Which of the following is NOT a special design of a vacuum flask to keep water warm for long time?
A The outer glass wall is painted in silvery.
B The layer between the double-glass wall is a vacuum.
C The stopper is made of plastic.
D The inner glass wall is painted in black.
The correct answer and explanation is :
The correct answer is C. The stopper is made of plastic.
Explanation:
A vacuum flask, commonly known as a thermos, is designed to maintain the temperature of the contents inside, either hot or cold, for an extended period. The key mechanisms that help in achieving this thermal insulation include the structure of the flask and its material properties.
- A. The outer glass wall is painted in silvery:
This is a common feature in vacuum flasks. The outer glass wall is often coated with a reflective silvery paint, which helps in reducing the heat exchange between the flask and the outside environment. This reflective surface minimizes heat transfer by radiation. By reflecting radiant heat back inside, it helps maintain the internal temperature. - B. The layer between the double-glass wall is a vacuum:
This is one of the most critical design features of a vacuum flask. The space between the two glass walls is a vacuum, which eliminates air and thus significantly reduces heat transfer via conduction and convection. Without air molecules, the flask cannot easily transfer heat, keeping the contents warmer for a longer period. - C. The stopper is made of plastic:
While a plastic stopper may be used for ease and convenience in some vacuum flasks, it is not a special design feature aimed at heat retention. The stopper is primarily designed to seal the flask, preventing leaks. However, the material of the stopper (whether plastic or another insulating material like cork or rubber) does not significantly contribute to the thermal insulation properties of the flask. - D. The inner glass wall is painted in black:
The inner glass wall of the vacuum flask is often painted black. This design choice is intentional because black absorbs heat more efficiently than other colors. If the flask is used to store hot liquids, the black inner surface helps absorb and retain heat, thereby preventing heat loss.
Thus, the correct answer is C because the material of the stopper is not directly related to the thermal insulation function of the vacuum flask.