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ThermodynamicsTHERMODYNAMICS
PROPERTIES OF PURE SUBSTANCES
Pure Substance
A substance that has a fixed chemical composition throughout is called
pure substance. Water, helium carbon dioxide, nitrogen are examples.
It does not have to be a single chemical element just as long as it is
homogeneous throughout, like air. A mixture of phases of two or more
substance is can still a pure substance if it is homogeneous, like ice and
water (solid and liquid) or water and steam (liquid and gas).
Vapor Vapor
Liquid Liquid
Water Air
(Pure substance) (Not a pure substance
because the composition of
liquid air is different from the
composition of vapor air)
Phases of a Pure Substance
There are three principle phases – solid, liquid and gas, but a substance
can have several other phases within the principle phase. Examples
include solid carbon (diamond and graphite) and iron (three solid phases).
Nevertheless, thermodynamics deals with the primary phases only.
In general:
- Solids have strongest molecular bonds.
- Solids are closely packed three dimensional crystals.
- Their molecules do not move relative to each other
- Intermediate molecular bond strength
- Liquid molecular spacing is comparable to solids but their
molecules can float about in groups.
- There is molecular order within the groups
- Weakest molecular bond strength.
- Molecules in the gas phases are far apart, they have no ordered
structure
- The molecules move randomly and collide with each other.
- Their molecules are at higher energy levels, they must release
large amounts of energy to condense or freeze.
THERMODYNAMICS 1 PROPERTIES OF PURE SUBSTANCES
,Thermodynamics
Phase – Change Processes Of Pure Substances
At this point, it is important to consider the liquid to solid phase change
process. Not so much solid to liquid because thermodynamics deals only
with liquid to gases (or vice versa) to generate power.
Consider water at room temperature (20°C) and normal atmospheric
pressure (1 atm) in a piston-cylinder device. The water is in liquid phase,
and it is called compressed liquid or subcooled liquid (not about to
vaporize).
Compressed Liquid
liquid
(Point 1)
If we add heat to water, its temperature will increase; let us say until
50°C. Due to the increase in temperature, the specific volume v will
increase. As a consequence, the piston will move slightly upward
therefore maintaining constant pressure (1 atm).
Compressed Liquid
liquid
Now, if we continue to add heat to the water, the temperature will
increase further until 100°C. At this point, any additional addition of heat
will vaporize some water. This specific point where water starts to
vaporize is called saturated liquid.
(Point 2)
THERMODYNAMICS 2 PROPERTIES OF PURE SUBSTANCES
, Thermodynamics
Liquid
Saturated
liquid
If we continue to add heat to water, more and more vapor will be created,
while the temperature and the pressure remain constant (T = 100°C and P
= 1 atm). The only property that changes is the specific volume. These
conditions will remain the same until the last drop of liquid is vaporized.
At this point, the entire cylinder is filled with vapor at 100°C. This state
is called saturated vapor
(Point 4)
The state between saturated liquid (only liquid) and saturated vapor (only
vapor) where two phases exist is called saturated liquid-vapor mixture.
(Point 3)
Vapor
Saturated
liquid-vapor Liquid
mixture
.
THERMODYNAMICS 3 PROPERTIES OF PURE SUBSTANCES
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