Teacher sheet · Ages 15–16 · about 45–50 minutes
The ocean in numbers: pressure, light and salt
Lesson page: echoesof.science/school/ocean/ages-15-16/ · Pupil worksheet: echoesof.science/school/ocean/ages-15-16/print/
Learning aims
- Use NOAA's depth rule to calculate pressure and check it against published values.
- Calculate density from mass and volume and relate it to salinity.
- Explain why producers live in the sunlit layer.
- Evaluate the uncertainty in estimates of ocean oxygen production.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: how much pressure do you think there is at the bottom of the Mariana Trench? Write a guess. |
| 10 min | Read the text and work through the pressure check together. |
| 20 min | Density practical; mean and range for each solution. |
| 5 min | Graph and uncertainty discussion. |
| 5 min | Game and quiz. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Pressure in the deep ocean comes from the salt. | Pressure comes from the weight of the water (and air) above; it increases about one atmosphere per 10 m. |
| The ocean gives us half our oxygen to breathe, so its plankton feed the air directly. | At least half of all oxygen produced comes from the ocean, but marine life uses about as much as it makes. |
| Light stops sharply at 200 m. | Light fades gradually; some can reach about 1,000 m, but rarely significantly beyond 200 m. |
Worksheet answers
- p ≈ 1 + 2,134 ÷ 10.06 ≈ 1 + 212 = 213 atm; 213 × 14.7 ≈ 3,130 psi.
- ρ = 102.6 g ÷ 100 ml = 1.026 g/ml = 1,026 kg/m³.
- Any two: amounts change all the time (seasons, nutrients, temperature, time of day, tides); satellite images do not capture the full picture; different organisms and places vary; production and use by marine life must both be estimated.
- River water is fresh and so less dense than salty sea water; the less dense water floats on top until it mixes.
Quiz answers
- Using NOAA's rule, what is the total pressure at a depth of about 50 m? About 6 atm. 1 + 50 ÷ 10.06 ≈ 6 atm.[6]
- Which figure supports the claim that about 97% of Earth's water is in the ocean? 1,335,000,000 km³ out of more than 1,386,000,000 km³. The ratio is about 0.96–0.97.[1]
- Why do photosynthesising plankton live near the surface? Below about 200 m there is rarely enough light for photosynthesis. Photosynthesis is no longer possible in the twilight zone.[5]
- Why is the ocean's net contribution of oxygen to the air much smaller than its production? Marine life uses about as much oxygen as it makes. Production and use are roughly in balance.[7]
- What is the salinity of average sea water? About 35 parts per thousand. That is about 3.5% salt by weight.[3]
- Why does fresh water float on sea water? It has a lower density. Denser salty water lies underneath.[4][9]
Going further
Use the pressure rule to make a graph of pressure against depth from 0 to 11,000 m and mark the zones, the Pisces V limit (about 2,000 m) and the Challenger Deep. Discuss why the rule is only approximate (sea water density changes with temperature, salinity and pressure).
Sources used in this lesson
- [1] How much water is in the ocean? (NOAA National Ocean Service): reliability High
- [3] Why is the ocean salty? (U.S. Geological Survey (FAQ)): reliability High
- [4] Discover your world: Please pass the salt (NOAA National Ocean Service (education activity)): reliability High
- [5] How far does light travel in the ocean? (NOAA National Ocean Service): reliability High
- [6] How does pressure change with ocean depth? (NOAA National Ocean Service): reliability High
- [7] How much oxygen comes from the ocean? (NOAA National Ocean Service): reliability High
- [8] Layers of the ocean (NOAA National Weather Service (JetStream online school)): reliability High
- [9] Introduction to density (Exploring Our Fluid Earth, University of Hawaiʻi at Mānoa (College of Education)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/ocean/sources/
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