Understanding Phase Change Differences between the organization of particles in solids, liquids and gases affect how phase change takes place and how much energy it requires Differences between the organization of particles in solids, liquids and gases affect how phase change takes place and how much energy it requires

Types of Solids Ionic crystals High melting points Held together by strong electrostatic forces Most ionic solids ex: NaCl Covalent Crystals High melting point Held together by strong covalent bonds Ex: diamond, sand SiO 2 Metallic Crystals High melting points Atoms are present in the lattice sites allowing movement of electrons Molecular Crystals Low melting point Held together by weak intermolecular forces

Structure of solids Most solids are crystallineMost solids are crystalline Crystal latticeCrystal lattice Orderly, repeating three dimensional patternOrderly, repeating three dimensional pattern Regular shapeRegular shape Type of bonding determines the melting pointType of bonding determines the melting point

Specific Types of Crystals Polymorphic Crystals of the same chemical make-up with different shapes Isomorphic Crystals with the same shape but slightly different chemical make-up Dessicant Uses absorption or adsorption to induce dryness Hydrated crystal Crystal that has absorbed water molecules Efflorescence Loss of water of crystallization or hydration

Unit Cells Can be thought of as a box, that when stacked together in 3 dimensions form the crystal latticeCan be thought of as a box, that when stacked together in 3 dimensions form the crystal lattice 3 types3 types Simple cubicSimple cubic One lattice point on each corner of the cube; each atom shared by 8 adjacent cubesOne lattice point on each corner of the cube; each atom shared by 8 adjacent cubesOne lattice point on each corner of the cube; each atom shared by 8 adjacent cubesOne lattice point on each corner of the cube; each atom shared by 8 adjacent cubes Body centered cubicBody centered cubic One additional lattice point in the center of the cellOne additional lattice point in the center of the cellOne additional lattice point in the center of the cellOne additional lattice point in the center of the cell Face centered cubicFace centered cubic Lattice points on the faces of the cube in addition to the cornersLattice points on the faces of the cube in addition to the cornersLattice points on the faces of the cube in addition to the cornersLattice points on the faces of the cube in addition to the corners

Relationship between Phases Using a phase diagram you can determine changes in melting and boiling point due to changes in pressure

Sublimation Change of a substance directly from solid to gasChange of a substance directly from solid to gas The vapor pressure of the solid is so high that they pass to gas or vapor without becoming a liquidThe vapor pressure of the solid is so high that they pass to gas or vapor without becoming a liquid

Special conditions Triple point – shown on a phase diagram where solid, liquid and vapor exist simultaneously Triple point – shown on a phase diagram where solid, liquid and vapor exist simultaneously Critical point – the point at which phase boundaries become blurred Critical point – the point at which phase boundaries become blurred

Critical temperature – the temperature above which a gas cannot be liquified no matter how great the pressure Critical temperature – the temperature above which a gas cannot be liquified no matter how great the pressure Critical pressure – the pressure it takes to liquify a gas at its critical temperature Critical pressure – the pressure it takes to liquify a gas at its critical temperature

Vapor Pressure In a closed container, gas formed by evaporation cannot escape In a closed container, gas formed by evaporation cannot escape Some of the gas molecules will strike the surface of the liquid or solid and condense back into it as more molecules move to the gas phase Some of the gas molecules will strike the surface of the liquid or solid and condense back into it as more molecules move to the gas phase When these two processes reach equilibrium, vapor pressure can be quantified When these two processes reach equilibrium, vapor pressure can be quantified

The higher the boiling point of the substance, the lower the vapor pressure Thus the substances with lower boiling points have higher vapor pressure

Factors affecting vapor pressure Surface area –Surface area – Greater surface area facilitates higher vapor pressureGreater surface area facilitates higher vapor pressure TemperatureTemperature Higher temperature – more molecules escape = higher pressureHigher temperature – more molecules escape = higher pressure Lower temperature oppositeLower temperature opposite

Type of moleculeType of molecule Strong intermolecular forces = low vapor pressureStrong intermolecular forces = low vapor pressure Weak intermolecular forces = high vapor pressureWeak intermolecular forces = high vapor pressure If intermolecular forces are relatively equal, the molecules of lower molecular mass will have the higher vapor pressureIf intermolecular forces are relatively equal, the molecules of lower molecular mass will have the higher vapor pressure

Vapor Pressure Diagram