Why does a liquid remain hot or cold for a long time inside a thermos flask?!
The presence of air, a poor conductor of heat, between the double glass wall of a thermos flask, keeps the liquid hot or cold inside a flask for a long time.
Well, in a technical sense, a Thermos bottle is really a vacuum bottle. Thermos is a brand name. Anyway, the glass envelope is a double wall. Imagine you are holding a round balloon in one hand and you use a finger to push in a depression, but not to the point where your finger reaches the other side of the balloon in your palm. THAT is what they do with the glass, only they make a bottle shape of the inside and outside. The outer wall is the part in the palm of your hand. The inner wall is the depression you make when you push in with your finger. With the balloon, the space if full of air, but they take the air out of the space to make a vacuum bottle. The vacuum is what makes the bottle work as an insulator. With nothing inside the space to carry heat, it insulates the inner wall from the outer wall. The only path for heat is around the top where the inner wall meets the outer wall. The area where heat conducts around the lip is tiny compared to the rest of the surface area which is insulated by the vacuum. Most are made from glass and as a result, are fragile. I have a rugged model which is all stainless steel.
good question. My guess is it like compresses the hot air, and the mist floats around keeping it warm. It stays cold because of the material i would say