how does a thermos flask retain water
Answers
A vacuum flask is a bit like a super-insulated jug. Most versions have an inner chamber and an outer plastic or metal case separated by two layers of glass with a vacuum in between. The glass is usually lined with a reflective metal layer. Unbreakable flasks do away with the glass. Instead, they have two layers of stainless steel with a vacuum and a reflecting layer in between them. There's also a tight, screw-down stopper on the top.
These few, simple features prevent virtually all heat transfer by either conduction, convection, or radiation. The vacuum prevents conduction. The tight stopper prevents air from entering or leaving the flask, so convection isn't possible either. What about radiation? When infrared radiation tries to leave the hot liquid, the reflective lining of the inner chamber reflects it straight back in again. There's virtually no way heat can escape from a vacuum flask and a hot drink stored inside will stay steaming hot for several hours.
Flasks also work for cold drinks. If heat can't escape from a vacuum flask, it follows that heat can't penetrate into a flask from outside either. The sealed stopper stops heat getting in by convection; the vacuum stops conduction, and the metal lining between the outer case and the inner chamber stops heat radiating in either.
Whether you like your coffee piping hot or icy cold, vacuum flasks are an absolutely brilliant way to keep your drinks just the way you want. Some heat still escapes (or gets in) eventually, mostly through the stopper, but flasks like this are still a vast improvement on virtually every other kind of insulated drinks container.
