All the objects
that we see and touch are in one of three states: they are solids,
liquids, or gases. We are used to rocks being solid and gasoline
being liquid, and we think that these things are always so. However,
the atoms in every object that we see or handle can exist in
all three states -- solid, liquid, and gas. The state that a
rock or an ice cube is in depends on its temperature. Each substance
has a melting point and a boiling point. The response of materials
to temperature tells us something about what the atoms in that
material are doing.
Objects in a solid state maintain their shapes.
This is because, at those particular temperatures, the atoms
of these objects are staying in more or less the same relationships
in space to one another. The atoms may be jiggling around a little,
but they are not going anywhere. Sometimes the atoms or molecules
in solids will even form crystals, which are beautiful, regular
lattices of atoms in orderly arrangements. The images of solids
on this page are examples of diagrams of crystalline structures
from a program called Crystal Maker. (You can download a Demo
(Mac only) from http://www.crystalmaker.com/
and try playing with these shapes yourself. They are much
more fun in three dimensions.)
When a substance
reaches its melting point, the atoms and/or molecules have absorbed
heat energy. When they reach their melting temperature the molecules
"let go" of their neighboring molecules and start moving
around. The molecules in compounds still maintain the bonds that
caused them to be the substance that they are. With the increased
heat energy the atoms can flow past each other and be poured,
like water. Temperature makes the difference: adding heat makes
the electrons move more quickly.
have to be very hot to melt: consider the melted rock that pours
out from a volcano as lava. Human beings have learned to heat
metals and to make glass, and to shape these materials when they
have been softened by heat. When the glass and metals cool, they
maintain their new shapes, making them useful to us.
that allow rocks and metals to be in a liquid state are too hot
for carbon-based life forms. Our bodies are made mostly of water,
and that water would boil away. Other chemicals in our bodies
would burn, which means that they would unite with oxygen molecules
in the air and be changed into other compounds -- into ashes.
are heated further, they reach their boiling points. At this
point the electrons have absorbed more heat energy and the atoms
begin to move faster and faster, bumping into each other, and
bouncing off each other. If they are in a confined space, they
will push against the container: balloons will grow larger and
pressure cookers that have no vents will explode.
We have all
seen water come to the boil, beginning with little bubbles of
gas on the bottom of the pot and becoming more and more turbulent
as more of the water turns into steam and belches up through
the liquid. Given enough heat, anything will boil and turn to
vapor, even stones. The gases that we are familiar with, such
as oxygen and nitrogen, have too much heat energy to solidify
on earth. At lower temperatures, oxygen and nitrogen can be liquids
and even solids. "Dry ice" is solidified carbon dioxide.
Plasma is still
another state of matter, one which
we do not observe naturally on
earth or on our sister planets because it requires a great deal
of energy. When lightning
comes down it may turn the
atmospheric molecules into a
plasma -- but only for am
instant. Scientists have been able to produce
very special conditions in their labs. In plasma, the atoms have
absorbed even more
energy than in they gaseous
state and are now ionized. This means
that many of the atoms have lost electrons, which are now loose
in the plasma. The
matter in stars is in the
We have seen
that, as atoms and molecules are heated, they become less fixed
in their spatial relationships to each other.
Atoms in solids stay close together
with not much movement, like people sitting in a theater.
As atoms become hotter they begin to flow
past each other like people in a crowd on the street, becoming
With more heat, the atoms separate from one
another and bounce around in space like fast-moving dancers.
This is the gaseous stage.
In plasma the atomic particles are
moving freely and rapidly, as if the parts of a body had all
separated and were being juggled in space.
1996,1997, 1998, 1999,
2000, 2002, 2003.
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