What Does Colorless Mean in Chemistry?

[SMD1990]

The other day, I went to a local library to look at the CRC Handbook. I was trying to find out about the solubility of silicon dioxide.

Anyway, I noticed that sodium chloride was listed as "colorless". This struck me as odd. I had always considered it as white.

The table seemed to differentiate between white and colorless. It listed sodium carbonate as white, yet sodium chloride as colorless.

So, my question is: What is "colorless"? I agree that Na2CO3 is white, but all the crystals if NaCl I have ever seen... I would not describe them as clear or colorless.

Silicon dioxide is also listed as colorless. However, large crystals of it appear much more transparent than those of sodium chloride.

 

[DaveC426913]

I suspect in the same sense that snowflakes appear white, yet are really colourless.

Truly white objects actually reflect all wavelengths of light (their atomic bonds do not absorb any frequencies). Colourless objects do not reflect the light, they are transparent to light, letting all frequencies pass - however due to their physical structure, that light is scattered so that it is not coherently transmitted.

So: colour white is a molecular thing, colourless-but-white is a scattering thing.

The difference is that, if you take the colourless material and alter its physical structure without any change to its chemical structure, you can see that it's colourless. In chemistry, we'd want to now its true colour when not masked by its temporary crystalline structure.

 

[Ygggdrasil]

If you grow a large enough crystal of NaCl, you will see that it is indeed transparent. As DaveC426913 mentioned, the small NaCl crystals you commonly encounter in salt shakers appear white because they scatter light.

 

[SMD1990]

That is great to hear! I was wondering if maybe there was some sort of impurity in my water, or something. Or else, that "colorless" did not mean what I would have thought.

Thus far, the largest crystals of sodium chloride I have grown have only been maybe 8 mm³. (Grown by accident in a glass of salt water that evaporated.) Still, quite white.

But, based on your replies, it sounds like an even larger one should be more "glass" like.

Come to think of it, the salt in that glass of salty water might not have been exactly pure...

 

[mrjeffy321]

But, based on your replies, it sounds like an even larger one should be more "glass" like.

It is not the size that is important, really, but the quality of the crystal. Defects in the crystal are what scatter the light.

Imagine you had a big piece of glass that is perfectly clear (and colorless). Now imagine that you crack the glass in many places, sort of like the cracks in the glass windshield in http://atlanticcustomsautoglass.com/wp-content/uploads/2010/05/My-Broken-Windshield.jpg" . Those cracks scatter light in all directions, making it look white, when in fact it is still the same clear glass it was before. The more cracks you have, the more it scatters light, and the more the clear object looks white.
"Cracks" are just one type of defect that a crystal might have that could scatter light.

If you are looking at a white crystal of sodium chloride, for example, then you know that there must be a lot of defects in its crystal structure that are acting to scatter the light, you just cannot see most of them because they are incredibly small.

 

[Ygggdrasil]

Consider the picture of salt crystals for wikipedia: http://en.wikipedia.org/wiki/File:Halit-Kristalle.jpg

Many of the crystals are transparent. Some of the crystal show transparent regions and cloudy white regions. The cloudy white regions contain the defects that mrjeffy321 is talking about. You can see similar phenomena occur with water ice cubes. Sometimes the ice cubes are transparent and sometimes they appear white and cloudy.

 

[aroc91]

If you've ever used an infrared spectrometer, you'd know that salt is colorless. They use ~2cm x 3cm salt plates that sandwich your sample. Of course, those plates are very carefully made so they're pure, as not to throw off the readings.

Google has plenty of pictures.

http://www.google.com/search?hl=en&...&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi

 

[SMD1990]

Hey, those salt plates are pretty neat! Kind of like what I was wanting to grow, only simpler in shape and much smaller in size.

How durable are they? Comparable to glass with similar dimensions?

 

[SpectraCat]

They are hygroscopic (absorb water readily from the air), and fairly prone to fracturing. I had NaCl windows on a vacuum chamber in my lab, and my grad student pumped the chamber down too rapidly causing one window to fracture. It was actually pretty cool .. the window cracked along crystal planes so that the window was segmented into 4 (almost) perfect quarters.

Anyway, I would say they are far less durable than glass ... the amorphous structure of glass makes it fairly flexible compared to pure salt crystals. In my experience, crystals involving divalent ions (like MgF₂, CaF_s, ZnS and ZnSe) are more robust than those involving only monovalent ions (NaCl, KBr, CsCl, CsI).