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topic 9716

Blue colour in copper sulphate solution! HELP!

A discussion started in 2001 & continuing through 2017


Q. Why does the colour blue enter the copper sulphate solution during the purification of copper? Can anyone please tell me? I really need to know!


Laura W [last name deleted for privacy by Editor]
-England, UK


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.

Ted Mooney, amadeya42.com Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey



Hi, I need to know why copper sulphate is a blue compound and why it turns to white colour after heating. Would it be possible to regain the blue colour of copper sulphate?

Parvin Z.
student - Toronto, Ontario, Canada


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.

Robert Smith
- Sheffield, England

thumbs up signThanks 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 :-)


Ted Mooney, amadeya42.com
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Q. As I read about copper (+2) Ion. The colour of this transition element with Chloride Ion is Green (CuCl2) but The colour of Copper Sulphate is Blue (CuSO4). Can You Solve My Problem? Why Does This Change Occur?

College DKV - Jamnagar, Gujarat and India


Q. Why does the colour of copper sulfate solution change from blue to white when you dehydrate the crystal?


February 11, 2008

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 sulfate there are no copper ions free, the copper is ionically bonded to the sulfate.

Samuel Pryor
- Adelaide, SA, Australia

October 2, 2008

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 is yellow, by itself. But when combined, their atoms work together to form an overall compound, copper sulfate. 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.

Putyrre Yertwsd
- Japan

June 28, 2015

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

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."

Natalia Lehr
- Goettingen, Germany

Why the color of copper is blue?

August 7, 2015

Q. Why the copper is blue in color? This is the question given to me on which I am having seminar I should explain it for 10-15 minutes in front of all.

Akansha prasad
student - secunderabad,telangana,india

August 2015

thumbs up signHi 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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

August 12, 2015

A. Akansha,
If you are well versed in physics you should get the idea quickly from this and the related articles:

If you are not well versed in physics then something written more for a layman audience is

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.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

August 2015
Max Planck

thumbs up signThanks 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 :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

January 13, 2016

Q. Why mercury subchloride and mercuric perchloride are white in color and red sulphate of mercury in red color, but these three compounds are derived from mercury?

Suganya devi
- chennai, tamilnadu, india.

... and for the next round of MadLibs, the participants picked mercury rather than copper, white in place of orange, and red in place of blue ...

thumbs up signHi 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!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

May 4, 2017

Q. A deep blue copper sulphate solution turns pale blue when water is added.
Name and describe the phenomenon which causes the change in colour from deep blue to pale blue.

Loreal p [last name deleted for privacy by Editor]
- Karachi,Sindh Pakistan

May 2017

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 :-(


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

May 10, 2017

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.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

May 11, 2017

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

geoff smith
Geoff Smith

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