Vacuum flask

A vacuum flask is a special container that is designed in such a way that heat cannot easily enter or exit. That way, hot drinks are kept hot longer, and cold food is kept cold. The flask is made of two containers which are separated. Between the containers there is almost no air, so heat does not pass easily.the flask is also know as dewar flask after it's inventor.

Laboratory Dewar flask, Deutsches Museum, Munich

Diagram of a vacuum flask

A cryogenic storage dewar of liquid nitrogen, used to supply a cryogenic freezer

Sir James Dewar invented the flask in 1892. It consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum. This significantly reduces heat transfer by conduction or convection. When used to hold cold liquids, this also eliminates most condensation on the outside of the flask.

In the household, vacuum flasks are used to keep beverages hot or cold for longer periods of time, and for keeping cooked food hot. They are also used for thermal cooking. Vacuum flasks are also used for many purposes in industry.