Chapter 8
Those Incredible Water
Molecules
Properties of
Water
8.1 Water Molecules Form an Open
Crystalline Structure
Ice formation
 Hydrogen
bonding
between adjacent
water molecules
increases as water
cools.
 Loss of thermal
energy causes
molecules to move
slower.
 More H-bonds form.
 To optimize # of Hbonds hexagonal
arrangement forms.
8.1 Water Molecules Form an Open
Crystalline Structure
 From
Most dense
at 4˚C
4°C to 0°C ice
formation occurs.
 Density decreases in
two directions! Why?
 Hexagonal crystals
formed during ice
formation are empty
– NO AIR!
 Above 4°C water
expands like other
substances (thermal
energy increases so
KE increases)
8.2 Freezing and Melting Go on at the Same Time
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Water freezes from the top down.
Water cools at surface, becomes more dense, and sinks.
Warmer water from below rises. This is called upwelling.
When entire body of water becomes 4°C, ice will start to
form at the surface where air temps are zero or below.
Ice formation at surface as hexagonal x’ls form (low
density)
Lowest water
temperature
at bottom of
lake rarely
below 4°C.
4˚C at bottom
b/c water is
most dense at
that temp!
0˚C
4˚C
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
Force of
attraction
between
molecules of
the SAME
substance.
Force of
attraction
between
molecules of two
DIFFERENT
substances.
For water, cohesive
forces are due to
hydrogen bonds.
For water,
which is polar,
adhesive forces
are due to its
interaction with
other polar
molecules.
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
meniscus
Between the glass
and the water.
Between the water
molecules.
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
Capillary Action
 The
Forces compete until the
adhesive force equals the
weight of the water.
combination of
adhesive and
cohesive forces
account for water
levels in each tube.
 What is the
relationship
between these
forces, water level,
and tube
diameter?
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
Adhesion is greater than
cohesion.
Cohesion is greater than
adhesion.
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
Hydrogen
bonds
Surface
Tension
Is the elastic tendency at the
surface of a liquid.
Cohesive forces pull water
molecules in all directions.
Water molecules at the surface
experience no upward pull.
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
Surface
molecules are
pulled inward.
• This causes the
surface area to be
as small as possible.
The smallest
amount of surface
area for a given
volume is a sphere!
•https://www.youtube.com/
watch?v=KFPvdNbftOY
https://www.youtube.com/w
atch?v=cNI-LIVs-to
8.3 The Behavior of Liquid Water is the Result
of the Stickiness of Water Molecules
Soap
molecules
collect at
the
surface of
the water
They
interfere
with the
hydrogen
bonds at
the
surface.
This allows
water to
spread
out
evenly
over a
surface.
Makes
cleaning
more
efficient.
8.4 Water Molecules Move Freely
Between the Liquid & Gaseous Phases
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1. Evaporation (vaporization) – liquid water molecules move
fast enough to overcome H bonds. Cooling effect. Lowers K.E.
& liquid is cooled.
2. Condensation – Opposite of evaporation. Removal of
slowest moving gas mol. condense into liquid. Energy is release
as H bonds are formed & increases temp. Warming process.
3. Sublimation – solid to gas phase
8.4 Water Molecules Move Freely
Between the Liquid & Gaseous Phases
 4.
Boiling – Cooling process Evaporation beneath a
liquid Surface – pressure of
water vapor inside bubbles
equals or exceeds the
atmospheric pressure +
water pressure. So bubbles
of water vapor come to
surface & escape. p. 249 More atm pressure raises
the b.p., less atm pressure
(like Denver, CO) lowers the
b.p. (95°C instead of 100C).
8.5 It Takes a Lot of Energy to Change the
Temperature of Liquid Water
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1. Specific Heat Capacity – quantity
of heat required to change temp.
of 1 g of substance by 1°C.
2. Water has a high SHC because of
how hard it is to break H bonds &
amount of heat consumed in the
breaking of the H bonds.
3. Temp – measure of K. E.
4. Liquid water resists changes in
temp. because of high SHC. Water
holds the heat longer, cools down
slower.
5. Islands and peninsulas have fewer
temp extremes cause surrounded
by water, than places surrounded
by land
8.6 A Phase Change Requires the
Input or Output of Energy
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1. Any phase change involves the
breaking or forming of molecular
attractions.
2. For every 4.184 joules absorbed,
water increases by 1°C until 100°C is
reached.
3. Heat of melting – solid to liquid –
heat energy gained –
Water = +335 joules/gram. (heat
added to ice)
4. Heat of freezing – liquid to solid –
heat energy released
Water = -335 joules/gram (heat
released from water as it freezes)
5. Heat of vaporization – liquid to gas
– Water = +2259 joules/gram (heat
energy gained water changes to
vapor)
6. Heat of condensation – gas to
liquid – Water = -2259 joules/gram
(heat energy released when gas
changes to liquid)
Properties of Water
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Most solids expand when they melt. Water contracts when it
melts. Most solids contract when they freeze. Water expands
when it freezes.
Most solids are more dense than the corresponding liquids. Ice
(0.917 g/cm3) is not as dense as water.
Water has a melting point at least 100°C higher than expected
on the basis of the melting points of H2S, H2Se, and H2Te.
Water has a boiling point almost 200°C higher than expected
from the boiling points of H2S, H2Se, and H2Te.
Water has the largest surface tension of any common liquid
except liquid mercury.
Water has an unusually large viscosity.
Water is an excellent solvent. It can dissolve compounds, such as
NaCl, that are insoluble or only slightly soluble in other liquids.
Water has an unusually high heat capacity. It takes more heat to
raise the temperature of 1 gram of water by 1oC than any other
liquid.