How does a thermos keep drinks hot or cold?
First, let me give you the answer in 30 seconds.A thermos cuts down all three ways heat can travel. The space between its double walls is a vacuum, which stops heat that passes by touch and heat carried by moving air. The walls are also shiny like a mirror, which reduces heat escaping as infrared light. A thermos does not warm what is inside; it slows heat moving in and out, so it works for cold drinks too.
Three ways heat travels
Heat travels in three ways: "conduction," where heat passes between things that touch; "convection," where moving air or water carries heat; and "radiation," where heat leaves as infrared, a kind of light our eyes can't see. A thermos is a container made to cut down all three at once.
Double walls and a vacuum
The part of a thermos that holds the drink has two walls, one inside the other, and the air between them has been removed to make a vacuum. Because the two walls do not touch, conduction has a hard time happening. And because there is almost no air between them, there is no convection, where moving air carries heat. Physics teaching material from the University of Sydney explains that even when the inside is hot, the outer wall stays at about room temperature.
Mirror-like walls cut radiation
The remaining one, radiation, is reduced by walls that shine like a mirror. According to an explanation from the University of Illinois, black things absorb and give off heat well, but a shiny surface reflects infrared and gives off very little of its own. So the heat from a hot drink is reflected back inward. The opening is closed with a lid that does not pass heat easily.
It slows heat instead of warming
A thermos is not a tool for warming its contents. It only slows heat moving in and out, so the same design also keeps cold drinks cold. However, it cannot stop heat completely. Heat slowly passes through the neck, where the two walls are joined, and through the support at the bottom.
Want to know more?
- A vacuum flask is also called a "Dewar flask," after its inventor, the Scottish physicist James Dewar.
- In research and science demonstrations, containers that work the same way are used to keep very cold liquid nitrogen cold.
- With a vacuum between the walls, the outside air pushes on them, but if the container is designed correctly with the strength of the materials in mind, the walls do not get crushed.
Quick quiz: test yourself
How does a thermos keep drinks hot or cold?
Show the answer
- It heats the drink
- It wraps it in thick metal
- It slows heat moving in or out
It slows heat moving in or out A vacuum between double walls and mirror-like surfaces cut down the ways heat can travel. That is why it keeps cold drinks cold, too.
A vacuum between double walls and mirror-like surfaces cut down the ways heat can travel. That is why it keeps cold drinks cold, too.
Next quiz ›Sources
- Characteristics of DewarsUniversity of Illinois Urbana-Champaign, Physics Van
- Thermos Flask (Workshop Tutorials for Physics, Thermal Physics Activities)The University of Sydney, School of Physics
This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.
