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Matter and its Changes
Phases of Matter
Phase Changes
Heating and Cooling Curves
Phases of Matter
• A phase is a state of existence, a description of how the atoms or molecules of a pure substance are attached to each other.
• Chemistry recognizes three (3) common phases: solid, liquid, gas.
• Each of the phases has its own characteristics…
Solids
• Individual particles of the substance are held tightly in place by connections to many other particles
• Explains why solids have a rigid shape, definite volume, are typically crystalline, and have all free surfaces.
Liquids
• Connections between particles are flexible and may be broken. However, as one connection is broken, another connection between the particle and another particle will form.
• Explains why liquids will take the shape of their container (flexible connections), still have a definite volume, and only have one free surface.
• Also explains why liquids can be poured. (fluid)
Gases
• There are no connections between individual atoms or molecules.
• Gaseous systems do not have a definite shape, do not have a definite volume, and have no free surfaces.
• Particles are in constant motion and a gas will expand to fill all available space.
Phase Changes
• This is the description of an event where a pure substance in one state of existence is changed to a different state of existence.
• Solid Liquid is called “Melting”• Liquid Solid is called “Freezing”• Liquid Gas is “Evaporation” or “Boiling”• Gas Liquid is called “Condensation”
How Phase Changes Occur
• Phase changes will occur when a sufficient quantity of heat has either been added (for melting and boiling) or removed (for freezing and condensation).
• Adding heat serves to weaken and/or break the connections between the particles.
• Removing heat serves to allow those connections to re-form.
Equations of Phase Changes
• Solid + heat Liquid(melting)• Liquid Solid + heat(freezing)
• Liquid + heat Gas (boiling)• Gas Liquid + heat (condensation)
• Notice that we do not use subtraction signs in chemistry equations describing events.
Melting Point
• A physical property.• Defined as the temperature at which a pure
substance will change from solid to liquid.• Is unique for all pure substances.• Is dependent upon the number, type, and
strength of the connections between the individual particles of a solid.
• The stronger the connections, the higher the melting point.
• Is exactly the same temperature as freezing point.
Boiling Point
• Is a physical property.• Is always higher than melting point.• Is unique for all pure substances.• Is defined as the temperature at which a pure
liquid will change from liquid to gas.• Also dependent upon the nature of the
connections between the particles of the substance.
• Is exactly equal to the temperature at which condensation occurs.
Phase Diagrams
• These are graphical presentations that display the different combinations of temperature and pressure that can permit a pure substance to exist in its different phases.
• While the specific details of a phase diagram will vary from pure substance to pure substance, the general pattern of all single component phase diagrams is the same for all pure substances.
A Typical Phase Diagram
Some Definitions
• Triple Point – the specific combination of pressure and temperature that will enable a pure substance to exist in all three phases simultaneously.– It can be determined in a phase diagram as the
point at which all three lines converge.
Definition #2
• Boiling Point – formally defined as the temperature at which a liquid will turn to vapor (gas) when the pressure is exactly 1.0 atm (which is the same as 760 mmHg or 760 torr – there are other conversions too).
• This can also be determined from a phase diagram.
The Phase Diagram for Water
A Final Quick “Trick”
By drawing a straight line extending vertically from the triple point, you can correctly describe which of the three phases is the most dense for a pure substance.
State and College Board Expectations
• The most common questions will ask you to describe what event will occur if either the temperature or the pressure are changed over a very specific line on the phase diagram.
Heating and Cooling Curves
• Graphical representations of how the temperature of a system changes as heat is added or removed through phase changes.
• It is observed that the temperature of a system remains constant during any phase change, even though heat is still being added or removed.
Heating Curve for Melting
Temp.
Heat added
S
SL
L
--- melting M.P.
Notice how the temperature remains constant during the phase change.
Cooling Curve for Condensation
G
GL
L
--- condensation
Temp.
--- Heat removed
?
Again, notice how the temperature remains constant during the phase change. Also notice that the “x” axis is measuring “heat removed” as the phase change is “downwards”.
An Overall Heating Curve
A
B
C
DE
FTemp.
-- Heat added
1
23
45
A Visual Slide of Phases
Sublimation• This is a “somewhat exotic” phase change.• In this process, a solid is changed directly
to a gas, by-passing the liquid phase. • Only a few substances will do this,
examples are dry ice (which is actually solid carbon dioxide), the element iodine, and the element sulfur.
Heating curve for Sublimation
S
SG
G
sublimation
temp
Heat added
Some final thoughts…
• Remember that phase changes are physical changes. You will still have the “same stuff”.
• Key thought is that the temperature of the system will remain constant while the phase change is occurring – this is why the temperature stayed the same for so long in your lab.
• A final note…there is an opposite process to sublimation. In a very few cases, a gas can be converted directly back to a solid – this change is called deposition.