Heat is transmitted in three ways: conduction, convection and radiation.
A flask keeps hot liquids hot and cold liquids cold by reducing the transmission of heat from inside the flask to the outside in the first case and from outside the flask to the inside in the second case.
- The vacuum or a lack of substance between the inner flask and the outer flask prevents the transmission of heat by conduction.
- The inner surface of the flask is silvered to reduce transmission of heat by radiation.
- The stopper prevents the transmission of heat by convection.
As there are only three ways by which heat is transmitted, the construction of a flask is such that heat transfer by all of them is reduced to a large extent.
A thermos is able to do what it does by using a several of physical and material properties to keep our hot chocolate hot and our ice tea icy. A thermos has an inner container made of glass. This container is really like one bottle inside of anouther bottle and sealed at the ends by melting the edges together. The air is removed out from the space between the two bottles to produce a vacuum, which is not a good conductor of heat and does a good job at slowing down the movement of heat. To slow down the other heat they coat the facing surfaces of the glass bottles with a silvery coating (like a mirror). This reflects the heat and helps slow down any losses that might get that way. They use a material like cork or rubber to make the bottle stopper and anything else that might touch the outside surface of the glass container. These materials are bad conductors of heat and slow down the heat loss. Whats left is what we can see on the outside which can be metal or plastic and is the covering for the glass bottles. The idea here (with the thermos) is to slow down the movement of heat from one place to the other. So if you have hot stuff in the thermos or cold stuff in the thermos, the end result is the same. Keep heat from moving to where you don't want it to go for as long as possible.