Notes: Expansion, Latent Heat, Phase Changes

1. Notes: Expansion, Latent Heat, Phase Changes

2. Other changes due to heat flow… Gases: If the volume of a gas is held constant, as the temperature goes up, the gas pressure goes up as well.

3. Thermal Expansion Generally, as temperatureincreases, the lengths and volumes of substances also increase. Examples: concrete sidewalks, metals, gases Expansion joints allow the metal bridge to expand without breaking or buckling.

4. Bimetallic strips used in thermostats work on the principle that the strip will curl because one metal expands more than the other

5. Volume Temperature One exception to the rule of thermal expansion is water As the temperature of water drops its volume decreases until 4°C is reached. Then its volumeexpandsas the temperature continues to drop to 0°C.

6. Result: ice is less dense than water so that ice floats. This protects underwater life in cold climates

7. Another consequence of the structure of water when frozen: • Have you ever put a canned soda in the freezer? There is a large part of the soda that is water. What happens to the can?

8. Phase Changes Evaporation: A change of phase from liquid to gas that takes place at the surface of a liquid. Boiling: a change of phase from liquid to gas that takes place beneath the surface of a liquid. Condensation: The change of phase from gas to liquid. Freezing: the change of phase from liquid to solid. Sublimation: the process in which a substance goes from solid to gas, bypassing the liquid state.

9. Condense Freeze Vaporize Melt PHASE CHANGES Gas Liquid Solid

10. Liquid Gas Solid Phase Change diagram temperature Liquid + gas Solid + liquid energy Phase changes occur when temperature stays the same. Every molecule must be converted to the new phase before the temperature will change again.

11. Changes of State • solid to liquid OR liquid to solid • melting point: the temperature at which a substance changes from solid to liquid OR liquid to solid “fusion” – putting together as in freezing or melting • latent heat of fusion, Lf - the amount of heat per kilogram that is required to be transferred to melt or freeze a substance

12. Liquid to vapor OR vapor to liquid • Boiling point: the temperature at which a substance changes from liquid to vapor OR vapor to liquid. • Latent heat of vaporization, Lv- the amount of heat per mass unit that is required to be transferred to boil or cause condensation

13. There is NO change in temperature during a change of state. The heat required when the temperature changes is given by Q = mcΔT The heat required during a change of state is given by Q = mL where L is the latent heat of either vaporization or fusion

14. For water: Lf = 80 cal/g Lv = 540 cal/g c = 1 cal/g Co Use these values to complete Concept Development Practice Page 23-1 The examples on the following slides involve Joules and kilograms.

15. Examples How much heat must be added to 3 kg ice at -7º C till it just begins to melt? Cice = 2100 J/kgK Q = mcDT Q = 3 (2100) (7) Q = 44100 J

16. How much heat must be added to 3 kg of ice at 0º C to melt all of it? Lfice = 3.33 x 105 J/kg Q = mLf Q = 3 (3.33 x 105 ) Q = 999000 J

17. How much heat must be added to 3 kg of water at 30º C so that all of it becomes steam at 100 º C ? Lv water = 22.6 x 105 J/kg First, the water must reach its boiling temperature, then it must all be boiled. Q = mcDT + mLv Q = 3 (4186)(70) + 3(22.6 x 105 ) Q = 7659060 J

18. How much heat must be added to 4 kg of water at 100º C so that all of it becomes steam at 120 º C ? Lv water = 22.6 x 105 J/kg Csteam = 2010 J/kgK First, the water must all be boiled, then the steam must heat up. Q = mLv + mcDT Q = 4(22.6 x 105 ) + 4(2010)(20) Q = 9200800 J

19. Water Plasma???? For water vapor to change into “water plasma” would require a temperature of approximately 12,000 degrees Celsius! By FAR, the most common form of matter in the universe both by mass and volume is...... PLASMA!