School › The ocean › Ages 15–16
The ocean in numbers: pressure, light and salt
Calculate the pressure at the bottom of the Mariana Trench, test NOAA's figures, measure the density of model sea water and discuss what we really know about the oxygen made in the ocean.
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NOAA estimates about 1,335,000,000 km³ of water in the ocean out of more than 1,386,000,000 km³ on Earth: 1.335 ÷ 1.386 ≈ 0.96, so roughly 96–97% of all water is ocean water (NOAA rounds it to 'about 97 percent'), covering more than 70% of the planet's surface.[1]
At sea level the atmosphere presses with 14.7 psi (about 1 bar, or 1 atmosphere). In water, the pressure rises by one atmosphere for every 10.06 m of depth, so the total pressure at depth d is roughly p ≈ (1 + d ÷ 10.06 m) atm.[6]
Check: at 4,000 m, p ≈ 1 + 4,000 ÷ 10.06 ≈ 399 atm ≈ 5,860 psi, which agrees with NOAA's 'over 5,850 psi'. At the bottom of the Mariana Trench (10,994 m) it is about 1,094 atm ≈ 16,000 psi, about 8 tons on every square inch, again close to NOAA's figure.[6][8]
Light: sunlight can reach about 1,000 m under the right conditions, but there is rarely significant light beyond 200 m, and photosynthesis stops in the twilight zone. That is why the ocean's producers live near the surface.[5][8]
At least half of the oxygen produced on Earth comes from ocean plankton, and one tiny bacterium, Prochlorococcus, makes up to 20% of the biosphere's oxygen. But marine life uses about as much oxygen as it makes, and the exact figures are uncertain: they are estimated from satellite images and change with season, nutrients and temperature.[7]
Sea water has a salinity of about 35 parts per thousand; sodium and chloride make up over 90% of the dissolved ions, which come from rocks weathered by slightly acidic rain. Salinity and temperature set the density of sea water, and denser water lies below less dense water, so the ocean is layered. When sea water freezes, the remaining water gets saltier.[3][9][4]
Try it: Measuring the density of model sea water
You need
- Electronic balance (0.1 g)
- 100 ml measuring cylinder
- Three beakers
- Table salt
- Distilled or tap water
- Stirring rod
- Graph paper or a spreadsheet
Safety
Wear eye protection as in any lab practical. Mop up spills straight away, especially near the balance; keep the balance dry. Do not taste the solutions.
What to do
- Make three solutions: 0 g, 17.5 g and 35 g of salt, each topped up with water to a total mass of 500 g (0, 35 and 70 parts per thousand). Stir until all the salt has dissolved.
- Weigh the empty, dry measuring cylinder and record its mass.
- Pour in exactly 100 ml of the first solution (read the bottom of the meniscus at eye level) and weigh again.
- Calculate the density: ρ = mass ÷ volume, in g/ml. Repeat for each solution, rinsing and drying the cylinder in between.
- Repeat each measurement three times and calculate the mean and the range.
- Plot density against salinity. Estimate the uncertainty from the ±1 ml reading of the cylinder and the ±0.1 g of the balance. Which one matters more?
What happens, and why?
Dissolved salt adds mass without adding much volume, so the density rises with salinity. That is why salt water lies under fresh water and why objects float more easily in salt water. In the ocean, differences in salinity and temperature create layers of different density. Table salt is mostly sodium chloride; real sea salt also contains other ions, but sodium and chloride make up over 90%.[4][9][3]
Play: Deeper: more or less?
As you go deeper in the ocean, does this go up or down? Sort each one.
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Quiz
Other ages: Ages 6–7 · Ages 10–11