Question

A thermo flask
(A) keeps hot liquid hot and cold liquid hot
(B) keeps cold liquid hot and hot liquid cold
(C) keeps both cold and hot liquid cold
(D) keeps cold liquid cold and hot liquid hot

Answer 1

Hint
This problem is related to heat transfer, generally heat is transferred from body to another body by three processes: they are conduction, convection and radiation. The main function of the thermo flask is to prevent the heat transfer.

Complete step by step solution
The thermo flask is the special kind of flask where the hot liquid like tea, coffee will remain hot for a long time and the cold liquids like cold water, juice will remain cold for a long time. Basically, the hot liquid will become cold by losing the heat to the nearby cold substance. For cold substances, it absorbs the heat from the nearby object. And the main function of the thermo flask is to prevent the heat transfer.
The thermo flask is made up of two layers and these two-layers have some gaps. Vacuum between these two layers of thermo flask helps to prevent the heat transfer through convection. And the glass walls are coated with silver because it reduces the heat transfer by means of radiation. The silver coated surface becomes shiny, so it reflects all the heat inside the wall, and it does not allow the heat to go out. And the layer of glass wall thermo flask is thin, so it prevents the loss of heat by conduction. Therefore, the thermo flask keeps cold liquid cold and hot liquid hot.
Hence, the option (D) is correct.

Note
The thermo flask is otherwise called an insulated flask. Since the vacuum is present in between the two walls of the flask, the flow of heat by convection is stopped. As a result, hot liquid kept in the thermo flask does not become cold and cold liquid does not become hot.