properties The of Water
Lecture 4A
Freshwater Ecosystems
EVPP/BIOL 350
Dr. Kim de Mutsert
Water
The abundance of
water is one of
the most unique
aspects of Planet
Earth The Blue
Planet
Properties of Water
1.
Molecular Structure
2.
Liquid Nature of Water
3.
Specific Heat
4.
Density vs. Temperature
5.
Solvent Properties
6.
Viscosity
7.
Surface Tension
Molecular Structure
Molecular Structure
Water is H2O, two atoms
of hydrogen attached to
one atom of oxygen
Because the O atom has
a greater mass the
electrons spend more
time near it than near the
H atoms
Thus, a charge
asymmetry is created
such that the O end of the
molecule has a negative
charge and the H end has
a positive charge, this a
called a dipole.
Hydrogen Bonds
This charge
asymmetry results in
weak bonding of
adjacent molecules
The net result is that
water has some
unusual properties
It is a liquid at room
temperature when it
should be a gas
The bonds between
adjacent water
molecules are called
hydrogen bonds
Liquid Nature of Water
Melting points (C) of related hydrides:
H4C -182 H3N -78
H2O 0
0
H2S -83 H2Se -64
HF -83
H2Te -48
Boiling points (C) of related hydrides:
H4C -161 H3N -33
H20
H2O 100
H20
100
H2S -60 H2Se -42
HF 19
H2Te -2
If water behaved as other related hydrides, it
would melt at about -50C and boil at about 0C,
making life as we know it on Planet Earth
impossible
Liquid Nature of Water
The explanation for waters abovepredicted boiling point lies in its
ability to form quasi-polymers
These hold the molecules closer to
one another as is required of a
liquid
It takes more energy (higher
temperature) to break these bonds
and convert water to a vapor
Solvent Properties
Water is considered
the universal solvent
because of its abilities
to dissolve a wide
range of chemicals
This ability is related to
its dipolar nature
The polar parts of the
water molecule can
bind to negative and
positive ions like
chloride (Cl-) and
sodium (Na+)
Heat
One calorie is defined as the amount of
heat required to raise the temperature of 1
g of water by 1C
High Heat Capacity: Additional energy
required to melt or evaporate 1 g of water
80 calories of energy (heat) required to
melt ice (while temperature remains
constant)
540 calories of energy (heat) required to
evaporate water (while temperature
Density by Temperature
Density is defined as the
weight or mass of a
substance per unit
volume (g/cm3)
The density of water
varies in an interesting
way with temperature
The first thing to notice is
that ice is a lot lighter
than liquid water
This is a good thing
because it mean ice
floats on water and is
much more easily
thawed in spring
Density by Temperature
The next thing to
notice is that water
reaches its maximum
density, not at 0oC,
but slightly higher at
4oC
This means that ice
and very cold water
both can float on
4oC water
Then after 4oC,
density decreases
with temperature, but
not as a straight line
Density by Temperature
In fact at higher
temperatures, the rate of
density decrease per unit
of temperature increase
becomes much higher
So, for example, the
density difference
between 30-31C would
be over 10x the
difference between, say,
4-5C
This will come into play
in lake stratification
Viscosity
Viscosity is a measure of liquids resistance to
flow of particles through that liquid
Related to frictional drag that objects experience
when moving through a liquid
Honey has a high viscosity, air has a low
viscosity (about 1/100th that of water)
Viscosity decreases with increasing temperature:
the viscosity of water is about 50% less at 30C
than at 0C
Viscosity has a significant effect on water
movement as well as movement by small
organisms and particles
Surface Tension
Surface tension occurs at the interface
between air and water
At that surface, water molecules can bind
in only one direction and the bonds
become much stronger
This creates a sort of membrane that can
be used by light organisms and particles
The surface layer is also a place where
hydrophobic materials accumulate
Water Quality Variables
Water quality refers to the basic chemical and
physical characteristics of water that determine
its suitability for life or for human uses
Some basic water quality variables include:
Transparency
Temperature
Conductivity
Dissolved oxygen
pH
Alkalinity
Water clarity and light
attenuation
Used routinely as an indicator of the
condition and productivity of lakes.
Three common methods of measuring
clarity are discussed:
Secchi depth: lakes, deep rivers, estuaries
Turbidity and transparency tube:
- streams, ponds/wetlands, coastal zones
Light meters (radiometers): lakes, deep
Water clarity - Secchi disk
A powerful tool in
detecting long-term
trends in lakes
because it is cheap,
easy to use and
usually linked to algal
growth, nutrient
levels or sediment
inflows.
Turbidity
Water clarity is generally
obscured by fine particles
(eg clays) suspended in
the water. This is turbidity.
The effect of turbidity is
partially aesthetic: the
water looks nicer
But water free of excessive
particles also is healthier
for most aquatic life
Turbidity reduced the light
available for plants and
algae
Light profiles radiometry
Chlorophyll - importance
Measuring chlorophyll-a
is a long-accepted
method for estimating the
amount of algae in lakes
Chlorophyll-a is the green
pigment that is
responsible for a plant's
ability to convert sunlight
into the chemical energy
needed to fix CO2 into
carbohydrates.
Chlorphyll a
concentration (density of
Water Quality: Temperature
Basic water quality variable
Determines suitability for
various aquatic life
E.g., trout and salmon
require cool T for survival
and reproduction, bass and
sunfish thrive at warmer T
Generally follows mean daily
air temperature.
Water Quality: Temperature
Water T does not vary as much diurnally as air T
The larger the water body the longer it takes to change its temperature
Small water bodies may be moderated by groundwater (cooler in
summer, warmer in winter)
Water Quality: Temperature
Development of watersheds generally
results in increased summer T
Sun heats pavement,
warms water flowing on it
When riparian vegetation
removed, sun heats streams,
lakes and wetlands directly
Water Quality: Conductivity
Measures the ability of water to
conduct an electrical current
Good way to determine ionic strength
of water because the ability of water to
conduct a current is proportional to the
number of ions in the water
Water Quality: Conductivity
Freshwater generally has low conductivity
measured in microSiemens (uS)
Estuarine and marine systems have much
higher conductivity measured in
milliSiemens (mS) which can easily be
converted to salinity
Humans and other terrestrial animals
require fresh water for survival as do plants
and animals normally found in freshwater
Water Quality: Conductivity
Fresh water = dilute solution of calcium
bicarbonate (CaCO3)
vs. salt water = concentrated solution of
mostly sodium chloride (NaCl)
Development of watersheds generally
increases conductivity. Why?
increase in chemicals applied to soils and
paved surfaces which wash into streams,
lakes and wetlands
Road salts can result in large increases in
NaCl in freshwaters which are detrimental
to freshwater life.
Specific Conductivity
Principle of measurement
A small voltage is applied between 2 parallel
metal rod shaped electrodes, usually 1 cm
apart.
Measured current flow is proportional to the
dissolved ion content of the water.
Conductivity increases with increasing
Temperature
If the sensor is temperature compensated to
25oC, conductivity is called Specific
Conductivity (SpCond)
Water Quality: Dissolved Oxygen
O2 is required for almost all
living things
In air, oxygen is very abundant
(20% = 20 parts per hundred)
In water, oxygen is more
scarce
Water bubbled with air can only
reach about 15 parts per million
(mg/L) O2 (i.e., saturated)
O2
Water Quality: Dissolved Oxygen
Solubility in water is
a function of
temperature
As T increases, the
amount of O2 that
the water can hold
decreases
Since most aquatic
organisms require
at least 3 mg/L of
oxygen, at warmer
temperature oxygen
can become a
limiting factor
Water Quality: Dissolved Oxygen
Decreases in DO generally
due to excess respiration in
the water
Excess respiration is caused
by input of organic matter
such as sewage, garbage,
and also by natural sources
such as decaying algae,
aquatic plants and leaves
As oxygen decreases, water
quality declines
Oxygen levels above 100%
indicate rapid photosynthesis
which may eventually lead to
other water quality problems
DO importance and reporting
Oxygen is produced during photosynthesis
and consumed during respiration and
decomposition.
Generally < 3-4 mg/L is stressful to aquatic
life.
Units of measurement are:
Concentration: mg/L = ppm; concentrations range 0.0
to 20 mg/L
% saturation used to determine if water is fully
saturated with oxygen at a particular temperature
Water Quality: pH
Measure of the acidic or basic characteristics of water
Specifically, pH = -log [H+]
So a pH of 7 means
the [H+]=10-7
A pH of 7 is
considered neutral,
below 7 is acidic and
above 7 is basic
Water Quality: pH
Organisms
generally prefer
a pH of 5-9 with
some being
even more picky
A near neutral
pH will allow the
greatest
diversity of life
The pH of a sample of
water is a measure of
the concentration of
hydrogen ions.
pH determines the
solubility and
biological availability
of chemical
constituents such as
nutrients
(phosphorus,
nitrogen, and carbon)
and heavy metals
(lead, copper,
cadmium, etc.).
pH
Image courtesy of USGS at http://www.usgs.gov/
Alkalinity
The ability of water to neutralize acids or
to resist lowering its pH when an acid is
added
It is also referred to as the buffering
capacity
It is determined by the composition of
sediment or bedrock
Carbonate rocks increase alkalinity (e.g.
CaCO3, MgCO3)
Hardness
Determined by the amount of carbonates
in the water
Related to alkalinity since the main source
causing high alkalinity is carbonate rocks
Soft water usually has low alkalinity and is
susceptible of high pH fluctuations