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Cheryl Forhan, teed 12, of Ottawa., Canada for her question:

How does a thermos keep liquids either hot or cold?

Suppose you put down good money then learned you had paid for some empty space. Would you had been fooled? Well, you may not have been, For a really empty space is very hard to make. What's more, it is very useful when you have it.

But surely, you think, the world is full of empty spaces, There’s a space under the sofa, a space up the chimney and plenty of free empty space outdoors. The fact is, these empty spaces are really full of air.. Air fills cracks deep underground. It mixes with ocean water and reaches high above our heads. And air is made of tiny particles called molecules.

A really empty space is without even molecules of air. We call it a vacuum It is very hard to make this kind of hole in the afro In face no one can remove all the air molecules to make a perfect vacuum. We must settle for an almost‑vacuum ‑ which works very well,

When you buy a thermos, part of your money goes for one of these working vacuums. The inner shell of the flask is made of a double wall of glass. The air ‑ or most of it ‑ has been removed from the space between the two walls. They are sealed together to prevent more air from rushing in to fill the vacuum.

The job of this kind of vacuum is to keep warm things warm and cool things cool. As you know, heat and cold are forever mixing together. A hot pie loses its heat in a cool kitchen. A hot radiator warms a whole room. Warm things tend to give away their heat to cooler things around them. Cool things tend to steal the heat from warmer things nearby, But this mixing job needs air.

Most of the heat from the pie was carried around by the air in the kitchen. Most of the heat from the radiator eras spread around by air, Without a blanket of air, the pie and radiator mould not give away their heat so easily.

Hot soup in a thermos flask is sealed within a thin wall that has no air. Its heat finds it hard to escape and mix with the cooler air  outside the flask. The heat inside cannot easily get through its vacuum packet. So it stays inside and the soup remains hot.

Nor can heat outside the flask cross the wall of vacuum and get into the inner whell. If iced tea is corked tightly in the flask, it cannot steal the warmth from the air in the room. So the iced drink inside stays cold. The little vacuum ,packet keeps the surrounding heat out. It keeps the inside cold in. For heat finds it very hard to get through a vacuum ,packet.

For this reason, the ice tea sealed inside the flask stays cold for hours. If it is corked tightly, hot soup inside the flask will stay hot for hours, There is no blanket of air to spread the heat from one object to another.

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