Physics of Water
By: Christa Jewett, Instructor
“For since the creation of the world God’s invisible qualities – His eternal
power and divine nature – have been clearly seen, being understood from
what has been made, so that people are without excuse.” Romans 1:20
Heat and Heat Capacity
• Water has a relatively narrow range of
temperature fluctuation.
• This is extremely important because marine
organisms generally tolerate only a narrow
temperature range.
Heat vs. Temperature
• Heat is the kinetic energy in the vibration of
individual atoms and molecules in a substance.
• Total heat measures both quantity and speed of
vibrating molecules.
• Temperature measures the degree of molecular
vibration.
– Example: If you have a bathtub of 80°F water and a
cup of 80°F water, introducing an ice cube to the
bathtub will not have a significant impact on the
temperature of the bathtub of water but it will on the
cup of water.
Heat Capacity
• Water has a very high heat capacity.
• Heat capacity: the amount of heat energy required to raise
a given amount of a substance by a given temperature.
– It is measured in calories per gram.
– 1 calorie = the amount of energy required to raise 1 gram of a
substance 1°C.
– The heat capacity of water is 1.00 calories per gram.
• Due to hydrogen bonds, more heat energy is required to
raise water’s temperature than that of most other
substances.
• Therefore, water can absorb or release a lot of heat with
little temperature change.
Water Temperature and Density
• Water is unusual because it becomes less dense as it freezes.
– Most substances become less dense as they warm and denser as they
cool.
– Water reaches its maximum density at 4°C (39°F).
• As water cools below the point of maximum density, it starts to
crystallize into ice and becomes less dense.
– This is because the crystal structure of the water molecules changes
the bond angle between the hydrogen and oxygen molecules from
105° to 109°.
– The crystal forms a hexagon structure that acts like a raft because it
increases the space the molecules take up by 9%. This space is why
the density decreases.
• This is why ice floats. This is also why ice forms on top of the water
(freezing over) whereas most liquids turn to solid from the bottom
up.
Ocean Water Density
• Seawater density varies with salinity and temperature.
– Seawater density normally varies between 1.02 g/cm3 and 1.03
g/cm3.
– Seawater weighs about 2-3% more than pure water which has a
density of 1 g/cm3.
• Seawater density increases with increasing salinity and
decreasing temperature.
• Because temperature and salinity affect water density,
seawater stratifies or form layers.
– Dense water is heavy and sinks below less dense layers.
– Waters with varying densities generally do not mix well.
– Ocean has three general zones based on temperature and
density: Surface zone (surface mixed layer), thermocline and
deep zone.
Ocean Stratification
• Surface mixed layer: temperature and salinity are relatively
constant because waves and currents continually mix the
water.
– Most biologically productive because it is exposed to sunlight
but only accounts for about 2% of the ocean’s volume.
• Thermocline: area where temperature rapidly changes with
depth.
– Does not exist in the Polar regions because the surface water
there is frigid and saline and it sinks.
• Deep layer: cold, dense and fairly uniform.
– Originates from the Polar regions.
– Makes up about 80% of the ocean’s volume.
Light
• Light only penetrates the upper area of the ocean, known
as the Photic Zone.
– Light is absorbed by water, suspended particles and algae.
– Light attenuation is the gradual loss in intensity of light through
water.
• White light is a combination of colors.
– Each of these colors has a different wavelength and radiant
energy.
– Colors at the red end of the spectrum have long wavelengths
and low levels of radiant energy so they are absorbed rather
quickly.
– Colors at the blue end of the spectrum have short wavelengths
and high levels of radiant energy so they are less easily
absorbed.
Light
• Water absorbs fractions of various colors as light travels through it.
– Most light is absorbed within 10 meters (33 feet) of the ocean’s
surface.
– Almost no light penetrates beyond 150 meters (490 feet).
– Almost complete darkness is achieved at 300 meters (~ 1,000 feet).
• Light attenuation varies based on turbidity but generally
wavelengths are filtered out at the following depths:
–
–
–
–
–
–
–
Red: 10 – 49 feet
Orange: 25 – 98 feet
Yellow: 35 – 165 feet
Green: approx. 300 feet
Blue: approx. 300 feet
Indigo: approx. 300 feet
Violet: approx. 330 feet
Other Physical Characteristics
• Temperature: Remains fairly constant somewhere
between 1.9°C to 30°C.
– This is a much narrower area of fluctuation than
experienced on land and is advantageous to marine life.
• Sound: Sound travels faster through water than air.
– Sound travels at about 1,500 meters (4,921 feet – nearly a
mile!) per second, which is about 5 times faster than in air.
– It travels through warm water faster than cool water and it
travels faster in deep water due to the pressure.
– This enables many animals to use echolocation or sound as
a means of communication.
Other Physical Characteristics
• Pressure: Water is heavy. 1ft3 of seawater is approx. 60 lbs.
– The weight of water exerts a force on marine life living in the
ocean.
• Buoyancy: Water is denser than air (about 800 times), so
buoyancy in water is a significant force.
– The density of water is almost exactly the same as that of most
living tissue.
– Most organisms in water are buoyed up by a force nearly the
same as their own weight.
– Some living tissue such as bone, teeth and shells have a greater
density than water and therefore sink.
– Organisms have various ways to adapt to this: gas bladders, light
skeletons, tissue with high oil or fat content.
Other Physical Characteristics
• Movement and Drag: Since water has a higher
viscosity than air, it resists movement through it
far more than air does.
– While it takes very little effort to push yourself off the
bottom of a pool thanks to buoyancy, it takes much
more effort to swim long distances due to drag.
– As a result, marine organisms vertically migrate
through the water rather than chase their prey
through the open ocean.
– Body shape, size and speed also determine how much
drag an organism experiences.