Why Do Cans of Diet Coke Float But Cans of Regular Coke Don’t?

Mark K. asks: Cans of Diet Coke float. Cans of regular Coke don’t.  The cans are the same size and they contain the same amount of liquid according to the label. What gives?

Fun fact: If you put a can of Coca-Cola into a pool of water, it would sink. However, if you put a can of Diet Coke into the same pool, it would float. (Another fun fact: Coca-Cola was invented by a man seeking a cure for the morphine addiction he acquired during the Civil War, and he marketed Coca-Cola as that.)

Image result for cans of coke

In any event, initially, this floating problem seems like something that breaks physics because both cans are the exact same size and contain exactly the same amount of liquid.  Of course, anyone who’s ever taken a physics class at some point probably knows what makes a given object float in the first place, in the simplest possible sense, is that an object will float provided it displaces an amount of liquid (or sufficiently dense gas) that weighs more than itself.


So getting back to cans of floating fizzy drinks- we know from all this that given a can of Diet Coke floats and a regular can of Coke sinks, despite the pair containing the same amount of liquid and in the same model of can, the regular Coke’s combined ingredients must be more dense. And, in fact, a 12 ounce can of Coke has more mass because of the 39 grams of sugar it contains, compared to the 125 milligrams of sweetener in a can of Diet Coke.  This minute difference in density between both cans just so happens to result in the total mass of either can falling on either side of the average density of pure water (just slightly less than 1 g/cc, varying slightly based on temperature, reaching its maximum density at 4 degrees Celsius or 39.2 degrees Fahrenheit). This results in one can floating and the other sinking and science teachers everywhere having a trick to go along with their often questionably accurate anecdotes.


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