Tuesday, 18 March 2014
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.