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Why does a can of soda "explode" when it's shaken? I looked online, but all the explanations were either very hand-wavy, or looked very wrong. Thanks!
CO2 is dissolved in the liquid. Liquids cannot dissolve as much gas when the liquid is agitated.
This concept is used in labs all the time. Gas can be removed from a liquid by shaking it at high speeds using ultrasound.
Best explanation I've heard is that shaking increases the kinetic energy of both the liquid and the gas. When the molecules are moving around faster, it's harder for them to grab each other. This makes sense because it's exactly what happens when a liquid is heated - hot liquids dissolve less gas.
CO2 is dissolved in the liquid. Liquids cannot dissolve as much gas when the liquid is agitated.
This concept is used in labs all the time. Gas can be removed from a liquid by shaking it at high speeds using ultrasound.
Best explanation I've heard is that shaking increases the kinetic energy of both the liquid and the gas. When the molecules are moving around faster, it's harder for them to grab each other. This makes sense because it's exactly what happens when a liquid is heated - hot liquids dissolve less gas.
Doesn't sound completely right to me.
Amount of gas dissolved in the liquid doesn't depend on whether the liquid is agitated or not. Speed of dissolution depends on the agitation, but concentration in a given temperature depends only on the pressure (Henry's law). Whatever happens when the can is shaken seems to be effect of oversaturation and nucleation.
When the solution is sonificated situation is a little bit different. Sonification produces high pressure gradients - in some places pressure in the solution is very high, in other very low. Where the pressure is low, gas can form bubbles. They get dissolved back, but as they tend to go up, they slowly leave the solution.
Borek, I think I may come up with a partial explanation.
The pressure inside the can increases with depth, so the lower portions of the can should hold more gas than the shallower parts. If the can is shaken, then conceivably the liquid from the bottom would rise to a point of lower pressure. Then, the pressure might not be enough to keep the gas in suspension, and bubbles would form. I don't know how this would lead to the can exploding, but I feel it's important. I read somewhere that whirlpools might play a role as well, but the explanation was very poor.
Axillary question: why does tapping the can help?
skeptic2
Nov22-10, 12:03 PM
The explanation I was given is that the CO2 in the liquid exerts a certain pressure per unit area of the surface. In shaking the liquid the surface area of the liquid increases dramatically and so does the total pressure.
But the can will still explode once the drink settles and the surface resumes its normal shape.
The pressure inside the can increases with depth, so the lower portions of the can should hold more gas than the shallower parts.
Pressure difference is negligible.
the pressure might not be enough to keep the gas in suspension
Gas is not suspended but dissolved, not sure what you are aiming at.
Axillary question: why does tapping the can help?
Please elaborate.
The explanation I was given is that the CO2 in the liquid exerts a certain pressure per unit area of the surface. In shaking the liquid the surface area of the liquid increases dramatically and so does the total pressure.
That looks so wrong I have no idea where to start. First of all, dissolved CO2 doesn't exert any pressure, pressure is exerted by the gas above the liquid. Total FORCE exerted by the pressure depends on the surface, but pressure does not. And surface that is important here - internal surface of the can - is constant.
but concentration in a given temperature depends only on the pressure (Henry's law). Whatever happens when the can is shaken seems to be effect of oversaturation and nucleation.
When something is in a oversaturated condition and some agitation causes it to crash out, isn't that usually a process that doesn't reverse itself? If I boil some water, dissolve salt in it, start to cool it. It's oversaturated. I tap the glass and the salt crashes out. If I leave it at that temperature, it doesn't go back to being oversaturated. With the soda can, the gas does go back into solution if left alone.
It's perfectly soluble as long as it's not being agitated, and it returns to this state when left alone. There's just something about shaking it that dramatically lowers solubility.
why does tapping the can help?
It won't. When people pop open a bottle of bubbly wine, they will often tap the glass with a spoon to build pressure. That's also a jerk move to do to someone's beer; tap the side of it when they leave the table so it goes flat very quickly.
When something is in a oversaturated condition and some agitation causes it to crash out, isn't that usually a process that doesn't reverse itself?
Never stated that. I guess what happens is that whether there is a sharp rise in the pressure after shaking the can or not depends on the can history. If it was kept in cold place and later allowed to warm up, it can be oversaturated, if it was warm all the time (or since it was shaken), pressure inside is constant.
chemisttree
Nov22-10, 03:40 PM
It's about nucleation and pressure. At the high pressure in the can before you open it, the CO2 is in eqilibrium between the gas phase and liquid. Opening the can lowers the pressure and puts the CO2 dissolved in the liquid out of equilibrium. CO2 starts to come out. The speed at which it comes out depends on any nucleation processes that might accelerate it like producing lots of fine bubbles when the can is shaken. These small bubbles adhere to the walls of the container and stick. Opening the can causes a rapid nucleation and effervescence. Tapping the walls of the can and waiting for the fine bubbles to dislodge, float to the surface and pop (all three!) will reduce the tendency to vigorously effervesce. But you have to wait for the small bubbles to rise to the surface and pop, otherwise it will still foam. If you don't tap hard enough, the bubbles won't dislodge at all.
Try it yourself and see.
Robot B9
Nov23-10, 10:30 AM
Does anyone know:
1. What is the pressure in an un opened can at room temperature and in an unshaken condition?
2. What is the pressure in an un opened can at room temperature when it is "shook up"?
Soda can bottlers and can manufacturers must know about this or there would be exploding cans all over the place. There has to be a reasonable factor of safety on this sort of thing.
Sounds like one for Myth Busters...
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