Ask A Scientist Chemistry Archive

name         Joe B.
status       educator
age          30s

Question -   Why does pop explode when it is shaken?  When you open the pop not shaken it 
just makes a fizz sound as the pressure is released.  When the pop is shaken no additional 
pressure is created from outside the can yet the contents will explode out when opened.
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Joe,

It is all a matter of a gas/liquid solubility equilibrium. When the can of pop is sitting 
undisturbed, most of the gaseous carbon dioxide "fizz" is dissolved in the liquid. Shaking 
the can creates bubbles in the liquid that come from the headspace in the can. These 
bubbles provide nucleation sites in which soluble carbon dioxide can escape from its 
dissolved state. In essence, the gas/liquid equilibrium is disturbed when the can is 
shaken. As a result, the carbon dioxide moves from a soluble condition to a gaseous 
condition in the headspace of the can. When the can is suddenly opened, much of the 
remaining dissolved gas comes out of solution in a gush. Thus, the mess.

If the can is shaken, but not immediately opened, in time the equilibrium will 
re-establish itself and the can will not erupt when opened. It will behave just as 
an unshaken can would if it were to be opened.

Regards,
ProfHoff 774
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When a soda is bottled, it is bottled under a pressure of CO2 that exceeds the solubility 
of CO2 in the rest of the formula (mostly water). When the can is opened without shaking 
and without other seeds for the dissolved CO2 to form gas the soda slowly loses CO2 until 
the solubility of CO2 is achieved. When the unopened can is shaken, the supersaturated 
solution evolves its CO2 with the result that the pressure of CO2 is significantly greater 
than the normal pressure would be -- hence the "explosive" evolution
of CO2 gas. A similar thing happens if you freeze the soda because CO2 is not very soluble 
in ice.

Vince Calder
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Joe B.,

There are three main ingredients in soda pop: water, syrup and carbon 
dioxide gas. The carbon dioxide gas is responsible for the fizz.

When the ingredients are combined, the carbon dioxide gas, syrup and water 
form a solution.  The ingredients are mixed in a container and the empty 
space at the top of the container is filled with carbon dioxide gas.  The 
bottle or can is then sealed.  The carbon dioxide at the top of the 
container is under pressure and prevents any of the dissolved carbon 
dioxide from escaping from the solution.

When the bottle is opened, a familiar hiss is heard and the carbon dioxide 
gas in the top of the bottle is released.  The dissolved carbon dioxide 
can now escape from the solution; the result is the familiar bubbling 
carbonated drink.

When you vigorously shake a can or bottle of soda, this gives the carbon 
dioxide gas more kinetic energy (motion) and causes the pop to spray or as 
you say "explode" from the bottle.  I hope that this helps.

Sincerely,

Bob Trach
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