The Kinetic Theory of Matter is the statement of how we believe atoms and molecules, particularly in gas form, behave and how it relates to the ways we have to look at the things around us. The Kinetic Theory is a good way to relate the 'micro world' with the 'macro world.'

A statement of the Kinetic Theory is:
1. All matter is made of atoms, the smallest bit of each element. A particle of a gas could be an atom or a group of atoms.
2. Atoms have an energy of motion that we feel as temperature. The motion of atoms or molecules can be in the form of linear motion of translation, the vibration of atoms or molecules against one another or pulling against a bond, and the rotation of individual atoms or groups of atoms.
3. There is a temperature to which we can extrapolate, absolute zero, at which, theoretically, the motion of the atoms and molecules would stop.
4. The pressure of a gas is due to the motion of the atoms or molecules of gas striking the object bearing that pressure. Against the side of the container and other particles of the gas, the collisions are elastic (with no friction).
5. There is a very large distance between the particles of a gas compared to the size of the particles such that the size of the particle can be considered negligible.

Since light, electromagnetic radiation, is required to see an object and light hitting an object gives it energy, as soon as one is able to see an object at absolute zero, it is not at absolute zero anymore from the new energy. Any other means of detection would add energy to the material at absolute zero. An object at absolute zero would be as hard to keep as a lump of antimatter. What would you keep it in? Practically, we can cool something down to temperatures approaching absolute zero, but we cannot get to that theoretical point, nor can we achieve temperatures below that point. There is no such thing as a temperature below absolute zero.

The Kinetic Theory of Matter does not, and is not intended to, take into account the energy of atoms due to excitation of electrons as you might see in glowing neon in a neon light or the bright redness of molten iron. In fact, objects cooler than molten iron and less excited than electrified neon will give off electromagnetic radiation, but that is another story.

In the view at this level, it is useful to look at atoms as if they were close to the hard little balls that Dalton considered. With this very mechanical view of atoms and molecules, we are losing some important facts to get an instructive thought on matter.

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SOLIDS

Solids are materials in which the atoms or molecules are set in place. In ionic solids such as table salt crystals, the ions are connected to their neighbors by electrical attraction. Covalently linked crystals such as diamonds produce the hardest materials. In other solids, each unit may have its own spot in which it fits (as in sugar crystals) or it may be just a jumble of molecules as in glass that have decreased energy. Crystalline solids have characteristic angles and can be cleaved along lines defined by the aligning of atoms or molecules of the crystal. Amorphous (without crystal shape) solids can be like carbon black or linked as in plastics. The common point about solids is that the atoms or molecules are in place. The temperature that can be shown by solid materials is due to the movement in place of the atoms or molecules. They have no independent linear motion of translation because they are attached to one another. Solids can have molecular energy due to vibration and rotation. Picture a class of second graders glued to their seat. Each student can jump up and down and sideways and turn the chair around, but they can’t move out of place. Another useful mental picture is a junkyard for springs. The springs have all been tied to each other in one enormous mass. Each spring can twist and vibrate, but it can’t get loose from its neighbor.

It is now necessary to change from being able to see and understand each atom or molecule to our larger world. Solids show a definite shape and a definite volume. Unless forces are used that are not commonly found near the earth’s surface, solids can not be compressed.

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LIQUIDS

Liquids are materials in which the atoms or molecules are as close to each other as solids, but the materials can slip over each other to change places. If you were only a few magnitudes larger than atoms, you might view liquids as B-B’s in a dump truck. Consider a large dump truck going fast down a very bumpy road. The B-B’s have some energy from the bumpy road. The top of the load is level. A few B-B’s are always in the process of getting enough energy to hop out of the dump truck. (This is a picture of vapor pressure of a liquid.) The B-B’s can be poured out of the dump truck. If there were a hole in the bottom of the dump truck, the B-B’s would leak out onto the ground. Like the B-B’s, liquids have no shape except for the shape of the container. B-B’s and liquids can not be compressed under common pressures. In a liquid the forces that hold the particles of liquid close to each other are greater than the forces due to motion that would force the particles away from each other.

The property of liquids of incompressibility is useful to us in hydraulic machines. A simple system of automobile hydraulic brakes are a good example of this. The brake pedal pushes a master cylinder. The travel (A description of distance (!) See Units and Measures.) of the brake petal is a few inches. The master cylinder pushes a small area of a liquid (hydraulic fluid) down a small tube (the brake lines) to the wheel cylinders. The wheel cylinders have a much larger area, but they go a shorter distance to push the brake pad against the drum or rotor, depending on what kind of brakes you have. The brake system cannot work correctly if there is any air (gas) in the system because the gas is compressible.

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GASES

Gas, or vapor, is the most energetic phase of matter commonly found here on earth. The particles of gas, either atoms or molecules, have too much energy to settle down attached to each other or to come close to other particles to be attracted by them. Material in the vapor phase have no shape of their own, that is, they take on the shape of the container. Gases have no given volume. A certain amount of gas at a pressure of one atmosphere and a volume of ten liters could become five liters if the pressure was increased or would become more than ten liters if the pressure was decreased.. The gas expands to fill the container. The Gas Law that covers the calculations of the pressure, volume, and temperature of gases is in a later chapter.

How can you picture the materials as a gas? A pool table is only in two dimensions, but what if the balls kept moving and the pool table were in three dimensions? Such a pool table would be like a gas. The rails of the 3-D pool table would be the sides of the container. The billiard balls would bounce off each other in completely elastic collisions and would bounce off the sides of the table to produce a constant pressure. The real hallmark of the gas is that the motion of the particles is so great that the forces of attraction between the particles are not able to hold any of them together.