I had a thermos flask that I had never used since I had bought it. Today, I decided to take it out and use it for storing hot water, so that I don't have to heat water every time I feel thirsty.

An interesting incident occurred. I heated water to near about boiling temperature, and filled the thermos flask with it. Then I tightened the lid. And then I started to hear this sound from the flask.

I realised that the sound is coming only if I put in hot water, not cold water. It's a weird sound. It is stopping if the flask is kept undisturbed for some time, but starting again if I move the flask.

Can anyone account for this sound?

The room temperature is about 15°C.

  • $\begingroup$ does your flask consist of a metal liner surrounded by a plastic cylindrical housing? $\endgroup$ Jan 12, 2018 at 19:59
  • $\begingroup$ It has a metal cylinder inside, then a gap filled with air (because I cannot expect vaccum at the price I bought it), and then a semi-transparent blue fiber glass outer covering. $\endgroup$ Jan 12, 2018 at 20:09

1 Answer 1


What is probably taking place is this: when something hot is inside the thermos, the air trapped between the outside of the thermos liner and the inside of the plastic sleeve surrounding it begins expanding due to (slow) heat loss from the thermos liner. where the liner meets the sleeve around the upper end of the assembly there is a press-fit joint which is probably 1) not totally airtight and 2) has a little water, tea, coffee, etc. sitting in it. when the pressure inside this space rises to a certain level, it pushes air out through that space against the surface tension of the water in that crevice, the fluid rebuilds the closure, it gets pushed out, etc. creating that brief bubbling noise. once the internal pressure falls below that threshold, the bubbling stops and the pressure builds up again.

You can test this idea by filling the thermos with something hot, capping it, and immersing it in a tub of water. watch for bubbles coming from one of the joints.

  • $\begingroup$ Tested it and it works. $\endgroup$ Jan 12, 2018 at 20:42
  • $\begingroup$ this was fun to think about! $\endgroup$ Jan 12, 2018 at 20:49

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