Understanding Phase Change

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

2. 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 SiO2 • 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

3. Structure of solids • Most solids are crystalline • Crystal lattice • Orderly, repeating three dimensional pattern • Regular shape • Type of bonding determines the melting point

4. 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

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

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

8. Sublimation • Change 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 liquid

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

10. 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

12. Vapor Pressure • 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 • When these two processes reach equilibrium, vapor pressure can be quantified

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

14. Factors affecting vapor pressure • Surface area – • Greater surface area facilitates higher vapor pressure • Temperature • Higher temperature – more molecules escape = higher pressure • Lower temperature opposite

15. Type of molecule • Strong intermolecular forces = low vapor pressure • Weak intermolecular forces = high vapor pressure • If intermolecular forces are relatively equal, the molecules of lower molecular mass will have the higher vapor pressure

16. Vapor Pressure Diagram