Phases of Matter Phase Changes Heating and Cooling Curves 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. L --- melting  S M.P. L S Heat added  Notice how the temperature remains constant during the phase change.

Cooling Curve for Condensation Temp. --- condensation  L ? G L --- 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 Temp. F 5 4 D E 3 B 2 C 1 A -- Heat added 

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 temp G sublimation S G S 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.