Jeffrey Fox, of Ardgowan School, asks :-

Where do the bubbles come from in the bottom of a glass of fizz or beer?

Pat Langhorne, a physicist at Otago University and who works on bubbles in sea ice in the Antarctic, responded.

A gas called carbon dioxide (CO2) is dissolved in beer and fizz. Provided the top of the bottle is on tightly, this gas also occupies the `space' in the bottle above the drink. All the time molecules of CO2 are coming out of the beer and into the `space', while other molecules leave the `space' to become dissolved in the beer. On average the same number of molecules leave the beer as enter it, so that the pressure in the `space' above the beer is higher than the pressure outside the bottle. This is why the gas rushes out when you loosen the top.

So loosening the top also reduces the pressure above the beer. This causes some of the dissolved CO2 to come out of solution since it is no longer `squashed' into the beer by the higher pressure. We then say the beer is supersaturated, i.e. it can?t wait to get rid of some of the dissolved gas. But it can't do this explosively. For the following reason it must do so slowly, by creating bubbles.

A bubble consists of gas surrounded by a liquid, the two phases separated by a definite surface. It takes energy to create this surface. So it takes energy to create a bubble. The pressure difference between the inside and outside of a bubble gets larger as the bubble gets smaller. Here we have a problem - it takes enormous amounts of energy to create the first tiny bubble. We are helped over this hurdle by the fact that a glass containing beer is not perfectly smooth. There are hundreds of tiny cracks and imperfections hiding tiny pockets of air that may contain hundreds of molecules even though we can't see them. These act as sites where bubbles can grow in the supersaturated beer. They are called nucleation sites. This is why bubbles form on the glass, or on bits of dirt or hair. Once the tiny bubble forms it can grow by other CO2 molecules diffusing towards it.

Incidentally this is all explained very clearly in a lovely wee book called Clouds in a Glass of Beer : Simple Experiments in Atmospheric Physics by Craig F. Bowen.