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Tuesday, October 12, 2010

Phase Change Diagrams

What is a phase diagram?

A phase diagram is a convenient way of representing the phases of a substance as a function of temperature and pressure. Below is the phase diagram for water.


Phase Diagram of Water


As seen above, one can determine from a phase diagram what state the water will be in at a certain temperature and pressure. For example, at 1 atm and 100ยบ Celsius both liquid and solid water will exist.

**Note that a phase diagram describes conditions and events in a closed system where no material can escape into the surrondings and air is present


The phase diagram of water is very different that the phase diagrams of other substances. One unique thing about water is that the boundary line between the solid/liquid sections has a negative slope, meaning that the melting point of ice decreases as the external pressure increases. This occurs because the the density of ice is less than the density of water, which is not true for other substances. For example, below is the phase diagram for Carbon Dioxide.

C02 phase diagram

Note that unlike water the line between the liquid/solid sections has a positive slope unlike water.

Below is the phase diagram for carbon. By defintition a phase diagram shows the different states that a substance could be in, and the temperatures and pressures at which it occurs. Carbon is different from the previous two because it can exist in different states as a solid:

The common graphite:     graphite  
And the more expensive diamond: Diamond


The reason diamond is more expensive is because of its rarity, which is clearly evident in its phase diagram.

diamond



As shown in the phase diagram above it takes huge amounts of pressure and temperature to form a diamond. While diamond can be formed artificially, it is too expensive to compete with the natural stores of the stones found in various parts of the world.
from http://bhs.smuhsd.org/science-dept/marcan/apchemistry/cool_phase_changes_diagram.html

When done go through these skill building questions at this URL http://www.science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html

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