How does a thermos keep cold things cold and hot things hot?
A thermos is also called a vacuum bottle because it uses a vacuum—a space that has no air in it—to keep heat from escaping from hot things inside; it also keeps heat from getting inside to make cold things warmer. The vacuum is located in a thin space between the thermos liner and its outer wall, where it stops the movement of heat to and from the outside air. A vacuum works in a thermos because it is empty of air (and molecules) and therefore has no conductivity. (Heat is caused by the motion of molecules.) Because the opening of a thermos is also tightly sealed with a stopper or lid made of a non-conductive material, no heat can escape or enter there, either. Hot food stored in a thermos can keep its heat for many hours; in the same way, cold food can remain cold because the vacuum insulates it from the warm air surrounding the thermos. The linings of thermoses used to be made of glass, which is a good insulator. The linings were also coated with silver, which made them shiny and reflective. Such mirrored liners worked very well—they were able to efficiently bounce back the invisible rays of heat energy (radiation) given off by all hot things. But there was one problem: glass thermos bottles broke easily. Today, most thermos containers are made of metal or plastic, which don’t break easily but also generally don’t work quite as well. Also, because vacuums in thermoses aren’t perfect—they contain some air—and because their lids don’t seal perfectly, they cannot keep cold things cold and hot things hot forever.