The color of Carbon

[lilmc211]

Why is carbon black?

[magicdice]

it isn't always - think diamonds; people, animals & plants are also mostly made of carbon & aren't black *waits for someone to make a snide comment about racism - rolls eyes*

[trilobite]

Saying 'people, animals & plants are also mostly made of carbon & aren't black' is a little tenuous in relation to the question, and also incorrect. It's about as valid as saying 'milk is mostly water but isn't clear and colourless'

Without being specific to carbon, the question may be posed 'why is any substance the colour it is?' It's due to the reflective and refractive properties of that substance.

Soot, a mixture of hydrocarbons and other impurities, appears black because it absorbs much of the light falling on it and reflects very little.

A green substance absorbs red and blue parts of the spectrum and refflects in the green.

An individual snowflake is clear and colourless, (pollutants aside), but is highly refractive, therefore they appear white en masse.

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Specific to carbon, there are only three forms of pure carbon. (With the third, the fullerenes, there is still some debate about whether they occur naturally, so we'll ignore them.) But the other two are diamond and graphite.

Diamond is colourless and transparent (though impurities such as iron, cobalt etc can alter this colour to anything from red, yellow to brown or black. A microcrystalline structure can make them appear opaque)

Graphite is a different form of pure carbon which is a black, opaque substance with a dull, metallic lustre.

For an idea of this, think of the lead in a very soft pencil (as these have less clay and more graphite than a harder pencil) and the slight 'shininess' (lustre) when you draw with it.

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So what we generally refer to as 'carbon'  (soot, coal, charcoal etc) are mixtures of various hydrocarbons and other impurities.

[Ralph]

If you're talking about carbon fibre laminated into cool looking stuff for your car/ bike/ house/ boat, that carbon is a graphite derivative, do see trilobites answer.

[magicdice]

Saying 'people, animals & plants are also mostly made of carbon & aren't black' is a little tenuous in relation to the question, and also incorrect.

okay first point i get, but incorrect? people, animals & plants are not black. they also mostly consist of carbon.

[trilobite]

Sorry, 'incorrect'  was in reference to the "mostly carbon" bit, I wasn't questioning the bit about 'plants and animals are not black' - that's just stating the bleeding obvious.

The average human body is around 55 - 60% water (by weight), depending on sex, age etc.

Or, alternatively broken down by element, contains some 65% oxygen (by weight) against 18.5% carbon.

Most (mammalian) bodies vary only marginally from these figures.

 

Plant tissue is more difficult to assay - and will vary from species to species, soil conditions, and the growth stage of the plant itself. However, the variances are greater with regard to trace elements, rather than the main tissue constituents.

An analysis of corn tissue revealed an elemental composition by dry weight (ie after the removal of all water which, if included, would bump up the % oxygen content) of;

45% Oxygen

44% Carbon

 

(From Troeh, Frederick and L. M. Thompson. 1993. Soils and soil fertility. Oxford University Press.)

 

So, I would still maintain that plants and animals are not 'mostly made of carbon'.

[magicdice]

Or, alternatively broken down by element, contains some 65% oxygen (by weight) against 18.5% carbon.

in which case we should be 65% colourless and 18.5% black?!

anyway i know full well it's all about the compounds, but even so my point about carbon not always being black still stands

[Hamish]

Carbon is a special type of semi conductor where the valence and conduction bands are in contact. The material will absorbe light if the energy of the light is greater than that of the band gap, ie 0. Therefore the carbon (in graphite form) will absorbe all wavelengths of light. As a result no light is reflected/transmitted and therefore no colour is observed. This can be seen in other smiconductors such as cadmium selenide wich is yellow orange because is absorbes light with a wavelength less then 515nm.