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.