Why does tapping the top of a shaken can stop the contents from spraying everywhere

A.� The tapping creates a vibration that releases the carbon dioxide bubbles from the sides and bottom of the can and they rise to the top. Here they join up, either forming bigger bubbles or mixing with the carbon dioxide gas that forms in the small empty space at the top of the can.

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So, when the can is opened all the carbon dioxide gas is already all at the top and escapes by itself, leaving the liquid in the can, where it belongs, rather than all over you.

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Q.� Why does a shaken can spray liquid

A.� Ordinarily, the gas bubbles in a carbonated beverage effervesce gently, rising slowly to the surface any time the can or bottle is opened or poured slowly.

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Shaking a can agitates the carbon dioxide gas bubbles, which are forced under high pressure to dissolve in the liquid during manufacture, dispersing them throughout the liquid. When the shaken can is opened this pressure is release and the dispersed bubbles expand and rush to the area of low pressure, i.e. the top of the can, pushing the contents ahead of them and out through the opening.

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Q.� Why does waiting a while have the same effect as tapping

A.� Shaking the can disperses carbon dioxide bubbles through the liquid, giving it thousands of places where it can change from its dissolved state back into a gas.

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But if the can isn't opened and therefore none of the gas can escape the high pressure inside the can eventually forces the gas to re-dissolve back into the liquid. This eventually lowers the pressure so that the can can be opened without the contents spraying everywhere.

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by Lisa Cardy