Physical Chemists: Why does a can of pop fizz (why does gas escape from the solution) once it is opened?

Question

I came across this application question on an exam and I couldn't quite muster up the explanation for this. I need an answer that is actually directly pertinent to physical chemistry, i.e. an explanation involving vapor pressure and any physical reactions occurring.

Answer 1

I took PChem a while ago, little rusty. I believe this is what happens.

In a closed pop container, little CO2 needs to evaporate in order to achieve equilibrium with CO2 dissolved; in other words, the gas above the liquid is saturated with CO2 and there is equilibrium between CO2 evaporating and being dissolved back in the soda.

Once the container is open, you can no longer saturate the gas above the liquid, as the soda is now exposed to the atmosphere.

Answer 2

Look up Henry's law and recognize that the CO2 gas pressure in the head space of the soda bottle is about 4 atmospheres.  You'll figure it out.

Answer 3

It doesn't involve vapor pressure much.  The fizz in carbonated beverages is carbon dioxide (which is why they are called carbonated).  Historically, the CO2 got there by fermentation of sugar by yeast in a sealed bottle.  So it was pressurized because the yeast produced the gas in a sealed container.  Now, most of the time the drink is treated with pressurized CO2 from a gas cylinder.  But it IS pressurized.  The water's vapor pressure is trapped in the can, but that's not much.  The CO2 dissolves in the water mostly, so its pressure is much lower than it would be if it were just in the air space.  So it acts somewhat as if it is a liquid with a vapor pressure, even though it is never actually a liquid, it's a dissolved gas.  Those pressures build up and you get the initial fizz when you open the can.  After that, the dissolved gas has a tendency to go back to equilibrium and leave the solution, it has that 'vapor' pressure with which the concentration of gas in the solution would be at equilibrium.  If you seal the container, it will pressurize to that equilibrium pressure and then stop.  But with the drink in a glass, the gas escapes.  So it keeps coming out of solution until there's none left, and the can goes flat.