A vacuum flask or Dewar flask has the remarkable capacity to hold extremely cold liquids like liquid nitrogen for a considerable length of time. The boiling point of liquid nitrogen is 77K compared to a room temperature of about 300K. You can hold liquid nitrogen for about a day without even a cover on the flask! You might expect such a cold liquid to explosively evaporate on contact with air in such an open container. How does it manage this containment?

Clearly, the flask is able to effectively block heat transfer from the outside either through the walls or through the open top.

The blocking of heat transfer into the liquid nitrogen from the top of a vacuum flask involves the insulating properties of gases and the ideal gas law. The gas above the liquid surface can be presumed to be dry nitrogen. When the ideal gas law is applied at approximately constant atmospheric pressure:

So the cold nitrogen above the liquid occupies about one-fourth of the space it would occupy in the surrounding room- temperature air. The dry nitrogen at about four times normal density forms an effective insulating "stopper" on top of the liquid.