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### The State Transition Diagram

Understanding the freeze drying process requires you to understand the State Transition Diagram for water.  Virtually all of the process can be mapped to this diagram.

We all understand that water exists in 3 states; liquid, solid and gas.  It is commonly understood that water converts to ice when it gets below 32F (0C).  Likewise it converts to gas at 212F (100C).

This is pretty common knowledge because that is how it works at atmospheric pressure.  But it is less common to know how water reacts under pressures other than atmospheric. The state transition diagram helps us understand how the states of water change when both temperature and water are considered.

The diagram charts the states of water at all temperatures and all pressures.  By looking at the appropriate temperature and pressure on the state transition diagram, we can easily determine if water is a solid, a liquid, or a gas.

A few notes about the diagram:

1. Pressure is on the vertical axis.
2. Temperature is on the horizontal axis.
3. Pressure is measured as absolute pressure.  (with a perfect vacuum being 0)
4. Lower pressures are at the bottom of the diagram
5. Pressures are plotted on a log scale
6. Temperatures are plotted on a linear scale
7. Lower temperatures are at the left of the diagram
8. The red horizontal line represents atmospheric pressure
9. The two vertical red lines represent the boiling point of water (right line) and the freezing point of water (left line)
10. The critical point represents the temperature and pressure that liquid water and water vapor become the same.
11. The triple point represents the temperature and pressure in which water can exist indefinitely in all three states without changing.

The diagram is divided into 3 distinct regions.  The blue region is the solid phase of water, the green region is the liquid phase of water and the peach region is the gaseous phase of water.  Between each of these is a boundary.  At this boundary, phase change begins.  All energy put into the system will be used to change phases of the water. This energy is known as "latent heat."

For instance, if you plot a temperature diagram of water as you heat it, the water will increase in temperature until it starts to boil.  Then it will stay at the boiling point until it is all boiled away.

If you do the same thing with ice water, you will see the temperature of the water drop and drop until it reaches the freezing point of water, then it will stay at that temperature until all of the ice is melted.  Then, it will begin warming up.

Crossing the boundary between each region pair has a name:

1. Melting – crossing from solid to liquid
2. Freezing – crossing from liquid to solid
3. Evaporation – crossing from liquid to gas
4. Condensation – crossing from gas to liquid
5. Sublimation – crossing from solid to gas
6. Deposition – crossing from gas to solid