Changes of State: the change of a
substance from one physical state to
another
Energy’s Role in Changes of State
• Physical change
– The identity of the substance does NOT change
• Particle Arrangement changes
– The particles of a substance move differently
depending on the state of the substance.
Adding Energy
• Melting occurs when energy is added to a
substance
– Melting: the change of state in which a solid becomes
a liquid by adding energy
– Particles must overcome some attraction to each
other
– Endothermic (endo- means inside)
• Solid to liquid
• Temperature increase
– Melting point: the temp at which a substance changes
from a solid to a liquid
Removing Energy
• Freezing occurs when energy is removed from
a substance
– Particles’ attraction must overcome the motion of
the particles.
• Liquid to Solid
– Freezing point: the temperature at which liquid
turns to a solid
• Freezing is the reverse of Melting
– Exothermic (exo- means outside)
Evaporation: the change of substance
from a liquid to a gas
• Surface
– Below boiling point
• Boiling: is the change of a liquid to a vapor, or
gas throughout the liquid
– Pressure inside bubbles =atmospheric pressure
– Boiling point: the temp at which a liquid boils
• Atmospheric pressure: varies depending on
the relation to sea level
Condensation: the change of state
from a gas to a liquid
• Condensation point: the temperature at which
the gas becomes a liquid
• Particles Clump together
– Attraction between them overcomes their motion
• Energy is removed
– Exothermic change
Sublimation: the process in which a
solid changes directly into gas
• Particles go from being tightly packed
together to being spread far apart
• Attractions between particles must be
completely overcome
• Energy is gained
• http://youtu.be/XIdX9woG7yI
Temperature vs Change of State
Temperature
Change of State
•The speed of the substance’s particle
•Temperature change is secondary
•Change of State occurs first
Quiz…
1. What are two changes of state that are
endothermic?
2. Do the particles of a substance move faster
or slower as the substance is heated?
3. When water evaporates, has a chemical of
physical change occurred?