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how does a vacuum flask keep liquids hot or cold

In a world where convenience and efficiency are paramount, thermos bottles have become an everyday necessity for many. These innovative containers, also known as thermoses or travel mugs, have the incredible ability to keep our favorite beverages hot or cold for extended periods of time. But how does the thermos work its magic? Let’s dive into the intriguing science behind these precious companions’ amazing insulating abilities.

Principle explanation

To truly understand the inner workings of a thermos, we must understand the fundamentals of heat transfer. Heat transfer occurs in three ways: conduction, convection and radiation. Thermos uses all of these methods to ensure insulation.

First, the inner chamber of the flask is usually made of double glass or stainless steel. This design minimizes conduction, preventing heat from moving between the liquid and the external environment. The space between the two walls is evacuated, creating a vacuum. This vacuum is an important insulator against conduction and convection heat transfer.

In addition, the inner surface of the container is coated with a thin layer of reflective material, such as silver or aluminum. This reflective coating minimizes radiative heat transfer because it reflects heat energy trying to escape.


The combination of vacuum and reflective coating significantly slows the loss of heat from the liquid inside the flask. When hot liquid is poured into a thermos, it remains hot because of the lack of air or particles to transfer the heat, effectively trapping the heat inside. Conversely, when storing cold liquids, the thermos prevents the infiltration of heat from the surrounding environment, thereby maintaining the desired temperature for a long time.

Additional features

Manufacturers often employ additional insulation to enhance the functionality of the flask. Some flasks may have copper-plated outer walls, which help to further reduce external heat transfer. Additionally, modern thermos bottles often have screw-on caps or lids with silicone gaskets to create a tight seal. This feature prevents any heat transfer by convection and ensures no spillage, making the flask portable and convenient.

Thermoses have revolutionized the way we enjoy hot or cold beverages on the go. By combining various insulating technologies such as vacuum, reflective coatings and additional insulation, these remarkable devices can keep our beverages hot or cold for hours, making them an indispensable tool in our modern fast-paced lifestyles.

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Post time: Jul-26-2023