A2. An alternate explanation from my high school chemistry book is that the Cu⁺⁺ ions which are in the Cu⁺⁺SO4^-2 solution are blue, even though Cu⁰, metallic copper, is orange-ish. Happy homework! What grade are you in; it will help with getting better answers. Thanks.

A. copper sulphate is blue, not because of air, but because it has been oxidised to the Cu²⁺ ion which is a d-block metal ion. The electrons in the metal ion can transition between different energy states and therefore the ion absorbs light energy and can be seen as being BLUE.

A lot of Cu²⁺ compounds will be blue because of these transitions, but the ligand (the thing attached to the metal ion) can effect colour.

Thanks Robert, you're probably right! The forum is a one-room schoolhouse with some readers 3rd-graders, and some post-docs, so it's sometimes hard to know what depth students require and can understand :-)

Regards,

A. OK. I've been studying this in year 12 chemistry at the moment and it's due to its emission spectra. The theory is that when a copper salt is in aqueous form the ions split up resulting in a Cu 2+ ion.

Energy from light and other outside sources is absorbed which can then bump electrons to a shell of higher energy level. When the electrons fall back to their original shell due to not having enough energy to orbit the nucleus in that shell they must release some energy and they do so by giving off light, in the case of copper ions the amount of energy given off produces blue light. Hence the colour (that's how they spell color in Australia where I'm from) of the copper solutions. In the case of dehydrated copper sulphate there are no copper ions free, the copper is ionically bonded to the sulphate.

A. Light reflects off of things, but some colors reflect more than others. Which colors reflect the best has to do with the material. We usually just memorize this -- oranges are orange, apples are red. But the "why" question is not all that easy to answer.

It has to do with the atoms that make up the material, and how they are arranged among themselves. Copper is indeed red-ish by itself, and sulfur [affil links] is yellow, by itself. But when combined, their atoms work together to form an overall compound, copper sulphate. This compound is different from the atoms that formed it. It is blue. It does not conduct electricity very well (copper does....), it cannot be pulled into a wire (like copper), it cannot be mixed with charcoal to make gun powder (sulfur....). The electrons which circle the copper atom interact with the light in a way to make copper color. Same with sulfur. But when the compound is formed, then the electrons do not circle just one single copper, but work their way around the copper, the sulfur, some oxygen atoms etc etc. The electrons see a bigger picture, and interact with incoming light in a different way. We see this as a color change.

You can actually watch a chemical reaction take place -- mix two things together, each of which has no color, and watch some colored liquid form.

"... why copper sulphate is a blue compound and why it turns to white colour after heating?

A.
energy levels are different in:
-a free ion Cu2+ ,
- or ion Cu connected to sulphate.

"When a copper salt is in aqueous form, the ions split up resulting in a Cu 2+ ion.
In dehydrated copper sulphate there are no copper ions free, the copper is ionically bonded to the sulphate."

Hi Akansha. I'm afraid your lecture on this topic will be excruciating for you and the listeners if you don't put effort into it. Please read the page, ask a followup question, and engage us in a practice run for your lecture :-)

Alternately, look up "black body radiation", study it a bit, and I'm betting you'll deliver a fascinating 10-15 minutes!

Good luck, and Regards,

A. Akansha,
If you are well versed in physics you should get the idea quickly from this and the related articles:
https://en.wikipedia.org/wiki/Spectral_line

If you are not well versed in physics then something written more for a layman audience is
http://www.irregularwebcomic.net/3259.html

Once you get that down, the reason that metals present different colors at different oxidation states is what several people previously suggested above. The oxidation state refers to how many electrons an atom has gained or lost compared to its neutral state. With one or two or three electrons missing the available energy states (empty electron shells) are different and the electron energy transition (jumping to a lower energy state and emitting a photon of a specific wavelength) that is the primary contributor to the observable color will be different.

Max Planck

Thanks Ray. It may be beyond Akansha's pay grade, I know it's beyond mine, but this funny business about the color of glowing metals is what forced Max Planck, completely against his will and contrary to his sense of logic, to drag the whole damn world into the wacky age of quantum mechanics :-)

Regards,

Hi Suganya. Please try your best to detail what you understood and didn't understand from the previous discussion so we can keep moving forward. The specific colors white & red rather than orange & blue are not the heart of this matter. Thanks!

Regards,

Sorry, Loreal, but I can't get my brain going 'til I've had a proper morning coffee, and I think my wife made the pot with 1 scoop of ground coffee instead of 6. It tastes like dishwater and looks very pale from dilution as well.
I'll be back after I make a fresh pot of strong, deep black, coffee :-(

Regards,

A. Oddly, Cu2+ is the most stable state, even though it is a univalent metal. This is because of the Jahn-Teller effect, because the electrons crowd each other.

A. Hi Lorial

The amount of light that passes through a solution (the colour you see) depend upon the concentration of the solution. The more dilute the solution (by adding water) the more light (in this case blue) that passes and the colour is lighter.

This can be used to measure the concentration.

I suggest that you do a search on "Beer's Law" and "Absorption spectroscopy" for a more detailed explanation

A. Hi Chasity. In one sense, all of the explanations here are fine because the actual reason that copper chloride is blue is the same for you as it is for anyone else in the world whether they be kindergarteners or post-docs. But what may be different for you, depending on your grade, is to what depth you must explain the blue color.

My suspicion from the style of your posting is that you are at a grade level where the proper depth of response would be that copper chloride contains Cu⁺⁺ and Cl^- ions, and the color of Cu⁺⁺ ions is blue while Cl^- ions have no color, as you know from dissolving table salt NaCl into water. Good luck.

Regards,

! Then one would like to know why Cu++ is blue!

A. Thanks Prabhkara. That may be the case -- and if it is, then Chasity can talk about d-block metal ions per Robert Smith's posting, or electrons bumping to higher energy levels per Samuel Pryor's posting, etc., etc. But one of the world's greatest physicists and teachers, Richard Feynman, frequently instructed us that you need a starting point (what you already know) and an ending point (a depth beyond which you will not try to go) for all such questions or you never get anywhere :-)

I think (but obviously don't know), that the chief curiosity for many students in this matter is that they know copper is orange colored, so they initially expect that copper should dissolve as an orange colored solution. The color change tells them that something different and more interesting than dilution is going on.

Regards,

Thanks Ted. That great teacher-scientist Richard Feynman has made the point pretty clear. That is how science progresses. Progress stops the moment questions cease.

A. Touché, sir, the questions must go on.

Regards,