Straws float in soda?

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Why do straws float in cups with soda but not in water? Is it the carbonation? If so, why?
Also, why does soda sometimes come out of the straw after drinking from the straw? Is it a vacuum that hasn’t been properly unsealed?

In: Physics
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The dissolved carbon dioxide forms (nucleates) in bubbles that affix themselves to the inside and outside of the straw, particularly where the straw’s surface isn’t totally smooth. These act as little buoyancy devices, and make the straw act as though it’s lighter than the soda around it.

Also straws are generally made from polypropylene which has a density close to one. Thus it floats in water/soda

If you put your finger over the top of the straw before putting it in the soda, then it won’t float even when you remove your finger

Even when the soda is flat (the CO2 is gone) straws will still float in some sodas because the sugared syrup in the water makes it thicker than normal water, and therefore slightly more dense than the plastic in the straw. The lighter material floats to the top.