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Hot Topics, Cool Stuff:

Hot Topics, Cool Stuff:. Phase Changes. When heat energy is added to a solid, liquid, or gas Molecules gain kinetic energy, move faster , and temperature increases. When heat energy is removed from a solid, liquid, or gas

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Hot Topics, Cool Stuff:

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  1. Hot Topics, Cool Stuff: Phase Changes

  2. When heat energy is added to a solid, liquid, or gas • Molecules gain kinetic energy, move faster, and temperature increases. • When heat energy is removed from a solid, liquid, or gas • Molecules lose kinetic energy, move slower, and temperature decreases

  3. During a Phase Change… • Ex. Solid liquid • attractive forces can no longer hold defined shape of solid due to greater movement of molecules: forces are broken • Heat energy is used to convert solid m.c. to liquid m.c.

  4. During a Phase Change… • Ex. Liquid  solid • attractive forces need to be restored to bring molecules back to closely-packed, defined shape • Heat energy used to convert liquid m.c. to solid m.c.

  5. When heat energy is either added or removed during a phase change: • Heat energy is used to break or restore forces between molecules so that the phase change can occur • This causes the molecules to become either organized or disorganized • Practical Application: As ice in a glass of lemonade melts on a hot day, the temperature inside of the glass will not change.

  6. During a phase change… energy is NOT used to speed up/ slow down the molecules. The temperature DOES NOT change! • Practical Application: As ice in a glass of lemonade is melting, the temperature inside of the glass will not change until ALL the solid ice has changed to liquid water.

  7. Parts of a Phase Diagram

  8. Melting Points and Freezing Points Are… • Equal • melting and freezing occur at the same temperature: reversible process

  9. Melting Points and Freezing Points Are… • Unique for specific, pure substances • nitrogen has a melting point of –209 oC • water has a melting point of 0oC • sodium metal has a melting point of 98 oC

  10. Melting Points and Freezing Points Are… • The same temperature for pure substances regardless of the amount of substance • 10 grams of water has a melting point of 0 oC • 100 grams of water has a melting point of 0 oC, but it will take longer for all of the solid to melt

  11. Heating Curve of 1 ice cube vs. Heating Curve of 8 ice cubes

  12. Phase Change: Liquid to Gas • Boiling occurs when the vapor pressure of the liquid is equal to the atmospheric pressure

  13. Phase Change: Liquid to Gas • In other words, the liquid pushes against the air just as much as the air pushes on the liquid • Vapor pressure (a.k.a- “Equilibrium Pressure”) = pressure that is exerted by liquid molecules and relates to the tendency of molecules and atoms to escape from a liquid • The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point

  14. How do we calculate heat? Q = mCΔt Q = Heat (Joules) m = mass (grams) C = Specific Heat Capacity ( J/goC) Δt = change in Temperature (oC) = tf – ti ti= initial temperature tf =final temperature

  15. Specific Heat Capacity • Specific Heat Capacity = the amount of energy required to increase the temperature of one gram of substance by one degree Celsius. • Unique for each substance • The higher the specific heat capacity, the more energy required to change its temperature. • Practical application: This is why our metal ring stand gets hotter than the water inside of a beaker

  16. Specific Heat Capacity

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