Properties of
Water
Three major properties of water
¾Thermal- density
properties
¾High specific heat
¾Liquid- solid characteristics
Molecular Structure - Basis of
Properties
¾ At equilibrium - H2O molecule forms
isosceles triangle with obtuse angle of
104.5° at oxygen atom
¾ Continual state of vibration
¾ Several ionized states
¾
¾
¾
¾
H3O+
OHH4O++
O--
Molecular Structure - Basis of
Properties
¾ Weak coulombic characteristics of
hydrogen bonded to weakly
electronegative oxygen
¾ Both ionized and covalent states occur
¾ Water is almost the only known compound
that exhibits these characteristics
Density Relationships
¾ Specific gravity - Ratio of the mass of a
substance to the mass of an equal volume
of a reference substance (usually water)
¾ Specific gravity of pure ice at 0°C is
0.91680
¾ Specific gravity of water at 0°C is 0.99987
¾ From 0°C, specific gravity of water
increases to a maximum of 1.0000 at
3.98°C
Important !!!!!
Figure 2-3, page 12
Density Relationships
¾
Density difference per degree lowering - the
difference in density between water at a given
temperature and water at a temperature 1°C
lower
¾
Physical work is required to mix fluids of
different density
¾
Work is proportional to difference in density
Density Relationships
¾ Work required to mix water
between 29°C and 30°C is
40 times greater
than work required to mix water
between 4°C and 5°C (24°C and
25°C - 30 times)
Effect of salinity on density
¾ Density increases as concentration of
dissolved salts (salinity) increases
¾ Increase is approximately linear
¾ Salinity of lakes is generally in the range
of 0.01 - 1.0 g/l
¾ Range is most commonly 0.01 - 0.5 g/l
¾ Saline lakes can exceed 60 g/l
¾ Salinity of seawater is 35 g/l
Effect of salinity on density
¾ Temperature of maximum density
decreases as salinity increases - rate
of 0.2°C/g/l
¾ Maximum density of seawater is at 3.52°C
Effect of Pressure
¾ Pressure can compress water and lower
temperature of maximum density
¾ Pressure increases by 1 atmosphere per
10 m of depth
¾ Temperature of maximum density
decreases 0.1°C/100 m of depth
Structure of Ice
¾ Each molecule is H-bonded to nearest
four neighbors
¾ Every oxygen atom is at the center of a
tetrahedron formed by four oxygen
atoms
¾ O-H bonds are directed toward
electrons of adjacent oxygen molecule
Structure of Ice
¾ Its lone pairs of electrons are directed
toward O-H bonds of adjacent
molecules
¾ Results in open lattice with both
perpendicular and parallel voids
¾ Voids allow ice to float on water
Structure of Ice
¾ Ice molecules vibrate (translational
and reorientation movement) at a rate
of 106 movements/sec at 0°C
¾ Water at 0°C vibrates at 1012
movements/sec
¾ Rate of vibration increases as
temperature increases and results in
decreasing viscosity
Structure of Ice
¾ As water melts, H-bonds are disrupted
and water fills the voids and density
increases
¾ Above 3.98°C, vibrations increase and
the distance between bonds increases
and density decreases
Water is a liquid crystal
¾
¾
¾
¾
Hydrogen bonds create lattice even in liquid
phase
Water freezes at 0°C and boils at 100°C
On the basis of van der Waals forces alone,
water would be expected to freeze at -100°C
and boil at -80°C
Water and mercury are the only inorganic
compounds that exist as liquids at the
temperature and pressure found at the earth's
surface
Specific Heat
¾ Specific heat is the amount of heat in
calories that is required to raise the
temperature 1°C of a unit mass (usually
1 gram) of a substance
¾ Specific heat of water is 1.0 - very high
Specific Heat
¾ Few substances have a higher specific
heat
Liquid ammonia - 1.23
¾
Liquid hydrogen - 3.4
¾
¾ Most substances have much lower
specific heats
Rocks - 0.2
¾
Latent Heats
¾ Water also has a high latent heat of
evaporation - heat energy required to
evaporate a unit mass of liquid at a
constant temperature
¾ Latent heat of evaporation - 540 cal/g
¾ Latent heat of melting - 79.72 cal/g
¾ Latent heat of sublimation - 679 cal/g
Latent Heats
¾ Water also has a high latent heat of
evaporation ¾ Heat energy required to evaporate a unit
mass of liquid at a constant temperature
¾ Latent heat of evaporation - 540 cal/g
Latent Heats
¾ Latent heat of evaporation - 540 cal/g
¾ Latent heat of melting - 79.72 cal/g
¾ Latent heat of sublimation - 679 cal/g
Specific Heat and Latent Heats
¾
These heat characteristics are a function of the
heat energy required to disrupt hydrogen bonding
Environmental Consequences
¾ Heat-requiring and heat-retaining
properties of water provide a much more
stable environment in aquatic systems
than in terrestrial systems
¾ Fluctuations in temperature are gradual
¾ Seasonal and diurnal extremes are small
compared to terrestrial systems
Environmental Consequences
¾ Climate is strongly affected by bodies of
water
¾ Areas near major bodies of water have
mild winters with higher preciptitation rates
and moist, cool summers.
¾ Steaming lakes - water vapor from
evaporation condenses in cooler overlying
air masses
Environmental Consequences
¾ Latent heat of melting is high - water
warms slowly and large energy losses are
required to form ice cover
¾ Latent heat of evaporation is high - much
heat is expended in evaporation therefore
water warms slowly
Viscosity-Density
Relationships
¾ Viscosity is the tendency of a solution to
resist flow
¾ Viscosity increases as density increases
¾ Density of water is 775 times greater that
air at STP
¾ Viscosity changes as temperature
changes
Viscosity-Density
Relationships
¾ Viscosity is related to buoyancy
¾ Aquatic organisms require less supportive
tissue
¾ Sinking rates are regulated by buoyancy
and therefore viscosity
Surface Tension
¾ Bonding properties are disrupted at the
air-water interface
¾ Molecular attractions are directed
inward toward liquid phase
¾ Surface tension of water is higher than
that for any other liquid except mercury
Surface Tension
¾ Surface tension decreases with
increasing temperature
¾ Surface tension increases slightly with
dissolved salts
¾ Surface tension is greatly reduced by
the addition of organic compounds