How does a thermos flask keeps a hot liquid hot and a cold liquid cold?

Answer 1

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 another 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. What's 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.

NB It's worth mentioning that modern vacuum flasks (Thermos being a trade name) are made almost entirely of stainless steel, inside and out - it doesn't shatter and is naturally reflective, although not polished to a mirror shine when used as a flask inner. Modern stoppers are also made of plastic.

Answer 2

Thermos is made in a double layered structure with space for air in between. The interior and exterior are both made of heat insulating materials. The air in between these two layers slows down the heat transfer inside and outside substantially too. Thus hot contents remain hot and cold contents remain cold.

Answer 3

They are specially designed to limit the amount of heat conduction between the liquid and the surroundings. Air is a very good insulator, better still is a vacuum. By having a smaller vessel in a larger vessel, an airspace or a vacuum can be between the smaller vessel within the flask and the outer vessel. The thermos flask also limits the amount of conduction through the solid part buy limiting the contact points in the design. Some flasks also have a silvery surface to limit transfer of heat through radiation.

Answer 4

The flask is two cylinders with insulation in between.

The insulation prevents the interior cylinder (flask) from changing temperature as quickly as the outer cylinder (skin) and so the liquid retains it original temperature for a longer period of time.

The quantity and quality of the insulator determines how well the thermos works.

This is exactly the same concept as a picnic ice chest, or the insulated mugs sold for coffee or soft drinks.

Answer 5

Insulation!

High quality (and expensive) thermos bottles use something called a dewar flask to provide this insulation. A dewar flask is a double walled glass container with vacuum between the walls and both walls silvered like mirror.

• heat conduction is insulated by the glass
• convective heat transfer is insulated by the vacuum
• radiative heat transfer is insulated by the mirror like silvering on the glass, which reflects heat back where it came from

Unfortunately dewar flasks being made of glass are very fragile, don't drop one.

Medium quality thermos bottles use a double walled metal flask to provide this insulation. The metal flask does not insulate well against heat conduction, but is far more rugged than a dewar flask. The metal flask may or may not have vacuum between the walls, so it may or may not insulate against convective heat transfer like the dewar flask. Silvering is not needed as the metal is already shiny and reflective to insulate against radiative heat transfer.

Inexpensive thermos bottles just use a single walled metal or plastic flask surrounded by expanded plastic foam to provide this insulation. The plastic foam is fairly good insulation against conduction and convection but not radiation. It is usually even more rugged than a double walled metal flask as the foam provides shock absorbing. But heat loss is far more rapid than in the other types of thermos.

Special purpose dewar flasks have been used to hold liquefied gasses at close to absolute zero and even molten metals at several thousand degrees (these use double walled silvered ceramic flasks not glass) for significant periods of time.

Answer 6

they have an inner and outer wall in between the walls, some thermoses use a vacuum, since the only form of heat that can travel through a vacuum is radiation. other use dead air space

Answer 7

Helps keep heat energy from getting into the thermos.