Kinetic Theory of Matter and States of Matter—Solids, Liquids, Gases
All matter in all creation responds to changes in temperature in several ways. One of those ways it in regard to its physical state. Depending on temperature, matter on earth will usually be either solid, liquid or gas (though two other states are possible under extreme conditions.)
To understand why matter changes state, it is necessary to first understand temperature. To understand temperature, it is necessary to understand the Kinetic Theory of Matter.
The Kinetic Theory of Matter says that all particles of matter are in constant motion. This means that in any sample, the molecules are moving. The faster an object (of any size) moves, the more kinetic energy it has.
There is a formula for kinetic energy that relates kinetic energy to mass and velocity:
KE = 1/2MV2
This formula models the observations that KE (kinetic energy) goes up as velocity increases. The Kinetic Theory of Matter says that the warmer something is, the faster the molecules go. Therefore, warmer molecules have more kinetic energy.
This is a key understanding for much of what follows with regard to states of matter.
The understanding of the Kinetic Theory of Matter leads directly to the definition of temperature.
Temperature is a basic measurement that is related to many of the concepts that will follow in the exploration of states of matter.
The average kinetic energy of the molecules in a sample.
We measure temperature with a thermometer (A meter of the thermo.). A thermometer is placed in contact with a substance and the molecules of the substance bump into the thermometer until its molecules have the same average kinetic energy as the substance. When this occurs, the thermometer will indicate a temperature in one or more standard temperature scales.
Temperature scales that are commonly used include Fahrenheit, Celsius, and Kelvin. Most science relies on Celsius and Kelvin.
To convert from Celsius to Kelvin:
Kelvin = Celsius + 273.15
Celsius = Kelvin - 273.15
To convert from Celsius to Fahrenheit:
Fahrenheit = Celsius * (9/5) + 32
Celsius = (Fahrenheit - 32) * 5/9
So, temperature can be understood as the average kinetic energy of the molecules as they move in constant motion.
The state of matter depends on its temperature. When the average kinetic energy of the molecules is relatively low, then the matter will be observed as a solid. Extremely low temperatures beyond what is normal can result in another state of matter called a Bose-Einstein condensate, which will be discussed later. As the temperature goes up, matter will change into a liquid, then into a gas. At ultimately high temperatures, matter will be a plasma.
When in the solid state, matter has a definite shape and volume.
Solids, at any given temperature, will not change in shape or volume. The size and shape of a block of metal does not change unexpectedly.
This is because the molecules are bound to each other by forces that are stronger than the average kinetic energy of the individual molecules at that particular temperature. Though the molecules "wiggle" in place, they are not moving with sufficient energy to break the bonds that hold them in place.
As the temperature goes up, the wiggling will take up more space, and most solids will expand, but the shape will remain uniform and the increase in size is small by comparison to the size of the original.
When in the liquid state, matter has a definite volume, but not a definite shape.
Liquids, at any given temperature, change shape to fit the container in which they are kept, but they retain their volume. Pouring a liquid from one container to another changes its shape, but does not change its volume.
The reason that liquids change shape is because the kinetic energy of the molecules exceeds the force that holds them together in the solid. But those forces still are strong enough to keep them from simply scattering anywhere. The liquid sticks to itself, but not so strongly that it has a fixed shape.
As the temperature goes up, molecules in a liquid (as in a solid) move more rapidly. The faster they move, the further they can get from the other molecules, and thus, they take up more space. As with solids, higher temperatures result in most liquids taking up more space, but the volume (as with solids) increase is small by comparison to the volume of the original.
When in the gas state, matter has neither a definite volume nor a definite shape.
Gases, at any given temperature, change shape and volume to fit and fill the container in which they are kept. Changing the size or shape of the container results in the gas rearranging to fill it.
This is because the molecules in a gas are moving so rapidly that they have exceeded the force that holds solids and liquids together. They have "escaped" and move freely within whatever space they are contained within.
As the temperature continues to go up, they will continue to move more and more rapidly. This change of kinetic energy will affect how hard they crash into the walls of the container—that is, if the volume does not change, as temperature goes up, the pressure in the container will go up.
A state of matter where molecular kinetic energy is nearly zero (thus, the molecules are nearly motionless).
When in the Bose-Einstein condensate state, all of the atoms of a substance behave as if they were a single particle. This occurs at near -273 C (0 K).
When you get to a temperature near absolute zero, something special happens. Atoms begin to clump. The whole process happens at temperatures within a few billionths of a degree, so you won't see this at home. When the temperature becomes that low, the atomic parts can't move at all. They lose almost all of their energy.
Since there is no more energy to transfer (as in solids or liquids), all of the atoms have exactly the same levels, like twins. The result of this clumping is the BEC. The group of rubidium atoms sits in the same place, creating a "super atom." There are no longer thousands of separate atoms. They all take on the same qualities and, for our purposes, become one blob.
At extremely high temperatures or low pressures, the kinetic energy of the molecules of a gas will be so high the electrons become free.
Plasmas are created when conditions are such that the atoms "shake off" their electrons. The space is filled up with the atom's nuclei and its electrons, but they are not bound to each other. From the framework of the kinetic theory of matter, plasmas exist when the kinetic energy exceeds the force that binds the electrons to the atom. This happens at very high temperatures, very low pressures, or some combination of the two.
Viewing states of matter through the lens of the kinetic theory of matter offers a powerful view of how and why solids, liquids, and gases behave as they do. Understanding the way they respond to increasing temperature (which is a measure of increasing kinetic energy) is more clear when understood from the model of moving molecules.