Why are chemicals the color(s) they are?

[Genecks]

I suspect this is more of a physics question, but I don't really know why a solution of a copper ion looks blue.

 

Does anyone know a good book (academic or thorough) or website that really goes into this kind of stuff?

[ecoli]

Its sort of a cross-topical question, so its not out of place in chemistry. It has to do with the way chemicals absorb light. Photons hit the chemical particles, which introduces energy. This energy causes electrons to jump up into a different state. When the electrons fall back down, the energy is released as photons of a specific wavelength, which we perceive as specific color.

[insane_alien]

its the electron orbitals and the possible energy levels they have. this will determine whether they absorb a photn or let it pass.

[swansont]

No, no no, it's ALL physics!

 

In general, it depends on the situation.

 

In some cases the material will absorb certain colors, and you see what's left over — the material will reradiate the light in all directions, so it's weaker than the reflected light. And the excited state you get from the absorbed light my give up its energy (or "relax") through methods that don't immediately involve giving up a photon, known as nonradiative decay, where you get phonon (vibrational state) excitations. This is often the case with solids. Chlorophyll, for example, absorbs strongly in the blue and red part of the spectrum, so it looks green.

 

In other cases, you get absorptions at higher energies and you have some radiative or nonradiative decays, and then the atom or molecule has a strong transition that gives up a photon, and you see that color. (fluorescence) More often the case with with polyatomic gases.

[Klaynos]

No, no no, it's ALL physics!

 

I can't believe I resisted posting something very similar to that!

 

 

I wonder if you could manufacture a gas that absorbs a small fraction a high intensity laser and reradiates in all directions in the visible to show beam paths...

[swansont]

I wonder if you could manufacture a gas that absorbs a small fraction a high intensity laser and reradiates in all directions in the visible to show beam paths...

 

Dry ice or liquid nitrogen works OK, using scatter instead of fluorescence.

[Klaynos]

Yeah we've used liquid nitrogen to do it with a white light source, and been depressed at just how diverging our collimated light is

[John Cuthber]

"No, no no, it's ALL physics!"

A solution of copper sulphate in water looks blue. This is because there is an energy level that the electrons in the copper ion can be excited to by light in the red or infra red region.

However that transition is between two states of d orbitals. The physics tells us that such a transition is forbidden and therfore the copper ion is colourless.

 

The chemistry tells us its blue.*

 

More seriously if that stuff is physics how come I'm a spectroscopist and I'm a chemist?

 

And even more seriously, yes it's perfectly possible to see fluorescense from a gas but you need to get the laser at exactly the right wavelength.

There are a number of ways, for example you can use the green light from a mercury lamp to excite iodine molecules in the vapour state (coincidentally these happen at the same wavelength). Or you can use a tuneable laser source (which would be expensive) or a broadband source.

It might be interesting to try this with a blue LED and Br2 or NO2 but only if you know how to play with toxic gases safely.

* I know it's down to vibrational effects really, but I learned it in chemistry, not physics.

[YT2095]

All Science is Chemistry, everything else (like physics) is Stamp Collecting!

[swansont]

"No, no no, it's ALL physics!"

A solution of copper sulphate in water looks blue. This is because there is an energy level that the electrons in the copper ion can be excited to by light in the red or infra red region.

However that transition is between two states of d orbitals. The physics tells us that such a transition is forbidden and therfore the copper ion is colourless.

 

The physics also tells us that red or infrared photons have less energy than blue photons, so I'm curious as to where the extra energy comes from?

 

The chemistry tells us its blue.*

 

More seriously if that stuff is physics how come I'm a spectroscopist and I'm a chemist?

 

And even more seriously, yes it's perfectly possible to see fluorescense from a gas but you need to get the laser at exactly the right wavelength.

 

 

Define "exactly." Doppler and collisional broadening can be GHz — huge!

 

All Science is Chemistry, everything else (like physics) is Stamp Collecting!

 

Beware the ghost of Rutherford!

[John Cuthber]

"The physics also tells us that red or infrared photons have less energy than blue photons, so I'm curious as to where the extra energy comes from?"

I think you need to think about that again. Hint, if something were to absorb blue light, what colour would it be?

 

"Define "exactly." Doppler and collisional broadening can be GHz — huge!"

Typically "exactly" here means correct to about 4 sig fig. doppler and colisional broadeneing just alter the "exact" wavelength that gets absorbed by a particular atom.

 

Question 1

A simplified model of the helium atom is that it consists of a nucleus with 2 units of charge and two electrons each with minus 1 unit of charge. The initial conditions are such that the system remains bound ie the system's energy is less than the ionisation energy

a (for some marks) write down the Schrodinger equation for this model of the helium atom.

b (for all the tea in China) solve that 3 body equation analytically and without aproximations.

[swansont]

"The physics also tells us that red or infrared photons have less energy than blue photons, so I'm curious as to where the extra energy comes from?"

I think you need to think about that again. Hint, if something were to absorb blue light, what colour would it be?

 

OK, I was in the laser mindset and thinking it was only IR photons being absorbed.

 

This is exactly the same as the case I listed earlier, with the chlorophyll example.

[John Cuthber]

What, no solution to the 3 body problem.

Surely that means the physicists can't do the maths on the second simplest atom in the universe (never mind anything more complex).

 

Doesn't that rather limit things?

 

Incidentally, as a chemist I can solve that equation exactly, but I do use a rather odd looking computer.

[Klaynos]

I'll have to look up the reference because they're on my desktop, but we can find the energy bands in systems. Without having to solve a 3 body problem, good strawman though.

[John Cuthber]

Much of physics is about producing mathematical models of the universe that predict how things will behave.

I don't think that pointing out the fact that the equations for thhose models are not soluble without making aproximations is a strawman, I think it's a geunine rebuttal of the idea that "physics is everything".

[AbeMichelson]

To answer the Copper ion question directly and using Chemistry(!):

 

The copper ions form a loose bond with four surrounding water molecules, forming [Cu(H2O)4]2+. This has an octahedral (think 8 sided die) shape. This causes the electronic structure of the copper complex to have a gap between unoccupied and occupied electronic states that is the same energy as blue light!

 

This explaination can be found in Inorganic Chemistry Textbooks.

 

For a really neat experiment that accents the solvent (water) effect on electronic configuration, put cobalt ions (cobalt chloride will work fine) into 3 different solutions: water, acetone, and ethanol. Note the color change. This can be explained by the spectrochemical series in ligand field theory.

 

I always give physicist a hard time because they say everything can be explained by physics, but often can't make the leap from Physics to Chemistry themselves personally. This is all based on electric fields changing the energy level/configuration around an atom. I used to joke when I taught that almost all of chemistry can be explained through Coulumb's Law. (everything else is Thermodynamics).

Sorry, may have assumed some things:

1. Occupied states= where the electrons are (at least a high probability of being), unoccupied = where they are allowed to be, if given enough energy.

2. To go from one to another, you have to add a specific amount of energy (in your case the energy of blue light).

3. AAnd one correction: In water it is tetrahedral, not octahedral, sorry.