There are a couple of things you could try, but
only one of them is safe to do in a
classroom and is relatively unlikely to work.
What is happening is that in the water, the
baking soda (sodium bicarbonate) is breaking up
into sodium ion and bicarbonate ion. Meanwhile,
the acetic acid in the vinegar is breaking up into
hydrogen ion and acetate ion. The rate at which
this happens depends on the strength of the
dissociation; sodium bicarbonate is a soluble
salt, which means that it dissociates very
quickly. Acetic acid however is a weak acid, which
means that (by chemical reaction standards) the
breakup is much slower. Once the hydrogen ion from
the acetic acid meets up with the bicarbonate ion
from the baking soda, however, they combine to
form carbonic acid. Carbonic acid then dissociates
into carbon dioxide and water. Once there is too
much carbon dioxide, it comes out of solution and
becomes bubbles of carbon dioxide gas. This
increase in volume from the transition from liquid
to gas is what causes the explosion.
Thus there are three ways to slow this reaction
down. They are:
1. Take the bicarbonate ion out of solution so
that it cannot combine with the hydrogen ion.
2. Take the hydrogen ion out of solution so that
it cannot combine with the bicarbonate ion.
3. Keep the carbon dioxide dissolved in the water
and not become a gas.
(1) and (2) are difficult. For (1), you would
need a metal ion that would grab onto bicarbonate.
Aluminum would probably be your best bet, but I
wouldn't want to play with aluminum ions because
they're poisonous (and other ions, like lead, are
worse). For (2), you would need a strong base to
gobble up the hydrogen ions like sodium hydroxide.
Unfortunately, that burns skin if it makes
contact, so you don't want that either.
(3) is probably the easiest: dump ice chips
into the water. Carbon dioxide, like most gasses,
is more soluble in water at colder temperatures.
By chilling the water to freezing by adding ice,
you might keep less of the carbon dioxide from
bubbling off. However, if there is still too much
carbon dioxide, then it will still bubble off (and
release heat as it does so, which will melt the