What is the primary purpose of a Dewar container?

The primary purpose of a Dewar container is to preserve the temperature of cryogenic liquids. Dewar containers are specially designed vacuum flasks that minimize heat transfer, allowing them to maintain extremely low temperatures for substances such as liquefied gases. The vacuum between the inner and outer walls of the container acts as an insulating layer, significantly reducing heat transfer through conduction and convection. As a result, they are ideal for storing cryogenic materials like liquid nitrogen or liquid helium, which must remain at very low temperatures to maintain their liquid state.

This unique feature is why the correct answer emphasizes the importance of temperature preservation in cryogenic applications.