Re: Ice cream sodas

[William Beaty <billb@ESKIMO.COM>]

On Mon, 5 Mar 2001, PETER CRAFT wrote:

> Forgive my ignorance, but the day some chemistry students were making
> ice cream sodas (dropping ice cream into some cola) and noted that when
> the ice cream hits the soft drink it rapidly "fizzes" up.  They were
> curious about the cause of this sudden gas evolution as the same thing
> did not occur when blocks of ice were dropped into a drink.  This, they
> reasoned, excludes temperature as being the only mechanism involved when
> the ice cream was added.  After consulting my fellow teachers the best
> we could summize was that the rapidly melting ice cream my provide sites
> of nucleation. We were uncertain about this as we are more familiar with
> nucleation as an explanation in the process of boiling.  Could anyone
> set us right?

This was always a mystery to me as a child.  I finally figured out the
answer a couple of years ago, and put it on my kid-lore collection:

  http://www.amasci.com/~billb/cgi-bin/instr/instr.html

  FOAM BLAST
  Mix some whipped cream with a little water, stir well, then pour some
  warm cola onto this milky liquid.  FOOSH! Giant explosion of foam all
  over. If you do the same with milk, very little happens. The microscopic
  bubbles in the whipped cream are the cause. The same thing happens when
  melted icecream hits warm cola, thus explaining why "rootbeer floats"
  make foam, yet pouring milk into rootbeer creates only boredom. Icecream
  is "miniaturized foam," and it only needs some carbonated water to let
  it expand back its "true size."

The "nuclei" are small bubbles.  WHen they rise through a supersaturated
carbonated liquid, they provide a liquid/gas interface and CO2 inflates
them rapidly.   It's a miniature version of the Nyos/Camaroon disaster.

A similar "joke":  put some melted icecream (or some diluted whipcream)
into an opaque cup.  When a carbonated beverage is poured into the cup by
your unsuspecting victim, it creates a huge volume of foam.

Ice cubes will do the same thing if their surface temperature is
significantly below 0C and their surfaces are dry and dusted with frost
crystals.  No doubt the surfaces take a moment to become wetted, and until
they do, they trap a layer of air which rapidly becomes filled with CO2
from the surrounding liquid.  Warm wet ice cubes don't do this.

Also try making your own foam.  Make tiny bubbles with a blender or
electric mixer (a bit of detergent in the water), then inject it into cola
with a pipette or a soda straw.

Note that this explains why a can of cola will "explode" if shaken.
Shaking the cola generates tiny bubbles.  Most of them will rise to the
surface quickly, but some of them cling to the sides.  Open the can, and
those bubbles grow very large.  Dark beer and root beer have surfactants
which allw smaller bubbles to persist, which makes the problem worse.  To
fix the problem, whack the can on a hard surface to dislodge the bubbles,
then wait again before opening it.  Or, to make the problem WORSE, leave
the can turned on its side after shaking it, and LOTS MORE bubbles will
cling to the sides.


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William J. Beaty                                  SCIENCE HOBBYIST website
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