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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.

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.

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.

Experiment: How much salt floats an egg? A fair test

1. Pour exactly 250 ml of water into the jar and gently put in the egg. It should sink.
2. Take the egg out. Add one level teaspoon of salt and stir until it dissolves.
3. Put the egg back in. Does it sink, hover in the middle or float? Write it down.
4. Repeat, one level teaspoon at a time, until the egg floats. Count the spoonfuls.
5. Repeat the whole test twice with fresh water and the same egg, and work out the average.
6. Bonus: float the egg, then very slowly pour a little fresh water down the side of the jar. Where does the egg end up?

Test	Teaspoons of salt until the egg floats
Test 1	Prediction
Test 2	Prediction
Test 3	Prediction
Average	Prediction

Questions

1. Explain in two or three sentences why the ocean is salty.

2. Name the three ocean zones from the lesson and give one fact about each.

3. In your egg test, what did you change, what did you measure and what did you keep the same?

4. Why would a deep-sea diver need a strong submarine instead of just a wetsuit?

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

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