Why a vacuum flask slows temperature change
A vacuum flask has two walls with very little air between them. This gap greatly reduces heat transfer by conduction and convection, while reflective surfaces reduce transfer by radiation. The flask slows heat moving in either direction, so the same container can help keep soup hot or water cold.
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A barrier to heat transfer
Heat can travel through direct contact, moving fluids and radiation. A vacuum flask uses several features to slow those different routes.
The space between its walls contains very little matter. That greatly reduces conduction through the gap and prevents ordinary air circulation there. Reflective surfaces help reduce the exchange of thermal radiation between the walls.
The vacuum does not remove every route for heat. Heat can still pass through the neck, lid and other connections, so the contents gradually approach the surrounding temperature.
- Hot soup loses heat more slowly.
- Cold water gains heat more slowly.
- Opening the lid gives heat another route in or out.
The flask does not actively heat or cool its contents. Its job is to slow the exchange, which explains why starting temperature and lid design both affect how long a drink stays as intended.