boiling liquid expanding vapour explosion

Thanksgiving turkey and drinking water

Vasudevan Mukunth

— 2 min read

Saw this 20-something-second long video going around on Facebook and Twitter:

By itself, the video, produced by the US Consumer Product Safety Commission, tells me nothing apart from what I shouldn’t be doing, no reasons or explanations. Versions of the video were also carried by The Guardian, USA Today, Reuters, NBC News, Washington Post and Scroll. (Disclosure: I work for The Wire, which competes with Scroll.) So I went looking – and found the answer on mental_floss:

The instant the frozen food hits the oil, the ice crystals melt, then momentarily sink. This exerts an upwards force on the oil. An instant later, these small sinking bubbles of water boil, expanding as they heat up and adding further force to the oil. This bubbling and forcing the oil upwards creates an aerosol of boiling oil and air violently shooting up out of the pan and towards the other parts of the kitchen.

I believe this also has a technical term, though it seems forced: ‘boiling liquid expanding vapour explosion’. The explosiveness arises from a process called flash evaporation (*junior year thermodynamics memories*) – when the pressure surrounding a liquid is suddenly reduced significantly, causing some of the water to instantly turn, or ‘flash’, into vapour (Business Insider has the video explainer). Because gases occupy more volume than liquids, flashing can also be interpreted as an explosive expansion.

One useful application: In most desalination plants around the world, salty water is passed through a throttling valve that converts some of it into salt-free vapour, which is condensed into potable water. The remaining salty water is then sent through another throttling valve at a lower pressure to repeat the process. This is called multi-stage flash distillation. Other applications: fire extinguishers and pressure cookers being able to let off steam. Some unfortunate ‘applications’: boiler explosions and rapidly worsening accidents involving tankers.

Relevant to the case of the frozen turkey: water boils at 100º C and oil boils at 450º C – both at atmospheric pressure. So when the turkey is dipped into a vat of very hot oil, the ice crystals falling off into the container sink beneath the oil but begin to boil on the way because of the oil’s temperature. Because water is denser, the boiling water remains trapped under its hot oil ceiling, although it’s also expanding because of the heat. At one point, the ceiling ruptures and the water flashes out, carrying some oil with it. The real problem begins when the oil is splashed into the fire below the container. Then, as mental_floss writes, “in a matter of seconds after putting your dinner on, your dinner has destroyed a large amount of your property and a significant portion of your, well, life.”

Featured image credit: toasty/Flickr, CC BY 2.0.

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