Change of Phase
Level : Conceptual Physics
Instructor : Kim
Energy and Change of Phase
Change of phase means the state when a substance is changing from solid state to liquid
state or from liquid to gaseous state.
For example, when ice(solid state) is melting to water(liquid state) or when water(liquid
state) is vaporize to steam(gaseous state), this process is called change of phase.
To raise the temperature of a substance, heat energy must be added. To change the phase
of the substance, heat energy must also be added.
For example, if you have 1g of ice at 0ºC, there must be a certain amount of heat energy
needed to completely melt the ice into water. The amount of energy needed to completely
change the phase of 1g of solid ice into 1g of liquid water is 80cal.
solid ice at 0ºC
liquid water at 0ºC
80cal of heat
must be added!
However, if the temperature of the 1g of ice is T= −10ºC, the heat energy will be first
used to raise the temperature from −10ºC to 0ºC, then heat energy will be used to melt it.
To calculate the total amount of heat Q needed to first raise the temperature of ice and
then completely melt it, there must be two steps.
First calculate the amount of heat needed to raise the temperature by ∆T= 10ºC, then
added the amount of heat necessary to melt the ice, which is 80cal.
The specific capacity c=0.5cal/g∙ºC, m=1g and ∆T= 10ºC, so
=> Qtotal=cm∆T + heat needed to melt the ice = 0.5×1×10 + 80cal= 85cal
=> Qtot =85cal
- If you add heat to a solid, it will melt and become a liquid, and if you add heat to the
liquid, it will vaporize and become a gas
- Conversely, energy must be extracted from a substance to change it phase from gas to
liquid to solid
Heat Energy is absorbed when change of phase is in this direction
SOLID!
LIQUID!
GAS!
Heat Energy is released when change of phase is in this direction
Calculating the Heat needed to Melt Ice into Water
The amount of heat needed to melt solid to liquid is expressed as
Q=LF m
where LF is called the heat of fusion, where LF =80cal/g for solid ice
Q=cm∆T = cm(Tf -Ti )
specific capacity for liquid water c=1cal/g∙ºC
specific capacity for solid ice c=0.5cal/g∙ºC
Q=LF m
where LF =80cal/g
Q1) The mass of a piece of solid ice is 2g. How much heat energy is needed to melt it
completely to water?
a) 80cal
b) 160cal
c) 200cal
d) 250cal
Q2) The mass of a piece of solid ice is 3.125g. How much heat energy is needed to melt
it completely to water?
a) 80cal
b) 160cal
c) 200cal
d) 250cal
Q3) There is a 20g solid ice placed in an isolated chamber. The temperature of the ice is
T= −15ºC. How much heat energy Q is needed to raise the temperature to −5ºC?
a) 100cal
b) 150cal
c) 200cal
d) 250cal
Q=cm∆T = cm(Tf -Ti )
Q=LF m
specific capacity for liquid water c=1cal/g∙ºC
specific capacity for solid ice c=0.5cal/g∙ºC
where LF =80cal/g
Q4) There is a 20g solid ice placed in an isolated chamber. The temperature of the ice is
T= −15ºC. How much heat is needed to completely melt the ice?
a) 1100cal
b) 1340cal
c) 1750cal
d) 2150cal
Q5) There is a 20g solid ice placed in an isolated chamber. The temperature of the ice is
T= −15ºC. How much heat is needed to raise the temperature to 20ºC
a) 1100cal
b) 1340cal
c) 1750cal
d) 2150cal
Q6) How much heat must be added to ice of mass 720g at −10ºC to take it to liquid state
at 15ºC?
a)68000cal
b) 72000cal
c) 80000cal
d) 86000cal
Calculating the Heat needed to Vaporize Water into Steam
- If you need 80cal of heat to melt 1g of ice completely into water, then there must be a
certain amount of heat needed to vaporize liquid water entirely to steam
- The amount of heat needed to vaporize 1g of water is 540cals.
- The amount of heat needed to vaporize liquid into solid is expressed as
Q=LV m
where LV is called the heat of vaporization, where LV =540cal/g for solid ice
* Phase Change of liquid water*
i) In order for 1 gram of ice at 0ºC to completely melt, 80 calories of heat must be added.
(if you add only 70cal of heat to 1g of ice at 0ºC, then there will mixture of water and ice)
ii) All the heat is used to melt the ice, leaving no heat to raise the temperature
iii) The heat will continue to be used to melt the ice until it all melts
iv) When there is nothing more ice to melt, then the heat is used to raise the temperature
of the now liquid state water
v) Each additional calories of heat absorbed by the gram of water will increase its
temperature by 1ºC until the water reaches 100ºC
(remember that the heat energy needed to raise the temperature of 1g of water by 1ºC is 1cal)
vi) Again the heat is now used only to turn the water into gas, while the temperature does
not change
vii) To vaporize the whole 1g of water into steam, 540 calories of heat must be added
viii) Finally, when the whole 1g of water is vaporized into steam, the temperature rises
again
Heats of Transformation
- To melt a solid means to change it from the solid state to liquid state. The process
requires energy to be obtained because the molecules of the solid must be freed from
their rigid structure.
- To vaporize a liquid means to change it from the liquid state to the gas state. This
process requires energy to be obtained because the molecules must be freed from their
clusters
- To freeze a liquid to form a solid requires energy to be released from the liquid, so that
the molecules can settle into a rigid structure
- To condense a gas to form a liquid requires energy to be released from the gas, so that
the molecules can cluster instead of flying away from one another
The amount of energy per unit mass that must be transferred as heat when a sample
completely undergoes a phase change is called the heat of transformation L
Q = Lm
When phase change is from liquid to gas or gas to liquid, the heat of transformation is
called the heat of vaporization LV. For water
LV = 540cal/g
When phase change is from solid to liquid or liquid to solid, the heat of transformation is
called the heat of fusion LF. For water
LF = 80cal/g
Q=cm∆T = cm(Tf -Ti )
Q=LF m where LF =80cal/g
specific capacity for liquid water c=1cal/g∙ºC
specific capacity for solid ice c=0.5cal/g∙ºC
Q=LV m where LV =540cal/g
Q7) A container is filled with 20g of water at room temperature T=25ºC. How much heat
is needed to completely vaporize the water into steam?
a) 10800cal
b) 12300cal
c) 14500cal
d) 15700cal
Q8) A solid piece of ice has a mass of 10g at a temperature of −10ºC. How much heat
energy is needed to completely vaporize into steam?
Hint: Find the heat needed to raise the temperature of ice to 0ºC, then find the heat
needed to completely melt the ice, then find the heat needed to raise the temperature of
water from 0ºC to 100ºC(since ice is now melted) and finally find the heat needed to
completely vaporize the water. Then add all the heat.
a) 7250cal
b) 7800cal
c) 8210cal
d) 8500cal