Gases are all around us, but although many, such as
perfume, can be smelt, most gases are invisible. Like liquids, gases can flow
but, unlike solids or liquids, gases will not stay where they are put. They
have no set shape or volume, and they expand in every direction to fill
completely whatever container they are put into. If the container has no lid,
the gas escapes.
In hot air balloons, a burner heats the air inside. This causes the
particles of air to gain more energy and so they move faster and farther apart
from one another, pushing at the sides of the balloon. Heat always causes gases
to expand. If you left a balloon near a fire, the air inside could expand so
much that the balloon would burst.
Why does a champagne cork explode out of a shaken bottle? The
champagne inside the bottle contains lots of tiny bubbles of gas. Shaking the
bottle releases the gas, and the high-speed gas particles bang against the
cork. This creates an enormous pressure on the cork, and eventually forces the
cork out of the bottle.
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