School › The ocean › Ages 10–11
Inside the ocean: salt, layers and pressure
Why is the sea salty, why does fresh water float on salt water, and what are the sunlight, twilight and midnight zones? Then run a fair test: how much salt does it take to float an egg?
Read
The ocean covers more than 70% of Earth's surface and holds about 97% of all Earth's water. It is really one global ocean, divided into five named oceans: Pacific, Atlantic, Indian, Arctic and Southern.[1][2]
Sea water contains about 35 grams of salt in every kilogram (35 parts per thousand). Rain is slightly acidic, so it slowly breaks down rocks; rivers carry the dissolved salts to the sea, where many of them build up over time. Most of the salt is sodium and chloride, the same as in table salt.[3]
Salt water is denser than fresh water. That is why fresh river water floats on top of salty sea water, and why things float more easily in the sea.[4][9]
The ocean has layers. The sunlight zone is the top 200 m, where most of the light is. In the twilight zone (200–1,000 m) light fades fast and plants cannot make food. Below 1,000 m, in the midnight zone and deeper, there is no sunlight at all.[5][8]
In the dark, many animals make their own light (bioluminescence). The deepest point is about 10,994 m down in the Mariana Trench in the Pacific Ocean, and even there scientists have found tiny living things.[8]
The deeper you go, the more water presses on you: every 10 m adds about the same pressure as the whole atmosphere. At least half of the oxygen made on Earth comes from the ocean, mostly from tiny drifting plankton near the surface.[6][7]
Try it: How much salt floats an egg? A fair test
You need
- 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.
What to do
- Pour exactly 250 ml of water into the jar and gently put in the egg. It should sink.
- Take the egg out. Add one level teaspoon of salt and stir until it dissolves.
- Put the egg back in. Does it sink, hover in the middle or float? Write it down.
- Repeat, one level teaspoon at a time, until the egg floats. Count the spoonfuls.
- Repeat the whole test twice with fresh water and the same egg, and work out the average.
- Bonus: float the egg, then very slowly pour a little fresh water down the side of the jar. Where does the egg end up?
What happens, and why?
Each spoonful of salt makes the water denser. When the water is denser than the egg, the egg floats. Fresh water poured on top stays on top, because it is less dense than salt water, so the egg can hover between the two layers. To be fair, keep the same egg, the same amount of water and level spoonfuls, and only change the amount of salt.[4][9]
Play: Which ocean zone?
Sort each clue into the right ocean zone.
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Other ages: Ages 6–7 · Ages 15–16