How a Vacuum Flask Keeps Tea Hot for Hours

A vacuum flask (thermos) minimizes heat transfer through six design features:

1. Double-walled construction: Two layers (glass or stainless steel) with a gap between.
2. Vacuum layer: The gap is evacuated, eliminating conduction and convection (no molecules to transfer heat).
3. Reflective coating: Silver/aluminum on inner surfaces reflects radiant heat back inward.
4. Insulation synergy: Vacuum + reflection creates a near-ideal thermal barrier.
5. Tight-sealing stopper: Gasketed lid prevents heat loss through the opening.
6. Low-conductivity materials: Glass/steel conduct heat poorly, further reducing loss.

Together, these keep tea near boiling for 6–12+ hours, making the vacuum flask a masterpiece of passive thermal engineering.