Phase Changes

1. Phase Changes Physical Science Mr. Moss RHS

2. Phases • When 2 states are present at the same time, we describe each as a phase. • Here, we see 2 phases of water: • Solid Phase • Liquid Phase

3. Phase Change Definition • A Phase Change is the reversible physical change that occurs when a substance changes from one state of matter to another. • There are 6 common phase changes • Melting • Freezing • Vaporization • Condensation • Sublimation • Deposition

4. Deposition Condensation Vaporization Sublimation Melting Freezing

5. Phase Change • NOTE: • The temperature of the substance DOES NOT change during a phase change!!!! • All phase changes are related to energy and temperature.

6. Energy during Phase Changes • Law of Conservation of Energy • Neither created or destroyed. • Transferred or Transformed • In phase changes, energy is either absorbed or released. • Endothermic – energy is absorbed. • Ice melting • Exothermic – energy is released. • Water freezing

9. Heat of Fusion • 1g of ice absorbs 334 joules (J) of energy as it melts. • This amount of energy is the Heat of Fusion for water. • This is another term for the melting process.

10. Phase Diagrams

11. Phase Diagrams

13. Triple and Critical Point • The Triple Point is where the substance exists in all 3 states. • The Critical Point is where, under extreme high temps and pressure, the liquid and gaseous states are indistinguishable.

14. Melting & Freezing • Water is a molecule made up of 2 atoms of Hydrogen and 1 atom of Oxygen. • The arrangement of molecules becomes less orderly as water melts. • The arrangement of molecules becomes more orderly as water freezes.

15. Melting • Endothermic process • Heat energy is transformed into increased kinetic energy. • Causes molecules to vibrate more rapidly. • Some gain enough to overcome the attractive forces and move from the fixed locations. • This is the melting point. • Any energy gained after the phase change increases the average kinetic energy and the temperature rises.

16. Freezing • Exothermic • Average kinetic energy (Ke) decreases causing molecules to slow down. • At the freezing point, the attractive forces begin to draw the molecules into an orderly arrangement. • Continues until all have been fixed. • Any energy removed after the phase change is complete decreased the average kinetic energy of the molecules and the temperature decreases.

17. Vaporization • Vaporization happens when a substance changes from a liquid into a gas. • Endothermic • Heat of vaporization • Water gains 2258 J of energy when it vaporizes at 100° C. • 2 vaporization processes • Boiling • Evaporation • Takes place at the surface of a liquid • Occurs at temps below the boiling point

18. Evaporation • Process that changes a substance from a liquid to a gas at temps below the boiling point. • In a closed container: • Water vapor collects above the liquid • Pressure caused by molecules colliding with container is called vapor pressure. • Pressure increases as temp increases.

19. Boiling • As you apply heat energy, temperature and vapor pressure increase. • When vapor pressure = atmospheric pressure, the water boils. • Called the boiling point. • Depends on Atmospheric Pressure • Higher elevations have less atmospheric pressure • Takes longer to cook food.

20. Condensation • The phase change in which a substance changes from a gas to a liquid. • The water vapor transferred heat to the glass and condensed into a liquid.

21. Sublimation & Deposition • Sublimation is the phase change from a solid to a gas without going through the liquid phase. • Endothermic • Dry Ice • Deposition is the phase change from a gas to a solid without going through the liquid phase. • Exothermic • Frost on windows