Facts of the Matter
States of matter easily seen in water
ALL MATTER can exist in only four different states: solid, liquid, gas and plasma. This is true of all substances, although the relationships between the states can be quite different depending on the substance.
We only encounter plasma here on Earth in closed tubes such as those in fluorescent lights, xenon flash on our cameras, or in the tiny pixels of plasma TVs.
Plasma consists of oppositely charged atomic nuclei and electrons, having been energized to the point where electrons are completely stripped away from their atoms.
Virtually all substances can exist in the other three states, although the solid, liquid and gaseous forms of water are the ones we most commonly encounter.
Transitions between the three states always involve the absorption or release of energy in the form of heat, which is called latent heat.
Latent heat is defined as the amount of heat required to change a given amount of a substance from one state to another.
The calorie is a common unit of heat. It is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.
ZEE EVANS / NATIONAL SCIENCE FOUNDATION
Water is the common substance used to define and measure heat. It also is the classic model for understanding transitions of state. Above, a bank of clouds rolls into Arthur Harbor, Anvers Island, Antarctica.
This should not be confused with the Calorie (capital C) used in measuring the energy content of food. The Calorie is equivalent to 1,000 calories or 1 kilocalorie.
Water is a common substance that is used to define and measure heat, and we are familiar with all three states. For that reason it is also the classic model for understanding transitions of state.
Latent heats of fusion (freezing and melting) and vaporization (evaporation and condensation) are not the same for most substances.
For example, it takes 80 calories to melt one gram of ice but 550 calories to vaporize that same gram.
Compare that with 100 calories that would be required to raise the temperature of water from freezing (0 degrees Celsius) to boiling (100 degrees Celsius).
Rudolph Clausius formulated the kinetic theory of heat to explain the thermodynamic properties of matter in a classic paper published in 1857.
Kinetic theory is based upon the dual propositions that molecules have a great amount of space between them, and that heat and temperature are due to the motion of molecules.
When used as a model, it provides the basis to explain nearly all of the physical properties of matter. It allows for the motion to be translational, rotational or vibrational, which makes the theory apply equally well to all three states.
According to kinetic theory, the higher the temperature of a substance, the more heat it contains and the faster its molecules move.
From the molecular point of view, latent heat is the energy that is required to sever the bonds between atoms or molecules and allow them to escape.
When ice is melting, absorbed heat is breaking bonds between adjacent water molecules in the crystal lattice, and it becomes liquid.
Liquid water is made of free water molecules and clumps of water molecules. The clumps could be the tiny remnants of ice crystals or just chains of molecules.
Molecules are moving so fast that the clumps form and re-form a billion times per second as molecules attach and detach from one another.
At freezing temperature of 0 degrees Celsius, ice and water are in a state of dynamic equilibrium, and the number of molecules breaking free from the ice is the same as the number being captured by a crystal from the liquid. The clumps are large, and there are relatively few free molecules.
Adding heat causes the temperature to rise as the energized molecules move faster. Clumps disintegrate into smaller pieces and release more free molecules, until the water boils.
A Rayleigh function describes their motion, such that at boiling temperature the average energy of water molecules is such that slightly more than one-half of the molecules can escape as vapor.
Evaporation is a cooling process because molecules escaping from the liquid as vapor leave behind molecules with a lower average energy.
The electrical forces between water molecules are very strong, so it takes significant speed for a molecule to escape.
This is what gives water its high latent heat compared with other substances.
Temperature is not the only factor affecting changes of state. Pressure plays a role in changes of state as well.
For each substance there is a unique relationship between the pressure and the temperature at which transitions between states proceed.
A state diagram, which is a graph of pressure versus temperature, (also phase diagram), clarifies these relationships. The graph has the characteristic shape of a distorted, curved and tilted Y.
The actual shape and location of the Y on the graph are different for each substance, but all lean to the right.
The V-shaped portion of the Y represents the liquid state with the solid state below and gaseous state above. The point where the Y diverges is called the triple point. It is at that temperature and pressure and only at that temperature that all three states can coexist.
The triple point for water under 1 atmosphere pressure is 0 degrees Celsius, its freezing temperature. At this temperature and pressure, the vapor pressure of ice, water and water vapor are equal.
We are familiar with dry ice, which is frozen carbon dioxide, a case in which the solid sublimes directly to the gas without passing through a liquid state.
By comparison, the triple point of carbon dioxide occurs at a pressure of 5.2 atmospheres and minus 56.4 degrees Celsius. This is the lowest temperature and pressure at which there can be liquid carbon dioxide.
The vapor pressure of solid carbon dioxide is 1 atmosphere at minus 78.5 degrees Celsius. At room temperature there is no liquid state.
The same thing happens to water ice in the freezer, but the triple point of water is at much lower pressure and much higher temperature, so dry ice sublimates much more quickly than water ice.
Combining kinetic theory with the state diagram allows us to visualize, understand and explore many thermodynamic processes such as these state transitions.
It's one more example of the synergy of scientific syntheses that continue to expand our knowledge of how the universe works.
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at firstname.lastname@example.org