Phase Changes
Three States of Water
Water is an abundant substance on Earth. It can be found
as a solid, a liquid, and as a gas called water vapor.
1. How many words can you think of to describe
solid water?
2. Where is most of the liquid water on Earth found?
Where is most of the water vapor found?
3. Describe a natural event you have observed when
water changed from a liquid to a solid, and when
water changed from a liquid to a vapor.
1. How many words can you think of to describe solid
water?
Answers may include forms such as ice, sleet, snow,
and hail or formations such as glaciers, icebergs, and ice
caps.
2. Where is most of the liquid water on Earth found? Where
is most of the water vapor found?
Most liquid water is found in oceans (which cover 71% of
Earth’s surface). Water vapor is found in Earth’s
atmosphere.
3. Describe a natural event you have observed when water
changed from a liquid to a solid, and when water
changed from a liquid to a vapor.
Answers may include a pond freezing over or water
evaporating from a puddle.
Characteristics of Phase Changes
• States of matter (s, l, g) are also referred to as
phases.
• Phase Change: the reversible physical change
that occurs when a substance changes from one
state of matter to another
Phase Changes
Phase Changes
•
•
•
•
•
•
Melting
Freezing
Vaporization
Condensation
Sublimation
Deposition
Energy & Phase Changes
• During a phase Δ, E is transferred
between a substance and its surroundings
• E is either absorbed or released during a
phase Δ
– Endothermic Δ: system absorbs E from
surroundings (heat enters)
– Exothermic Δ: system releases E to its
surroundings (heat exits)
b. liquid
a. Liquid
b. Liquid
c. Gas
d. Liquid
e. Gas
f. Gas
Energy & Phase Changes, ctd.
When one freezes water, is energy (in the
form of heat) LEAVING or ENTERING the
system?
Melting & Freezing
• What happens to the arrangement of
molecules in water as it freezes? As it
melts?
• Are the freezing (melting) points of things
that are solid at room temperature higher
or lower than 25°C? (everyone think!)
MELTING/VAPORIZATION
Vaporization & Condensation
• Vaporization (l  g); is this endothermic or
exothermic?
• Condensation (g  l); is this endothermic
or exothermic?
Note: there are 2 processes
of vaporization; boiling
and evaporation…
what’s the difference?
Vaporization & Condensation, ctd.
• Evaporation – takes place at the surface of
a liquid and occurs at temperatures below
the boiling point
– The molecules near the surface are moving
fast enough to escape the liquid and become
water vapor.
• What happens to the molecules in water
as they are heated?
Sublimation & Deposition
• Sublimation – solid  gas
without changing
into a liquid first
• Deposition – gas  solid
without changing
into liquid first
Condensation/Freezing/Sublimation
Assessment and Review…
• Name six common phase changes.
• Melting, freezing, vaporization, condensation, sublimation, and
deposition.
• What happens to the temperature of a substance during a phase
change?
• The temperature of a substance does not change during a phase
change.
• How does the energy of a system change during a phase change?
• Energy is either released or absorbed during a phase change.
• What happens to the arrangement of water molecules as water
melts and freezes?
• The arrangement of molecules becomes less orderly as water melts
and more orderly as water freezes.
• What is the difference between evaporation and boiling?
• Evaporation takes place at the surface of a liquid and occurs at
temperatures below the boiling point.