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How can a heavy iron anvil float on liquid mercury? (Video)

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Posted May 31, 2018

Turn on your imagination and answer – what would happen if you would threw a 50 kg anvil in a bath of water? It would obviously make a huge splash and sink to the bottom immediately. But there is a liquid that would float the anvil fairly easily – it‘s mercury. So why does an anvil float on mercury, but sinks in water? And what does it sound like when you hit it with a hammer?

The size of the anvil doesn’t matter – as long as it’s made from iron (steel) it will float on mercury. Image credit: Santeri Viinamäki via Wikimedia (CC BY-SA 4.0)

Floating is actually a quite simple concept. Weight of things doesn’t even matter really. If you took a giant piece of cork it would still float on water no problem, regardless of how much it would weigh. In the same way, it doesn’t matter that the anvil weighs 50 kg – 100, 1000 or even 10 000 kg anvil would also float in mercury no problem. Meanwhile 10 grams of tungsten would immediately sink to the bottom of the bucket with liquid mercury. How come?

It is all about density and not the weight. Denser materials like staying closer to the ground and they can generate enough pressure to lift less dense materials. For example, density of pine is around 0.65 g/cm3, while water at room temperature is close to 0.998 g/cm3. This means that pine wood would easily float on water. And it definitely does – there are only several tree species that reach such density that their lumber doesn’t float on water. Meanwhile the density of steel is typically around 8 g/cm3 and so it cannot float on water. But the density of liquid mercury is around 13.5 g/cm3, which makes it easy for an anvil to float on it.

You might notice that the difference between densities of wood and water is actually smaller than between steel and mercury. This means that an anvil would be even harder to push down than a plank of pine. Tungsten is denser than mercury (19.3 g/cm3) and so it has no trouble reaching the bottom of the mercury bucket.

This was shown very nicely by a popular YouTube creator Cody don Reeder, who has a lot of experience with mercury. Handling such big amounts of mercury might be dangerous because of toxic vapours, which is why Reeder chose a nice breezy day for his anvil in mercury demonstration. He also hit the anvil with a hammer several times. When the anvil is out of the mercury bath it sounds much louder. In fact, a lot of people find that noise painful. But in mercury the anvil made a much quieter and muffled sound.

This is what science classes in school used to teach us, but they didn’t have good ways of showing it with real examples. That is the beauty of YouTube – you can find all sorts of visual information in shape of experiments, which you couldn’t do yourself.

 

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