Float or Sink: Don’t believe everything you see on the internet

Wow! A can of Diet Coke floats and a can of regular Coca Cola sinks! Really? Well No. Like so many things you see on the internet, it is not so simple.

While discussing recent news stories on the dietary ‘value’ of sugar, a colleague told me he had seen a surprising video on the internet.

It showed Steven Spengler putting cans of Coca Cola and Diet Coke into a tank and the can of Diet Coke floated while the can of Coca Cola sank. The celebrity scientist then related this to the amount of the sugar in the drink. See what you think:

I told my colleague that I didn’t believe anything I saw on the internet, but that the demo was convincing: it seemed as though nature itself was voting on the evils of sugar. A sort of ‘Witch Trial’ for harmful additives.

However being the person I am, I thought I would just check. I bought a can of each drink at lunchtime, went to the lab and did the test. This is what I saw:

This is what I saw when I put the two cans into water: they both floated.

Yes, that’s right: both cans floated. If you look closely you can see that the Coca Cola is lower in the water, but it did not sink. So how did I get the picture at the top of the page? Simple: I heated the water.

The density of water falls slightly with increasing temperature (See the graph at the bottom of this article) and when the water was around 36 °C at the top and 32 °C at the bottom, the Coca Cola sank. But when the water cooled to between 33 °C at the top and 30 °C at the bottom, the Coca Cola floated.

I made measurements of the mass of the cans, full and empty and found that the 330 ml of Coca Cola weighed 340.2 g while 330 ml of Diet Coke weighed 330.8 g. And hence I made an estimate for the density of the two fluids, and yes, Coca Cola appears to be 2.8% denser that Diet Coke.

But then the label tells you that 330 ml of Coca Cola contains 15.9 g of sugar, whereas Diet Coke contains Aspartame which is weight-for-weight 200 times sweeter. So there is only about  0.07 g of Aspartame in a can of Diet Coke. So this isn’t really ‘news’ of any kind.

I couldn’t understand the exact 9.4 g difference in mass because the density of the fluids is affected by all the other components ingredients which could differ between products.

However both fluids were denser than water (0.5% and 3.3.% respectively) . And whether a can floats or sinks depends only partially on the density  of the liquid in the container.

It also depends on the ‘air’ gap, and the weight of the can. So Steven Spengler’s demo just relies on a simple coincidence between the average density of the US-size cans and the density of water at about room temperature.

For larger containers, the mass of the container will make less difference and so I thought that for a 2 litre PET bottle, both fluids would probably sink. Was I right? No.

The Coca Cola weighed in at 2.129 kg and the Diet Coke was 85 g lighter at 2.044 kg. But in fact these bottles have a larger air gap and so – even when heated to 48  °C – both bottles floated.

Two 2 litre bottles of Coca Cola and Diet Coke floating in water – even when heated to 48 Celsius. The blue lines scrawled on the photograph show the water level around each bottle and the value on the thermometer.

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The density of pure water plotted as a function of temperature and fitted with a quadratic polynomial. The entire vertical range represents just 1% change of the density.

Tags: Coca Cola, Density, Diet Coke