People in white coats: Chemistry

Thursday, 1 May 2014

Age of bronze

Supplementary material

Bronze is an alloy of copper and tin, with arsenic sometimes added to the mix. It is stronger than copper alone, and obviously much, much stronger than tin (even if, paradoxically, bronze tends to grow stronger the more tin you put in it). Bronze is not to be confused with brass, an alloy of copper and zinc. The age of bronze (ca 1800-700 BCE) really came about with the invention of metallurgy; up to then, metals like copper and gold were hammered into shape to make jewelry or tools, but the majority of the latter were still made of stone (hence the name "stone age" and bone. The iron age would arrive after the bronze age, once technology (mostly meaning hotter fires, using coal) allowed the smithing of an even harder metal.

As for gold, well... it sure looks pretty and has its uses, but it is pretty soft. That's where the practice of biting a gold coin to ascertain its authenticity comes from: a gold-plated coin made of some baser metal would be much harder than a genuine gold one. (Having never owned nor tried to bite a gold coin, I have no idea how easy it would be to deform one with one's teeth. I probably wouldn't try).

Tuesday, 18 March 2014

PSSSHHHHT!!!

Supplementary material

Most liquids contain dissolved gases, and can contain a certain amount of molecules at a certain pressure (the limit being known as the saturation point). By increasing pressure, it is possible to increase the number of molecules dissolved in the liquid (making them supersaturated). However, if the pressure is then abruptly released, the excess dissolved molecules try to escape the liquid, shifting back into a gas. This is how we put the fizz in soft drinks: by supersaturating them with CO₂. Opening the bottle or can releases the pressure, and bubbles form.

Now it is clear that in a normal soft drink the gas escapes as a series of bubbles which clearly have only a few points of origins, called nucleation points: the CO₂ doesn't escape all at once from the liquid (otherwise, what would be the point?) These nucleation points, where bubbles start forming, are usually solid surfaces like a scratch on the inside of the bottle, a small piece of dust, a slice of lemon plunked into the glass or even local fluctuations in the liquid's molecular distribution.

In the famous Mentos and Diet Coke experiment, one or many Mentos mints are dropped into a soft drink (preferably into a 2L bottle). The surface of the candy, made of layers of sugars and gum arabic, is apparently home to a very large number of irregularities that serve as nucleation points. The CO₂, so eager to escape its dissolved state, has therefore the opportunity to start doing so simultaneously from many different points, creating an instant and thick foam of bubbles that violently bursts out of the bottle's collar.

Since the Mentos is also heavy, it sinks to the bottom of the bottle, making the reaction even more efficient (instead having the candy dance on the surface, floating on a bed of bubbles and seeing part of its surface isolated from the liquid). What's more, the gum arabic in the candy lowers the surface tension of water (it is a surfactant), meaning that the gas needs less energy to escape the liquid.