1. Energy Changes & Phase Changes
Heating & Cooling Curves

2. energy needed to heat/cool matter!

3. All P/C changes accompanied by energy changes
phase changes are physical changes:
sometimes energy is absorbed (endothermic)
sometimes energy is released (exothermic)
energy change for given phase change can be measured/calculated

4. calculate amount E needed to change pure substance:
from one single phase to another
Q = mcT
Q = energy in Joules
m = mass in grams
C = specific heat capacity
T = change in temperature = Tf - Ti

5. What about phase changes?
more than one phase substance present
melting ice:
both H2O(l) & H2O(s) present at same time
temperature is constant so T=0
even though ice is absorbing heat energy to change phase

6. 6 possible phase changes

7. PE: energy of relative position
molecules are always attracted to one another
energy put into system to pull molecules apart from one another
farther apart, the higher their PE

8. Melting Ice Ice on hot plate: ice is melting
ice absorbing heat from hot plate and using to change phase
temperature of ice-water mix is constant
heat energy from hot plate going into phase change
heat energy not going to KE of particle is going to PE during phase change

9. Identify a phase change as endothermic or exothermic
gas
freezing
condensation
deposition
melting
(fusion)
vaporization
sublimation PE
liquid
solid

10. Heating
& Cooling Curves
way to illustrate energy changes
measure temperature as function of time at constant heating/cooling rate

11. Temperature Time I II III IV V Liquid & Gas Solid & Liquid Solid
KE
KE
boil pt.
KE↔
KE↔
PE↔
KE
Temperature
PE
melt pt.
PE
PE↔
PE↔
Time

12. Melting & Boiling Points
Plateaus = Phase changes = PE changes
2 phases are present, temp is constant
melting point/boiling point:
value always given as exact temp (number with unit: ˚C, K)

13. What happens to temperature as heat is added at boiling point?
NOTHING …
temp doesn’t change until only 1 phase present!

14. To analyze a heating/cooling curve:
curve: does it go uphill or downhill?
label: phases present in each region
describe what happens to KE in each region
describe what happens to PE in each region
locate melting point and boiling point

15. Heating curve of H2O
Why does it take longer for water to vaporize than it does
to melt?