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 ExpansionGenerally, as
temperature increases, the lengths and volumes of substances also increase.
Examples: concrete sidewalks, metals, gases
Bimetallic strips: used in thermostats
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
Result: ice is less dense than water so that ice floats. This protects underwater life in cold climates
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?
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.Precipitation: rain, sleet, snow, hail that falls to Earth
when cloud particles become too heavy
Solid Liquid Gas
Melt Vaporize
CondenseFreezePHASE CHANGES
energy
tem
pera
ture
Solid
Liquid
Gas
Solid + liquid
Liquid + gas
Phase Change diagram
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” -- melt
• latent heat of fusion, Lf - the amount of heat per kilogram that is required to be transferred to melt or freeze a substance
– 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 kilogram that is required to be transferred to boil or cause condensation
There is NO change in temperature during a change of state.
The heat required when the temperature changes is given by
Q = mCDTThe heat required during a change of state is
given byQ = mL
where L is the latent heat of either vaporization or fusion
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
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/kg
Q = mLf
Q = 3 (3.33 x 105 ) Q = 999000 J
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
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
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!
Top Related