A vacuum flask is a bottle or flask made of two glass containers with a vacuum between them. The vacuum prevents heat from being transferred between the inner and outer containers by conduction, convection, or radiation. This allows the vacuum flask to keep its contents hot or cold for a long period of time.

Here are the characteristics of a vacuum flask:

• Double-walled construction: The vacuum flask is made of two glass containers, with a vacuum between them. This vacuum prevents heat from being transferred between the inner and outer containers by conduction.

• Silvered walls: The inner and outer walls of the vacuum flask are silvered. This reflects radiant heat back into the flask, preventing it from escaping.

• Vacuum seal: The vacuum flask is sealed tightly to prevent air from entering the vacuum space. This prevents convection currents from forming, which would also transfer heat.

• Insulating stopper: The vacuum flask is stoppered with a material that is a good insulator, such as cork or foam. This helps to prevent heat from escaping through the opening of the flask.

Vacuum flasks are used to keep beverages hot or cold for extended periods of time. They are also used to store cooked food hot, and to transport laboratory samples that need to be kept at a specific temperature.

Here are some additional facts about vacuum flasks:

• The vacuum flask was invented by Sir James Dewar in 1892.

• The vacuum flask is also known as a Dewar flask or Thermos flask.

• Vacuum flasks are made of a variety of materials, including glass, stainless steel, and plastic.

• Vacuum flasks can be used to store liquids, solids, and gases.

• Vacuum flasks are used in a variety of industries, including foodservice, laboratory, and medical.

I hope this helps! Let me know if you have any other questions.