no
The vacuum reduce heat loss by conduction and convection.
it is the cup that comes on a vacuum flask
Glass
There is a silvering in the vacuum flask in order to reduce the loss of heat through the means of radiation.
why are vacuum flasks called vacuum flask
Also known as a Buchner or vacuum flask, the flask uses vacuum to filter samples. A vacuum hose is attached to the hose barb and the funnel is placed on top. As the hose creates a vacuum, the sample is filtered through the funnel.
a vacuum flask is not 100% vacuum. it contains some particles or gas molecules which absorbs a small amount of heat.
When water freezes becoming ice it expands. In a vacuum flask as ice melts it will contract. This will cause the flask to crack.
There could be a crack or leakage that allows air to flow in or out of the flask. It may be time to replace the flask with a new one.
Thermos flask or vacuum flask.
A Thermos.
white