States of MatterStates of Matter

Section 12.4: Phase ChangesSection 12.4: Phase Changes

ObjectivesObjectives

►Explain how the addition and removal Explain how the addition and removal of energy can cause a phase change.of energy can cause a phase change.

► Interpret a phase diagram.Interpret a phase diagram.

Phase ChangesPhase Changes►Most substances can exist in 3 states Most substances can exist in 3 states

depending on the temperature and depending on the temperature and pressure.pressure.

►Water exists in 3 states on Earth - ice Water exists in 3 states on Earth - ice (solid), water (liquid), and water vapor (solid), water (liquid), and water vapor (gas).(gas).

►States are referred to as phases when States are referred to as phases when they coexist; for example, ice water has they coexist; for example, ice water has 2 phases, ice and liquid water.2 phases, ice and liquid water.

Phase Changes & EnergyPhase Changes & Energy•When energy is added or removed from a system, one phase can change into another.

•Melting, vaporization & sublimation require energy.

•In deposition, condensation, & freezing, energy must be removed.

Melting (or Fusion)Melting (or Fusion)► HeatHeat (the transfer of (the transfer of

energy from an object at a energy from an object at a higher temperature to an higher temperature to an object at a lower one) object at a lower one) moves from the water into moves from the water into the ice. the ice. In this case, when In this case, when it is assumed the it is assumed the temperature of the ice and temperature of the ice and water are the same at 0 water are the same at 0 00C, C, the energy absorbedthe energy absorbed by the ice is by the ice is NOTNOT used to used to raise its temperatureraise its temperature. .

Melting (or Fusion)Melting (or Fusion)►The energy The energy

absorbed by the ice absorbed by the ice goes into goes into breaking breaking the hydrogen the hydrogen bonds between the bonds between the water moleculeswater molecules in in the solid. The the solid. The molecules can then molecules can then move apart and move apart and become liquid.become liquid.

Melting (or Fusion)Melting (or Fusion)► Water has many hydrogen bonds to break, so Water has many hydrogen bonds to break, so

a lot of energy is needed to melt it (change a lot of energy is needed to melt it (change the solid to liquid). Different substances will the solid to liquid). Different substances will require different energies. Ionic compounds require different energies. Ionic compounds (with ionic bonds) will require more energy. (with ionic bonds) will require more energy.

► TheThe melting pointmelting point of a crystalline solid is the of a crystalline solid is the temperature at which the forces holding the temperature at which the forces holding the crystal lattice together are broken and it crystal lattice together are broken and it becomes a liquid.becomes a liquid.

► Every element and compound has a different Every element and compound has a different melting point.melting point.

The Heating Curve for WaterThe Heating Curve for Water► As the ice (in our ice As the ice (in our ice

water) melts, water) melts, ITS ITS TEMPERATURE TEMPERATURE REMAINS CONSTANTREMAINS CONSTANT..

► Once it is all melted, Once it is all melted, additional energy additional energy increases the increases the kinetic kinetic energyenergy of the of the molecules and the molecules and the water temperature water temperature increases.increases.

►Molecules with enough energy can Molecules with enough energy can escape the liquid and become a escape the liquid and become a gas.gas.

►VaporizationVaporization is the process by is the process by which a liquid changes to a gas or which a liquid changes to a gas or vapor.vapor.

VaporizationVaporization► As more energy is As more energy is

added, more molecules added, more molecules escape to the gas escape to the gas phase. As this occurs, phase. As this occurs, THE TEMPERATURE THE TEMPERATURE DOES NOT CHANGEDOES NOT CHANGE..

► When all molecules are When all molecules are in the gas phase, in the gas phase, additional energy will additional energy will cause an increase in cause an increase in temperature.temperature.

What is evaporation?What is evaporation?►When vaporization When vaporization

takes place only at takes place only at the surface of the of the surface of the of the liquid, the the liquid, the process is called process is called evaporation.evaporation.

►This can happen at This can happen at LOW temperatures, LOW temperatures, especially if there is especially if there is light energy.light energy.

VAPOR PRESSUREVAPOR PRESSURE►The pressure The pressure

exerted by a exerted by a vapor over a vapor over a liquid is called liquid is called vapor pressure.vapor pressure.

►An increase in temperature will increase the vapor pressure.

Vapor Pressure CurvesVapor Pressure Curves► A vapor pressure curve A vapor pressure curve

shows how the vapor shows how the vapor pressure of a liquid (y-pressure of a liquid (y-axis) changes as a axis) changes as a function of temperature function of temperature (x-axis).(x-axis).

► When the curve When the curve intersects the point intersects the point where the vapor where the vapor pressure equals the pressure equals the atmospheric pressure atmospheric pressure (760 mm Hg), the (760 mm Hg), the temperature is called the temperature is called the normal boiling point.normal boiling point.

What is boiling?What is boiling?

►The temperature The temperature at which the at which the vapor vapor pressurepressure of a liquid of a liquid equalsequals the the external or external or atmospheric atmospheric pressurepressure is called is called the the BOILING BOILING POINT POINT (or normal (or normal boiling point).boiling point).

What is boiling?What is boiling?►At the boiling point, At the boiling point,

molecules molecules throughout the throughout the liquid have enough liquid have enough energy to vaporize. energy to vaporize. Bubbles of vapor Bubbles of vapor can be seen in the can be seen in the liquid. liquid.

►Bubbles then rise to Bubbles then rise to the surface .the surface .

SublimationSublimation► SublimationSublimation is when a is when a

solid substance solid substance changes directly to a changes directly to a gas.gas.

► Solid iodine, carbon Solid iodine, carbon dioxide (dry ice), dioxide (dry ice), mothballs, and solid air mothballs, and solid air fresheners sublime at fresheners sublime at room temperature. Ice room temperature. Ice cubes sublime in your cubes sublime in your freezer.freezer.

CondensationCondensation► When water vapor When water vapor

molecules lose energy molecules lose energy (perhaps by contact with a (perhaps by contact with a cold object), their velocity cold object), their velocity decreases. They are then decreases. They are then more likely to form more likely to form hydrogen bonds when they hydrogen bonds when they collide with other water collide with other water molecules, thus forming a molecules, thus forming a liquid.liquid.

► The process by which a gas The process by which a gas or a vapor becomes a liquid or a vapor becomes a liquid is called is called condensation.condensation.

DepositionDeposition►DepositionDeposition is the is the

process by which a process by which a substance changes substance changes from a gas or vapor from a gas or vapor to a solid without first to a solid without first becoming a liquid.becoming a liquid.

►Energy is released to Energy is released to the surroundings as the surroundings as ice crystals form from ice crystals form from water vapor.water vapor.

Freezing (or Solidification or Freezing (or Solidification or Crystallization)Crystallization)

► As heat flows from water As heat flows from water to its surroundings, the to its surroundings, the water molecules water molecules lose kinetic lose kinetic energy. Formation of hydrogen energy. Formation of hydrogen bonds will take place which will bonds will take place which will keep the molecules fixed into keep the molecules fixed into set positions. A solid will form.set positions. A solid will form.

► TheThe freezing pointfreezing point is the is the temperature at which a temperature at which a liquid is converted into a liquid is converted into a crystalline solid.crystalline solid.

► ..

A New Look at the Heating A New Look at the Heating CurveCurve

► Read this curve from Read this curve from RIGHT TO LEFT - take RIGHT TO LEFT - take away heat instead of away heat instead of adding it.adding it.

► CondensationCondensation occurs occurs at the same at the same temperature as temperature as vaporizationvaporization..

► FreezingFreezing occurs at the occurs at the same temperature as same temperature as meltingmelting..

Phase DiagramsPhase Diagrams

►2 variables control the phase of a 2 variables control the phase of a substance.substance. TemperatureTemperature PressurePressure

►A A phase diagram phase diagram is a graph of is a graph of pressure vs. temperature. It will show pressure vs. temperature. It will show which phase a substance will be in at which phase a substance will be in at various conditions of temperature & various conditions of temperature & pressure.pressure.

Phase DiagramPhase Diagram

Different sections of the graph indicate different phases.

Phase DiagramPhase Diagram► Boundaries between the Boundaries between the sections are the sections are the temperature & pressure temperature & pressure conditions at which a conditions at which a phase change is phase change is occurring.occurring.

► The The triple point triple point is the is the point on a phase diagram point on a phase diagram that represents the that represents the temperature and temperature and pressure at which 3 pressure at which 3 phases of a substance phases of a substance can coexist.can coexist.

Phase DiagramPhase Diagram► The The critical point critical point

indicates the critical indicates the critical pressure and critical pressure and critical temperature above temperature above which water cannot which water cannot exist as a liquid.exist as a liquid.

► At the critical At the critical temperature, an temperature, an increase in pressure increase in pressure will not change a will not change a vapor into a liquid.vapor into a liquid.

Phase DiagramPhase Diagram

►The phase diagram for each substance The phase diagram for each substance is different because the normal boiling is different because the normal boiling and freezing points of substances are and freezing points of substances are different.different.

WaterWater

Carbon DioxideCarbon Dioxide