3.3 Phase Changes

1. 3.3 Phase Changes

2. What are we learning? • Define phase change • Explain how temperature can be used to recognize a phase change • Explain what happens to the motion, arrangement and average kinetic energy of water molecules during phase changes • Describe each of the 6 phase changes • Identify phase changes as endothermic or exothermic

3. Characteristics of a Phase Change • If 2 states of the same substance are present at the same time, we describe each different state as a phase. • Ex: an iceberg floating in the ocean- solid phase and liquid phase • A phase change is the reversiblephysical change that occurs when a substance changes from one state of matter to another • Melting, freezing, vaporization, condensation, sublimation and deposition are the 6 common phase changes

4. Temperature and Phase Changes • One way to recognize phase changes- measuring the temperature as it is heated or cooled • The temperature of a substance does not change during a phase change.

5. Ex Naphthalene • Naphthalene is used in mothballs. • This graph is of data collected when it is heated. • The temperature rises as it warms up, until it hits about 80C. (the melting point) • It will remain at 80C until it has all melted .

6. Energy and Phase Changes • During a phase change, energy is transferred between a substance and its surroundings. • Energy is either absorbed or released during a phase change. • Ex: Melting- endothermic change • During an Endothermic change, the system absorbsenergy from its surroundings. • The amount of energy absorbed varies • Ex: one gram of ice absorbs 334 joules of energy as it melts. (this is the heat of fusion of water) • Fusion is another term for melting

7. Energy and Phase Changes • When water freezes, it releases the same amount of energy (334J) to its surroundings. • Freezing is an exothermic change • In exothermic change the system releases energy to its surroundings.

8. Melting and Freezing • The arrangement of molecules in water becomes lessorderly as water melts and more orderly as water freezes. (remember the BB’s) • Attractions between water molecules in ice keep the molecules in fixed positions • As ice warms up the molecules vibrate more quickly • When ice gets near the melting point (OC), some molecules gain enough energy to over come the attraction with other molecules and move from their fixed positions. • (melting is complete when all the molecules can move).

9. Freezing • When water is placed in a freezer, energy flows from the water to the air in the freezer, and the water cools down. • The kinetic energy decreases and the molecules move more slowly • As the molecules slow down, forces of attraction have a greater effect • When all the molecules have been drawn into an orderly arrangement, freezing is complete

10. Vaporization and Condensation • Vaporization is the phase change where a substance changes from a liquid to a gas • it’s an endothermic process which means, that a substance must absorb energy in order to change from a liquid to a gas • One gram of water gains 2261 joules of energy- this is the heat of vaporization of water • There are 2 vaporization process, boiling and evaporation

11. Evaporation • Evaporation occurs at the surface of a liquid and at temperatures lower than the boiling point. • Evaporation is the process that changes a substance from a liquid to a gas at temp. below the boiling point. • When water evaporates, some molecules near the surface are moving fast enough to escape and become water vapor. • The greater the surface area, the faster it evaporates • Vapor pressureis the pressure caused by the collisions of water vapor and the walls of a closed container

12. Boiling • As you heat up a pot of water, the temperature and vapor pressure of water increases • The water boils when the vapor pressure becomes equal to the atmospheric pressure (the temp. at which this happens is the boiling point) • As the temp. increases during boiling, the molecules move faster • When the temp gets near 100C, some molecules below the surface have enough kinetic energy to overcome the attraction with other molecules • Bubbles rise to the surface, burst and release water vapor into the air • The boiling point depends on atmospheric pressure (higher altitude = lower BP)

13. Condensation • Condensation is the phase change where a substance changes from a gas/vapor to a liquid. (morning dew!) • Is an exothermic process (energy releasing)

14. Sublimation and deposition • Sublimation the phase change where a substance goes from a solid to a gas/vapor without changing to a liquid first. • When a gas /vapor changes directly to a solid without going through the liquid phase it’s called deposition. (exothermic) • Causes frost to form on windows