4,000 and 5,500 metres
The water molecules pack together tighter as pressure increases-the pressure increase with depth, due to the weight of the water above, and causes the greatest density changes in seawater with depth (greater than the density changes due to temperature and salinity changes). Seawater Properties that Control Density. Density is defined as the mass of water per unit volume and has units of grams per cubic centimeter (g /cm3), kilograms per liter (kg/ L) or kilograms per cubic meter (kg/m3). The density of fresh water at 4 C is 1.0000 g/cm3 or 1.000 kg/liter or 1000 kg/m3. Solution: The density of sea water is 1.03 X 10 3 kg/m 3 and the atmospheric pressure is 1.01 x 10 5 N/m 2. P total = P atmosphere + P fluid = (1.01 x 10 5) + (1.21 x 10 5) Pa = 2.22 x 10 2 kPa (kilo Pascals). Exercises :  At sea level, the air that surrounds us presses down on our bodies at. . You don't feel it because the fluids in your body are pushing outward with the same force. Dive down into the ocean even a few feet, though, and a noticeable change occurs. You can feel an increase of pressure on your eardrums. This is due to an increase in hydrostatic pressure, the force per unit area exerted by a liquid on an object. The deeper you go under the sea, the greater the pressure of the water pushing down on you. For every 33 feet (10.06 meters) you go down, the pressure increases by. .  Therefore pressure increases with increasing depth. The pressure at a given depth depends on the mass of water lying above that depth. (Hydrostatic equation given in class.) If the pressure change is 100 decibars (100 dbar), gravity g = 9.8 m/sec^2, and density is 1025 kg/m^3, then the depth change is 99.55 meter. The higher the temperature, the higher the sound speed. In most areas of the ocean, the warm water at the surface and the high pressure at the bottom produce a sound speed profile which is maximum at the surface and bottom, with a minimum in between. The mean sea level pressure (MSLP) is the atmospheric pressure at sea level or (when measured at a given elevation on land) the station pressure adjusted to sea level assuming that the temperature falls at a lapse rate of 6.5 K per km in the fictive layer of air between the station and sea level. Atmospheric pressure is the pressure exerted by the weight of air in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point Suppose you have a container open on one end that is inverted over water; as the container is lowered in the water the trapped air will be compressed by the water pressure (Figure 7). Assume the container holds one liter of air at sea level pressure (one atmosphere). PV = 1 liter x 1 atm. = 1.  The density of water also causes problems that increase dramatically with depth. The atmospheric pressure at the surface is 14.7 pounds per square inch or around 100 kPa. A comparable water pressure occurs at a depth of only 10 m (33 ft) (9.8 m (32 ft) for sea water). Thus, at about 10 m below the surface, the water exerts twice the pressure (2 atmospheres or 200 kPa) on the body as air at surface level. Three quarters of the planet Earth is covered by water. A majority of the planet's solid surface is abyssal plain, at depths between 4,000 and 5,500 metres (13,100 and 18,000 ft) below the surface of the oceans. Luckily, the change in the density of a gas is as predictable as the change in its volume. If, for example, you take a flexible container to 2 atm. (33 feet/10 meters) the volume will decrease by 1/2 but the density will double. In turn, at 3 atm the volume will be one-third and the density will triple; at 4 atm. the volume is now one-fourth and the density quadrupled and so on.
http://www.ric.edu/faculty/PSCI103/Seawater/Seawater_notes.htm http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/fluid_pressure.html http://oceanservice.noaa.gov/facts/pressure.html http://sam.ucsd.edu/sio210/lect_2/lecture_2.html https://en.wikipedia.org/wiki/Atmospheric_pressure http://www.lakesidepress.com/pulmonary/books/scuba/sectiond.htm https://en.wikipedia.org/wiki/Underwater http://www.deep-six.com/page70.htm
the greatest relative pressure change in sea water takes place between _____ and _____ meters.