Question

Explain why carbonated beverages must be stored in sealed containers.

Answer 1

To increase the solubility of CO2 in the soft drinks and soda water the beverage is kept sealed under high pressure

Answer 2

I'm still not understanding very well, could you explain with a little more detail as to what happens?

Answer 3

There is a dynamic equilibrium of dissolved and gaseous carbon dioxide at the water-air interface. By having a larger volume (not sealing the container), you reduce the partial pressure (i.e. concentration) of \(CO_{2~(g)}\), and by Le chateliers principle, you will shift the equilibrium concentrations effectively reducing the amount of dissolved carbon dioxide.

\(\sf CO_{2~(g)}\rightleftharpoons CO_{2~{(aq)}} \)

Answer 4

Okay. So I still don't get it. :/ Could you explain it to me as if I have never learned chemistry before? Like I know what a solute, solvent and solutions are. I also know that when you open a can of soda, it like sparks the release of CO2, or at least changes the pressure. Sorry, I just don't understand

Answer 5

hey @brittanyrolfe you asked a very good question.

Perhaps it would be easier to consider an analogy first.
What happens when you leave a cup of hot tea out in the open air?
After a while you notice than the tea will cool down until some pt it becomes roughly room temperature. This would be similar if you used a cup of iced tea, the temperature would increase with time and then stabilises. We call this 'stable' temperature a point of equilibrium. This means the rate of heat exchange is roughly constant in and out of the object.

The direction of the flow is explained by thermodynamics (especially the Second Law). As a physical principle, energy tends to flow from high energy to low energy, and matter flows from high concentration/pressure to low concentration/pressure. For example as I mentioned a cup of hot tea never spontaneously gets hotter on its own if you leave it out in open air. Nor will a block of ice get colder on its own. A ball will not spontaneous lift itself up, it tends to fall under gravity. The spontaneous direction of energy and matter transfer (also known as an energy/concentration/pressure GRADIENT) always occurs to create a 'balance' - equilibrium state whereby no net energy or matter change occurs further, just as after your cup of tea is left long enough, it won't increase or decrease its temperature substantially.

Now to our carbonated soft drink question.
In your sealed can, there is dissolved CO2 gas. Normally CO2 occurs in gaseous form in the air but in the carbonated drink, the CO2 is essentially trapped in solution. When you open the can the first thing you notice is the bubbles coming out, which tells you the CO2 is coming out of solution as a gas. This means the amount of dissolved CO2 is decreasing after you open it. The question is, why?

Remember to start with, our can had a high level of CO2 in dissolved form. Our thermodynamic law says once we allow exchange of gas with its surroundings, the CO2 is highly concentrated in the confined space of an unopened can and then has to distribute to a larger volume of open air according to the thermodynamic principle, until equilibrium is reached.

Another thing you notice is the temperature change, which is probably a more intuitive explanation. If you leave the refrigerated can in open air, the can temperature increases and will no longer be as cold. However gases like CO2, water and O2 are more soluble at cooler temperatures. Remember if you heat something you increase vibration of the particles making it more likely to enter the gas phase. So cooling a gas makes it more likely to enter solution. Thus increased temperature of carbonated drink decreases how well CO2 can stay in solution and will come out. This occurs on top of the pressure/concentration gradient we discussed.

If we put this together in chemical terms, you see the following changes when you open the can.

\[H^+ (aq)+HCO_3^{-}(aq) \rightarrow H_2CO_3(aq) \]
\[H_2CO_3 (aq)\rightarrow H_2O (l) + CO_2(g)\]

The first is acidity level decreases in the drink (you should be able to taste this difference) and secondly the level of carbon dioxide gas released increases. So overall your soft drink loses sourness (Acidity) and also becomes flat if you leave it in open air.

This occurs because CO2 is less soluble as temperature increases, so comes out of the solution, alongside the fact that the concentration/pressure of CO2 in the can must go from high to low (compared to the surrounding air).

Answer 6

Oh my goodness! THANK YOU SO MUCH! I really appreciate this explanation! @mww