Thermos Flask (Dewar Flask)

A thermos flask is used to keep hot liquids hot and cold liquids cold for a long time. Sir James Dewar was a Scottish scientist. He made the first thermos flask in 1892. That is why it is called the Dewar flask. Even today, a Dewar flask is used to keep a substance cold or hot.

• It has two glass tubes, one inside the other, with a gap between them.
• The surfaces of both tubes are coated with silver to make them shiny and reflect heat.
• The air between the tubes is removed, creating a vacuum, which prevents heat transfer.
• The flask is protected by an outer jar made of metal or plastic.
• Sponge or rubber is placed between the jar and flask to absorb shocks and prevent breakage.

The vacuum and shiny surfaces prevent heat from escaping or entering, keeping drinks hot or cold for a longer time.

Thermos flask

1. Prevents Radiation: The shiny surface inside reflects heat, stopping it from radiating out.
2. Prevents Conduction and Convection: The vacuum between the two walls stops heat transfer by conduction and convection.
3. Retains Heat or Cold: Since heat can’t escape or enter easily, the liquid inside stays hot or cold for a long time.
4. Some Heat Loss: A small amount of heat escapes through the lid and glass walls by conduction.
5. Temperature Change: After 2-3 hours, the hot substance may cool down slightly due to minor heat loss.