Question

The purpose of using vacuum flask is to store
(A) hot drinks.
(B) cold drinks.
(C) hot as well as cold drinks.
(D) None of these

Answer 1

Hint
The silver coating on the inner bottle prevents heat transfer by radiation, and the vacuum between its double wall prevents heat moving by convection. The thinness of the glass walls stops heat entering or leaving the flask by conduction. The case surrounding the flask provides additional insulation.

Complete step by step answer
The purpose of using the vacuum flask is to store hot as well as cold drinks.The basic concept behind using the vacuum flask is to keep the temperature of liquid constant which is being kept in the flask, no heat transfer would take place via conduction, convection or radiation. A thermos flask has double walls, which are evacuated and the vacuum bottle is silvered on the inside. The vacuum between the two walls prevents heat being transferred from the inside to the outside by conduction and convection.
Option (C) is correct.

Additional Information
A vacuum flask is an insulating storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck.

Note
The vacuum prevents conduction. The tight stopper prevents air from entering or leaving the flask, so convection isn't possible either. There's virtually no way heat can escape from a vacuum flask and a hot drink stored inside will stay steaming hot for several hours. Flasks also work for cold drinks.