Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: johnson039 on 27/11/2008 14:13:00

Title: What gives a substance its colour?
Post by: johnson039 on 27/11/2008 14:13:00

all things around us are made up of compounds, and all compounds are made up from atoms.
then, what makes the atoms/compounds got color?? [???]

is it sth related to the orbitals of atoms?? or the absorbance frequency of the energy levels??!~

hope someone can give me some explanation [:)]

Title: What gives a substance its colour?
Post by: srobert on 27/11/2008 22:25:28

You're quite right. For most things colour is due to the energy levels in the compound. When light strikes the substance specific frequencies of the light are absorbed when electrons in the compound are promoted to higher energy levels, called an excited state. The colour you see is what you get when the remaining light is reflected back off the surface. For most materials the electrons drop back down to the ground state (where they were before the light was absorbed) in a number of steps giving off thermal energy as they do. Fluorescent compounds absorb light of one frequency and then drop back down to the ground state in more than one step, one of which emits a photon of light of a different frequency from that which was absorbed. In phosphorescent compounds the electron cannot simply drop straight back down to it's ground state, and so the excited state hangs around for a period of time meaning the photon of light is emitted some time after the initial photon was absorbed.

I'm not sure about this but I think some things are coloured due to the physical surface of the object. Perhaps someone on here can explain that a little more.

Title: What gives a substance its colour?
Post by: johnson039 on 28/11/2008 07:10:26

thx srobert
So if the atoms absorb a frequency of wave in the light and the electrons are excited to higher energy levels, while the other frequency of visible light reflect so we can see the color (reflected light)
What abt the excited electrons?they re supposed to return totheir ground state,right? As the electrons go back to their ground state, they will release a quntum of energy, so are all substance releasing energy evey moment as there is light?

Title: What gives a substance its colour?
Post by: RD on 28/11/2008 07:14:17

Quote from: srobert on 27/11/2008 22:25:28

I'm not sure about this but I think some things are coloured due to the physical surface of the object.
 Perhaps someone on here can explain that a little more.

http://www.thenakedscientists.com/forum/index.php?topic=18177.0

Title: What gives a substance its colour?
Post by: srobert on 17/12/2008 10:27:59

Quote from: johnson039 on 28/11/2008 07:10:26

What abt the excited electrons?they re supposed to return totheir ground state,right? As the electrons go back to their ground state, they will release a quntum of energy, so are all substance releasing energy evey moment as there is light?

Essentially yes, although if they released it all in one quantum of energy it would have the same frequency as the light absorbed, and so the same colour and therefore rather than being absorbed this light too would be reflected and so the compound would be colourless. Instead the energu is generally dissipated as heat energy. This is why objects get hotter when under a light. Certainly most light sources also emit some infra-red (heat) and so tehre's the direct effect of that (particularly old fashioned tungtsen fillament incandescent lights), but even if you eliminate the infra-red you still get heating from the way I described.

Your new question has made me reconsider what I originally meant by reflected light. I said it very simply as if I undertood it, but when I think about it I'm not sure what exactly is going on when light is reflected. Is it absorbed and re-emitted? If it's not absorbed then why is the substance not transparent to light of that frequency. Is this elastic scattering (Rayleigh scattering if I remember correctly). What's the machanism of this process? Thanks

Title: What gives a substance its colour?
Post by: srobert on 17/12/2008 10:29:58

Quote from: RD on 28/11/2008 07:14:17

Quote from: srobert on 27/11/2008 22:25:28

I'm not sure about this but I think some things are coloured due to the physical surface of the object.
 Perhaps someone on here can explain that a little more.

http://www.thenakedscientists.com/forum/index.php?topic=18177.0

Thanks for this link RD.

Title: What gives a substance its colour?
Post by: lightarrow on 17/12/2008 11:55:01

Quote from: johnson039 on 27/11/2008 14:13:00

all things around us are made up of compounds, and all compounds are made up from atoms.
then, what makes the atoms/compounds got color?? [???]

is it sth related to the orbitals of atoms?? or the absorbance frequency of the energy levels??!~

hope someone can give me some explanation [:)]

The subject of colour is very complicated.
Atoms are colourless.
Colour is not an ojective property of things (example: the effect of "coloured shadows").
Some colours don't have a corrisponding wavelenght in the visible spectrum (e.g. magenta).
Colour can be the effect of interference, ecc..


There were a lot of threads about colour. These are just a sample:

http://www.thenakedscientists.com/forum/index.php?topic=13323.0

http://www.thenakedscientists.com/forum/index.php?topic=6614.0

http://www.thenakedscientists.com/forum/index.php?topic=9984.0

http://www.thenakedscientists.com/forum/index.php?topic=16329.msg189580#msg189580

http://www.thenakedscientists.com/forum/index.php?topic=13632.msg163896#msg163896

http://www.thenakedscientists.com/forum/index.php?topic=16656.0

http://www.thenakedscientists.com/forum/index.php?topic=16639.0

http://www.thenakedscientists.com/forum/index.php?topic=15472.0

http://www.thenakedscientists.com/forum/index.php?topic=11926.0

http://www.thenakedscientists.com/forum/index.php?topic=18177.0

http://www.thenakedscientists.com/forum/index.php?topic=6729.0

Title: What gives a substance its colour?
Post by: Make it Lady on 17/12/2008 19:49:49

I just love exciting electrons. My favorite releasers of pretty colours are the metal salts in fireworks that make everyone go oooooooh! As a chemistry teacher magnesium was such a pain.

Title: What gives a substance its colour?
Post by: lightarrow on 19/12/2008 13:04:59

Quote from: Make it  Lady on 17/12/2008 19:49:49

I just love exciting electrons.

Ehm.. "electron" is actually my second name... [;)]

(Bad Boy! hope you didn't take it bad [8)])

Quote

My favorite releasers of pretty colours are the metal salts in fireworks that make everyone go oooooooh! As a chemistry teacher magnesium was such a pain.

I would like very much to see thallium salts on the flame, the green colour seems very beautiful from photos. Do you have at school? (I don't think so, because of its toxicity...)

Title: What gives a substance its colour?
Post by: Make it Lady on 21/12/2008 20:05:32

Personally I would die of an asthma attack if I even looked at a thallium salt.

Title: What gives a substance its colour?
Post by: lightarrow on 22/12/2008 14:24:41

Quote from: Make it  Lady on 21/12/2008 20:05:32

Personally I would die of an asthma attack if I even looked at a thallium salt.

Why? Worried for your students' health or for what they could do to others with it?   [:)]