Thermodynamics/Thermal Properties Of Matter
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[edit] The phases of matter
Solid = definite size and shape.
Liquid = definite size but no definite shape.
Gas = neither definite size nor definite shape.
[edit] Change of Phase
It is associated with a single phase transition temperature (Tp).
All substances can exist in any of the 3 phases under the proper conditions of T and P.
Transition form one phase to another is accompanied by absorption or liberation of heat. Example:
o Temperature of an ice block heated with a coil increases (a to b) until it reaches 0°C. o At 0°C, some liquid water appears, i.e. ice begins to melt. Thus, ice and water exist together at 0°C. o T remains at 0°C until all ice is melted (c) if P is constant. o When all ice has melted, T rises again (c to d) until it reaches 100°C. o At 100°C (d), bubbles of steam start to escape from the liquid surface, i.e. water begins to boil. Thus, water and steam exist together at 100°C. o T remains at 100°C until all water has boiled away, if P is constant. o If the water vapor is trapped and not allowed to diffuse away, T increases (e to f) to form superheated steam.
Some substances decompose before reaching a melting or a boiling point.
Glass and pitch do not change phase at a definite T, but become gradually softer as T is raised. Thus, they behave like supercooled liquids of very high viscosity.
Crystalline substances like ice or metal melt at a definite T.
A gas returns to the liquid phase or condenses when its T falls to boiling T. Thus, boiling point and condensation point are at the same T.
Similarly, a liquid returns to the solid phase or freezes when cooled to melting point. Thus, melting point and freezing point are at the same T.
[edit] Phase Diagram
It is a graph between P and T showing
- The relation between different phases of material (solid, liquid, gas).
- What phase is present for each pair of values of P and T.
Each phase can exist for certain ranges of T and P.
At each point on the diagram, only 1 phase can exist. At phase changes however, 2 phases exist in equilibrium. This is shown as point on the lines.
If a substance is heated at constant P (line A), melting and boiling T are the T at which line A crosses the fusion and vaporization curves.
Similarly, if a substance is compressed at constant T (line B), condensation and freezing T are the T at which line B crosses the vaporization and fusion curves.
At a constant and low P, a material can be sublimated i.e. transformed from solid directly to vapor (line C).
Water phase diagram:
Water contracts on melting. Its fusion curve always slopes upwards to the left.
Melting point:
As P is reduced to p1, melting point of ice increases to TA’. Thus, ice melts at high T if P is reduced. Also, water freezes at lower T if P is increased. This is why skiers press on snow by weight of their body, to create a thin layer of water that helps in skiing.
Boiling point:
As P is reduced to p1, boiling point of water decreases to TB’. Thus, water boils at lower T if P is reduced. This is why a person living on high mountains cannot cook his food properly, because of the low boiling T of water. High pressure vessels are used to cook food in shorter time because the high P produced on water inside the vessel increases the boiling T of water.
Triple point and Critical Point
Triple point = point at which the 3 phases exist together. There is thus only 1 P and T at which all the 3 phases can coexist.
Critical point = end of vaporization curve, at which liquid and vapor has the same density.
Above critical T, no liquid-vapor phase transition occurs for any P.
Above critical P, no liquid-vapor phase transition occurs for any T. Thus, only 2 phases occur: crystalline solid phase and amorphous phase. As T is raised, the amorphous phase changes continuously from a state which behaves like a liquid to a state which behaves like a perfect gas.
H and He have very low critical T, so it is hard to liquefy them.
[edit] Thermal Expansion
Increase in length (∆L) is directly prop. to increase in T (∆T) if ∆T is not too large: ∆L = α L0 ∆T
Where α = coefficient of linear expansion, characteristic for each material, unit = (°C).
For isotropic solids, change in length for a given ∆T is the same for all the lines in the solid. Thus, change in area is given by:
∆A = 2 α A0 ∆T = β A0 ∆T
Where β = coefficient of area expansion, unit = (°C).
Change in volume is given by
∆V = 3 α V0 ∆T = γ V0 ∆T
Where γ = coefficient of volume expansion, unit = (°C).
Fluids have no definite shape, so their volume expansion is large with the T rise.
Liquids generally expand 10 times greater than solids with increasing T.
As for water, and above 4°C, it expands as T rises. But when T is lowered from 4°C to 0°C, it expands instead of contracting. Density of water is thus maximum at 4°C.
