notes: expansion, latent heat, phase changes
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Notes: Expansion, Latent Heat, Phase Changes. 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. Thermal Expansion. - PowerPoint PPT PresentationTRANSCRIPT
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Notes: Expansion, Latent Heat, Phase Changes
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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.
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Thermal ExpansionGenerally, as
temperature increases, 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.
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Bimetallic strips used in thermostats work on the principle that the strip will curl because one metal expands more than the other
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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 volume expands as the temperature continues to drop to 0°C.
Volu
me
Temperature
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Result: ice is less dense than water so that ice floats. This protects underwater life in cold climates
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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?
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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.
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Solid Liquid Gas
Melt Vaporize
CondenseFreezePHASE CHANGES
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energy
tem
pera
ture
Solid
LiquidGas
Solid + liquid
Liquid + gas
Phase Change diagram
Phase changes occur when temperature stays the same. Every molecule must be converted to the new phase before the temperature will change again.
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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
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– 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
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There is NO change in temperature during a change of state.
The heat required when the temperature changes is given by
Q = mcΔTThe heat required during a change of state is
given byQ = mL
where L is the latent heat of either vaporization or fusion
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For water:
Lf = 80 cal/g Lv = 540 cal/g c = 1 cal/g Co
Use these values to complete Concept Development Practice Page 23-1The examples on the following slides involve Joules and kilograms.
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Examples
How much heat must be added to 3 kg ice at -7º C till it just begins to melt?
Cice = 2100 J/kgKQ = mcDT Q = 3 (2100) (7)Q = 44100 J
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How much heat must be added to 3 kg of ice at 0º C to melt all of it?
Lf ice = 3.33 x 105 J/kgQ = mLf
Q = 3 (3.33 x 105 ) Q = 999000 J
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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/kgFirst, 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
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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/kgCsteam = 2010 J/kgKFirst, the water must all be boiled, then the
steam must heat up.Q = mLv + mcDTQ = 4(22.6 x 105 ) + 4(2010)(20)Q = 9200800 J
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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!