|MadSci Network: Chemistry|
I wouldn't trust a breakfast spread that does this! Perhaps you should change to vegemite! ;-) Seriously, though, this is an example of quite a general phenomenon. And curiosity driven experimentation and observation is a GOOD THING, even when it involves playing with your food. Have you looked at a lump of rosin that a violinist uses to prepare the bow? It is a block of yellow or orange material. But where the violinist has rubbed the bowstrings along it, it looks white. And the powder it produces definitely looks white. This is another example of essentially the same phenomenon. Marmite is a material that has visco-elastic flow properties, somewhere between a viscous liquid and a soft gel. When you pat the surface repeatedly with a spoon, what you are doing is generally roughening up the surface into a tangle of microscopic peaks and valleys. Some marmite adheres to the spoon, and is lifted into a sharp peak as the spoon is withdrawn. Because the material is not truly liquid, the peak stays there. Optically, marmite (as usually encountered with a smooth surface) is an indescribable brownish-black colour with a significant surface lustre. It can look quite shiny. What this means is that the light which reaches our eyes from a sample of marmite has travelled through a fair amount of light absorbing material before we get a little bit of it reflected back (the brownish-black colour that we see). But is also means that a small but significant proportion of the light -- perhaps 10 to 15% -- is reflected directly back from the surface (the lustrous or shiny appearance). When we roughen the surface, we increase the amount of that surface reflection, and we change things so that the reflection will be a diffuse one from several surfaces rather than a direct one. The result is that the material looks white, because most of the light we see coming from it has been reflected off its front surface in a diffuse way, without ever penetrating deeply into the material where it can be absorbed. There are many materials, like the rosin, that look quite highly coloured when in the form of large crystals or glasses, but lose most or all of their colour and look white, when ground to a fine powder. As a postscript, I really should direct North American readers to information on the products concerned. I am told that they may be unfamiliar to you. Both Vegemite and Marmite are different forms of yeast extract. They are brownish materials that are usually spread thinly on bread or toast, but can also be diluted with hot water to make drinks, or used as a flavouring in certain sorts of cooking. For information about
Marmite check here. Where "some English children are addicted to Marmite", which is a "strong flavoured, slightly salty" material, most Australian children are addicted to Vegemite, which is stronger flavoured, saltier, and somewhat sweeter. Vegemite is an Australian cultural icon, and a bit of an in joke. Marmite is available here, but does not sell well. There is also web information available about Vegemite
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