with carbon dioxide (CO2) all of the gas does not
escape spontaneously when the seal of the can or
bottle is broken and the drink is poured. If you
look closely, you can see that bubbles do not form
at random sites: they form at places where there
are imperfections or (in a poor-quality drinking
establishment) dirt on the surface of the glass.
Points where the CO2 changes phase (or nucleates)
and escapes from the beer are called nucleation
sites. Adding salt to the drink increases the number
of nucleation sites and produces noticeably more
bubbles (small glass beads can be used instead
to show that the release of CO2 is not due to a
chemical reaction with the salt). In effect, this
creates a ‘reverse cloud’ in the beer, in the sense
that there is a cloud of CO2 gas in the main body
of liquid. This experiment is explained in more
detail by Bohren (1987) in his book Clouds in a
Glass of Beer: Simple Experiments in Atmospheric
Physics.
Returning to the atmosphere, these ideas
can be used to show that water droplets are
similarly much more likely to form when there