Beer, glorious beer.
The colour of beer or of any translucent liquid depends on how much of it you look through. A single drop f beer looks colourless. A glass of it may, according to the brew, appear to be an amber yellow or dark brown. Beer that is yellow in a tumbler would look red in a barrel-sized container of clear glass. The forth or head on a glass of beer looks white because each bubble consists of carbon-dioxide gas inside a minute quantity of beer. The light reaching your eye passes through insufficient beer to turn yellow. In other words it doesn’t have time to change colour.
You may notice a similar phenomenon with water. A single drop of water is quiet clear, so should a glass of it be, if the supply is pure. However if you look through a tube of water several meters long, it has a tinge of blue, the longer the tube, the darker the shade of blue. If you went deep into the ocean, you would find it gradually getting darker, the water above would have absorbed all the light. In an ocean of beer, the light would change from yellow to read, then from dark red to black. But the foam of beer and that on the sea look equally white because the light from each has passed through too little liquid.
Take a pinch of salt, sprinkle it into a glass of beer, and immediately you will see bubbles forming along the path of each falling grain of salt. The bubble has nothing to do with any chemical reaction between the beer and the salt – as you can easily prove by dropping in grains of a substance that doesn’t dissolve.
The bubbles come from carbon dioxide dissolved in beer at about twice atmospheric pressure. They won’t form in the beer with ought any inducement. They need something to cling to – what scientist call a ‘nucleation site’. If your glass is flawed in some way or the beer contains a speck of cork or impurity, bubbles will form at the site. The principal applies equally to all kinds of carbonated drinks. You may have noticed that, in glasses designed with a pebble like finish, bubbles will stream from each nodule in the glass.
If you want to experiment, sprinkle some clean sand into the beer. As the grains fall, they will collect bubbles. Unlike the salt, which dissolves in the beer, the sand collects at the bottom of the glass and continues to send up bubbles. With ought some inducement, the dissolved carbon dioxide does not have the energy to start a bubble - In other words, to push aside a tiny volume of beer and fill the space with gas. Once a
bubble gets started at a nucleation site, more carbon dioxide comes to the party, pushing into the bubble and expanding it. It’s just like blowing up a balloon: the hardest part always is in getting it started.