Question

Thermos flask keeps a liquid hot because

Answer 1

Glass is used in a thermos flask. Glass is an insulator and obviously a very bad conductor of heat. Hence heat is never lost by conduction in a thermos flask. Hence things are always kept hot.

Answer 2

Thermo flask is like a bottle inside another bottle and sealed the ends by melting inside. the air is removed out from the bottles to produce a vaccum inside,which is not a good conductor and does a good job at slowing down the movement of heat andhence the liquid kept hot.