Teacher sheet · Ages 10–11 · about 45–50 minutes
Inside the ocean: salt, layers and pressure
Lesson page: echoesof.science/school/ocean/ages-10-11/ · Pupil worksheet: echoesof.science/school/ocean/ages-10-11/print/
Learning aims
- State that the ocean covers more than 70% of Earth and holds about 97% of its water.
- Explain where the salt in the ocean comes from.
- Describe the sunlight, twilight and midnight zones.
- Explain density layering and run a fair test with one variable changed.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why is the sea salty but rivers are not? Collect ideas. |
| 10 min | Read the text; draw a cross-section of the ocean with the zones and depths. |
| 20 min | Egg fair test in groups; class averages. |
| 5 min | Which ocean zone? game. |
| 5 min | Quiz. |
Materials for the experiment
- A clear jar or beaker
- 250 ml of tap water (measuring jug)
- Table salt
- A teaspoon (level spoonfuls)
- A raw egg
- A tablespoon to lower the egg
- Paper towels
Safety: Do not taste the water. An adult may handle the raw egg; everyone washes their hands afterwards. Wipe spills straight away. If an egg cracks, clean up and wash hands.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Rivers are fresh, so they cannot make the sea salty. | River water carries small amounts of dissolved salts; these build up in the ocean over a very long time. |
| Sunlight reaches the whole ocean. | There is rarely significant light below 200 m, and no sunlight below 1,000 m. |
| Nothing can live in the deepest trench. | Scientists found living single-celled organisms in the Challenger Deep. |
| Salt makes things heavier, so they sink. | Salt makes the water denser, so objects float more easily. |
Worksheet answers
- Rain is slightly acidic and slowly breaks down rocks on land. Rivers carry the dissolved salts to the sea, where many of them build up over time.
- Sunlight zone (0–200 m): most light, plankton photosynthesise. Twilight zone (200–1,000 m): faint light, bioluminescence begins, big-eyed fish. Midnight zone (below 1,000 m): always dark, about 4 °C, sperm whales hunt there.
- Changed: the number of spoonfuls of salt. Measured: whether the egg sinks or floats (the number of spoonfuls needed). Kept the same: the egg, 250 ml of water, level spoonfuls, stirring until dissolved.
- The water pressure increases by about one atmosphere every 10 m, so deep down it would crush a person; a submarine is built to withstand it.
Quiz answers
- About how much of Earth's water is in the ocean? About 97%. Almost all Earth's water is salty ocean water.[1]
- How do salts get into the ocean? Rain breaks down rocks and rivers carry the salts to the sea. Rain is slightly acidic and slowly breaks down rock.[3]
- Why does fresh river water float on top of sea water? Fresh water is less dense than salt water. Denser salt water sinks below fresh water.[4][9]
- How deep is the sunlight zone? The top 200 m. There is rarely much light below 200 m.[5]
- What happens to pressure as you dive deeper? It increases, about one atmosphere every 10 m. The weight of the water above presses down.[6]
- Where does at least half of the oxygen made on Earth come from? Tiny plankton in the ocean. Plankton photosynthesise in the sunlit water.[7]
Going further
Make a density column: colour salty water blue and fresh water red, and pour the fresh water slowly onto the salty water over the back of a spoon. Then try warm fresh water on cold fresh water and discuss how temperature also makes ocean layers.
Sources used in this lesson
- [1] How much water is in the ocean? (NOAA National Ocean Service): reliability High
- [2] How many oceans are there? (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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