Water – Physical Properties
Image: Brittany Ferries
H2O
• Elemental building blocks
Atoms and Molecules
• A (not) – tomos (cut)
– Fundamental building block of matter
• Molecule
– Two or more atoms sharing electrons
1
Water – H2O
• Polar molecule
– Electrons are unevenly shared, giving
apparent negative and positive ends
Hydrogen Bonds
• Polarity of water results in weak attraction
call hydrogen bonding. This is not a
molecular bond! It does result in an
ordered liquid with special properties.
Water’s Special Properties
• Cohesion and Surface Tension
• Capillarity
2
Thermal Properties of Water
• As a gas water is unordered – energy is
high enough to overcome hydrogen
bonding
Thermal Properties of Water
• Order provided by hydrogen bonds
provides more stability for liquid phase
Thermal Properties of Water
• Hexagonal crystals formed in solid phase
actually cause ice to be less dense than
liquid water
3
Liquid Water’s Heat Capacity
• Water can remain a liquid
despite the addition of more
energy per unit mass
Heat = Energy
• Heat
– Measured in calories (joules*)
– 1 kilocalorie is 1000 calories
– One calorie raises the temperature of water by 1
degree Celsius
• Heat Capacity
– The quantity of heat needed to produce a unit
change of temperature in a unit mass of material.
– The heat capacity of water is 1 calorie per gram
*1 calorie = 4.184 joules (J)
Phase Changes
• Heat of Fusion
– Change fresh water from a solid at 0oC to a
liquid at 0oC requires 80 calories
– This is the latent heat of fusion
– There is no change in temperature only a
change in physical state
– The heat is released again in changing liquid
water to ice.
Efus = 80 cal/g
4
Phase Changes
• Heat of Evaporation
– Change fresh water at 100oC to a vapor at
100oC requires 540 calories
– This is the latent heat of evaporation
– There is no change in temperature only a
change in physical state
– The heat is released again in changing from
vapor to liquid water.
Evap = 565 cal/g
Phase Changes
Phase Changes in Water
5
Evaporation and Precipitation
Salt and Phase Change
• Adding salt (dissolved ions) into water
changes physical properties
– Raises boiling temperature at a given
pressure
– Lowers freezing temperature at a given
temperature
Salt and Phase Change
6
Density
• Mass per unit volume
– Liquid water at room temperature = 1 g/cm3
• Density effected by temperature, pressure,
and salinity
– Thermal contraction
• Expansion with heat
• Contraction with lower temperatures
• But water is unusual…
Density of Water
• Density maximum at
4oC
• Ice less dense than
liquid water
– Ice floats!
USGS
Density of Seawater
7
Formation of Sea Ice
Coolantarctica.com
Brine Rejection
• Water away from
spheres of hydration
freezes first
• Salinity in liquid
increases
• Pockets of brine are
left over and expelled
to ocean
Sea Ice and Sea Level
• Does melting and freezing of sea ice
affect global sea level?
8
Pressure and Salinity
• Pressure
– Increasing pressure increases density
• Water is not very compressible
• Salinity
– Higher salinity increases density
• Cold salty waters are denser than warm
fresher waters.
Transmission of Heat
• Conduction
– This is a molecular process. When heat is
applied to one area, the molecules move
faster and the energy spreads to adjacent
ones.
• Convection
– Convection is a density driven process
• Radiation
– Direct transmission of energy by radiation
Temperature, Density, and Salinity
Distributions
9
Temperature, Density, and Salinity
Distributions
Temperature, Density, and Salinity Profiles
Seasonal Thermocline Variations
10
Seasonal Thermocline Variations
Halocline
• Rapid change in salinity with depth
Halocline
• Rapid change in salinity with depth
11
Light Transmission through Water
12
Light Transmission through Water
•
•
•
•
Absorption
Scattering
Attenuation
By 10m, only 10% of light is
transmitted
• By 100m, nearly no light is
being transmitted
Shallow Underwater
• Flash unable to cover large area
Shallow Underwater
• Close-up view allows flash to bring out
reds and oranges
13
Shallow Underwater
Natural light in background
Closer up, flash is more effective
(less absorption)
Sound Transmission in Water
• Speed of sound in water is 5 times faster
than in air
• Speed of sound increases with increasing
pressure, temperature, and salt
Sofar Channel
• Sound Fixing and Ranging Channel
14
Key Points
• Water Thermal Properties
– Heats of fusion, evaporation, and heat content
– Hydrogen bonding
– Phase changes
• Light transmissivity
• Sound transmissivity
• Depth profiles – all of the ‘clines.
15