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.

     Some substances 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.

     Temperatures 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.

  

     When substances 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 plasma under 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 plasma state.

     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 liquids.

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.