Wikipedia:Reference desk/Archives/Science/2022 October 24

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October 24

Inorganic chemistry: colors between states.

Nitrogen dioxide, with white solid at -196°C, coloured liquid and gas at 0-50°C)

So in the earlier discussion, solid oxygen and liquid oxygen have colors, oxygen in gas state is relatively colorless. I think this is the most common for substances. Are there things that have color in the gas and solid state, but no color in liquid state, or things that have color in liquid and gas state, but no colors in solid state? And I think I'll do the favor of cancelling out questions for some outstanding solids at room temperature, that have really liquid melting points, and therefore, even higher boiling points, let's disregard them out of the question. Thanks. 67.165.185.178 (talk) 01:45, 24 October 2022 (UTC).

There is a particular allotrope of solid nitrogen which is black, though most solid, liquid, and gaseous nitrogen is clear. See [1]. It only exists under some very extreme conditions. Allotropes of phosphorus also shows various colors under different conditions, one of which is white (so nearly colorless) while the others are red, violet, and black. The most common form of phosphorus gas is tetraphosphorus, which I believe is colorless or white. Liquid phosphorus is such an exotic thing, I can find no images of or descriptions of its color. --Jayron32 15:16, 24 October 2022 (UTC)

It's not very exotic. White phosphorus has a pretty low melting point, and there should be some liquid whenever you have a white phosphorus fire (granted, we could hope that this was more exotic than it is). You can see liquid phosphorus (without the fire) at about 2:42 of this entertaining YouTube video. Double sharp (talk) 16:10, 24 October 2022 (UTC)

Stuff that is gaseous at room temperature is mostly molecular (apart from monatomic). When these are cooled to liquids and solids, it is still molecular. Whatever causes colour also causes it whatever form the molecules are in. So normally the colour is quite similar, but solids and liquids could be more intensely coloured. For a list see list of gases. However some molecules are unstable and may change when the temperature changes, eg nitrogen dioxide which appears to have a white solid. (which is really dinitrogen tetroxide). Analogous N₂F₄ and NF₂ are both colourless. Graeme Bartlett (talk) 21:47, 24 October 2022 (UTC)

The property of polymorphism is quite common and can lead to multiple colors in different crystals of the same substance. The record is held by a compound called ROY, which has at least 13 forms, including red, orange and yellow versions. It is also piezochromic, which is another way solids can alter their color. In solution, many dyestuffs behave as pH indicators and change color. The record for low molecular weight fluorescence is held by 3-Hydroxyisonicotinaldehyde. Mike Turnbull (talk) 10:01, 25 October 2022 (UTC)

Allotropy in general is very common. The notion of "three phases" of matter (or even "four phases" when you bring in plasma) is a rather facile, non-technical understanding of phase. In reality, most substances have many more phases than that, and different phases AND different allotropes can each have different colors. Color is ultimately a quantum mechanical property of a substance, involving absorbance spectra with meaningful absorbance bands in the visible and near-visible range. This sort of stuff can be worked out empirically, (that is, you can measure the absorbance spectrum of anything) but the problem is theoretically irreducible for any atomic system larger than a 1-electron atom (H, He⁺¹, Li⁺²). Which is to say that no rigorously predictive theory could be done to explain the spectrum or the color of a substance a priori. We've noticed certain trends, that allows for us to have some understanding of things like how functional groups in some molecules will have ranges they tend to create absorbance bands, that allows for things like IR spectroscopy and UV-vis spectroscopy, but we still can't look at a chemical formula and say "that should look blue". It's complexity is beyond the ability of theory to make those kinds of quantitative predictions. --Jayron32 20:34, 26 October 2022 (UTC)

Some substances have different phases, but not necessarily more than 3. Many polymers for example, have a glass-transition temperature, as opposed to a melting point. 67.165.185.178 (talk) 04:40, 27 October 2022 (UTC).

Whereas water has at least 14. --Jayron32 15:23, 27 October 2022 (UTC)