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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.

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.

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.

Experiment: Measuring the density of model sea water

1. 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.
2. Weigh the empty, dry measuring cylinder and record its mass.
3. Pour in exactly 100 ml of the first solution (read the bottom of the meniscus at eye level) and weigh again.
4. Calculate the density: ρ = mass ÷ volume, in g/ml. Repeat for each solution, rinsing and drying the cylinder in between.
5. Repeat each measurement three times and calculate the mean and the range.
6. 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?

Salinity (parts per thousand)	Mass of 100 ml (g), trial 1	Trial 2	Trial 3	Mean density (g/ml)
0	Prediction
35	Prediction
70	Prediction

Questions

1. Calculate the total pressure, in atmospheres and in psi, at 2,134 m, where sperm whales hunt giant squid. Show your working.

2. A pupil measures 100 ml of salt water with a mass of 102.6 g. Calculate its density in g/ml and in kg/m³.

3. Give two reasons why the figure 'at least half of Earth's oxygen comes from the ocean' is uncertain.

4. Explain why fresh water from a river forms a layer on top of the sea where the river meets the ocean.

Sources for every fact on this sheet: echoesof.science/school/ocean/sources/

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