How Does a Vacuum Bottle Work?
A vacuum bottle is a bit like a super-insulated jug. A vacuum bottle works by insulating whatever is stored in it. A vacuum bottle (sometimes called a Thermos) is really a bottle within a bottle. Most of the air has been pumped out of the space between the bottles, thereby creating a vacuum.
Heat cannot travel easily through a vacuum. Some heat does get through the vacuum by radiation. Much of this heat, however, is blocked by the bottle’s shiny, silvery coating, which is designed to reflect heat. By shutting the heat in or out, the vacuum bottle helps keep cold liquids cold and hot liquids hot.
Whether you like your coffee piping hot or icy cold, vacuum bottles are an absolutely brilliant way to keep your drinks just the way you want. Some heat still escapes (or gets in) eventually, mostly through the stopper, but flasks or thermos like this are still a vast improvement on virtually every other kind of insulated drinks container.