Science Questions

Why rinse with water to prevent cola frothing over?

Mon, 9th May 2011

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Question

Anon, via text asked:

Hi, Love the show, downloaded all the podcasts! When you have a cup of cola, you're told to rinse the cup with water first to stop it frothing over. Why?

Answer

Dave -   This is an effect very related to the freezing water we were talking about earlier.  Basically, there’s lots of carbon dioxide gas dissolved in the cola, and as soon as you release the pressure, that’s much more stable as a gas, rather than dissolved in the cola.  But, in order to form a bubble you’ve got to create a whole lot of extra surface and that costs lots of energy.  This means bubbles are only stable when they’re above a certain size.

If you’ve got a very clean glass, there’s nowhere where a bubble can form, so it takes a long time for the bubbles to form, and so it doesn’t froth up.  If you’ve got lots of dirt on the bottom, some of that can be hydrophobic, which makes it easier for the bubbles to form.  You also might trap some gas, so you’ll have a bubble there to start with.  As soon as you’ve got lots of bubbles there, you’ll get lots more bubbles, they’ll grow very quickly, turn into a froth and make a horrible mess.

Chris -   And I guess the spin off from that is that if you do clean the glass first, your drink will stay fizzier for longer if the bubbles are forming more slowly?

Dave -   Totally.  There will be more gas in there for longer, because it’s escaping less quickly.

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