Denise Rupp, age 10, of Des Moines, Iowa, for her question:
How does a vacuum keep in both heat and cold?
A vacuum flask is a handy gadget to carry in your lunch box any day of the year. In winter, it can keep a helping of hot soup hot all day. In summer, it can stop a cold drink from becoming lukewarm. The magic vacuum, however, cannot work unless the flask is sealed securely.
A vacuum is no more than an empty space. There may, of course, be an empty space .on a shelf and another in the back of the bus. But these empty spaces are not vacuums because they are not really empty. True, there are no solid cane on the shelf and no solid body in the vacant bus seat. But these spaces, nevertheless, are filled up to the brim with air. A vacuum is an empty space with the air, or most of the air, removed.
Air is a mixture of gaseous molecules too small for our eyes to see. We cannot see clean, pure air or taste or smell it. Air seems to be nothing at all but you would be surprised at how many things it can do. It carries sounds. When you speak, your words are shoved along by busy molecules of air. An airless vacuum is a silent place. You could shout your head off and never be heard.
The air also carries heat. Gaseous air molecules spread heat from a hot radiator all around the room. The only difference between boiling water and ice cubes is heat. Without the help of busy air molecules, the hot water would stay hot and the ice would 'stay chilly. And a vacuum is an empty space with almost all of these busy air molecules removed.
To create a perfect vacuum, the last air molecule must be removed and this is no easy job. Scientists have almost done it, but not quite. However, an empty space with just a trace of air is a workable vacuum. One of these almost vacuums is sealed inside a vacuum flask to keep hot things hot and cold things cold. The silvery flask inside the outer shell has twin walls sealed together, one outside the other. Almost all the air molecules have been removed from the thin space between them. This is the almost vacuum that performs the magic.
Remember, heat cannot spread around without the help of air molecules. It cannot spread through the airless space between those two glassy walls. When you put a helping of hot soup inside the flask, virtually none of the heat can escape. It stays inside and keeps the soup hot. When you seal a cold drink inside the flask, heat from outside cannot get in to warm it up and it stays cold. The vacuum space between the walls stops heat from getting in and also from getting out of the flask. It seals heat in and the soup stays hot. It seals heat out and the cold drink stays cold.
The flask must be closed with a tight stopper, otherwise heat will come in or escape through the open top. If the stopper is loose, the magic will fail. But remember, the vacuum is not perfect. Gradually, heat seeps in or out; gradually, the cold drink gets warm and the hot soup gets cool. But a good flask with a tight stopper should keep your food either hot or cold all day long.