GeographyISC Class 11 · Salinity, Temperature and Density of Ocean Water

Composition and Salinity of Sea Water (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Sea water is about 96.5 per cent water and 3.5 per cent dissolved salts, mainly sodium chloride, giving an average salinity of about 35 parts per thousand. Salinity rises with evaporation and falls with rain, river inflow and melting ice; it peaks in the subtropics and is lowest in polar seas.

Chart: sea water is 96.5% water and 3.5% salts; the salts are mostly chloride (55%) and sodium (30.6%), with sulfate, magnesium, calcium and — Notice that salts are only 3.5% of sea water (35 g per kg), and that chloride and sodium, the parts of common salt, make up most of them.
Picture: Matt Herod · CC BY-SA 4.0 · Wikimedia Commons

Understand Composition and Salinity of Sea Water

Composition. About 96.5 per cent water and 3.5 per cent dissolved substances; about 35 g of salts per kg. Main salts as a share of total dissolved salts: sodium chloride about 77.8 per cent, magnesium chloride about 10.9, magnesium sulphate about 4.7, calcium sulphate about 3.6, potassium sulphate about 2.5, others the remainder. Chloride and sodium together make up about 85 per cent of the ions. Principle of constant proportions: relative proportions stay nearly the same in the open ocean, though total salinity varies. Sources: weathering of rocks (rivers), volcanoes and sea-floor vents, wind-blown dust; dissolved gases include oxygen, carbon dioxide and nitrogen.

Salinity is grams of salt in 1,000 g of sea water (o/oo or ppt).

Factors. Evaporation raises it (subtropical oceans, Red Sea); precipitation lowers it (equatorial belt); river inflow lowers it (Bay of Bengal, Black Sea, Baltic); freezing leaves salt behind (raises it) and melting dilutes; currents and winds (the North Atlantic Drift keeps the North Sea saline); enclosure: enclosed seas in hot dry regions are very salty (Red Sea, Persian Gulf, Mediterranean), those in cool humid river-fed regions are fresh (Baltic).

Horizontal distribution. Open ocean 33 to 37 o/oo, average 35. Highest in the subtropics (about 20 to 30 degrees N and S), slightly lower at the equator because of rain, lowest in polar seas. Red Sea up to about 41 o/oo. The Arabian Sea is more saline than the Bay of Bengal. Inland lakes with no outlet (Lake Van, Dead Sea, Great Salt Lake) exceed 200 o/oo.

Vertical distribution. In high latitudes salinity increases with depth; in the subtropics it decreases with depth; the halocline is the zone of rapid change; below about 1,000 m it is nearly uniform (about 34.5 to 35 o/oo).

Composition and Salinity of Sea Water: key facts

Salinity: $S a l ini t y (o / oo) = g r am so f s a l t / g r am so f se a w a t er x 1, 000∣ A v er a g e ab o u t 35 o / oo$
Main salts: NaCl about 77.8%
MgCl2 about 10.9%
MgSO4 about 4.7%
CaSO4 about 3.6%
K2SO4 about 2.5%
Mixing two masses: $M i x = (S 1 x m 1 + S 2 x m 2) / (m 1 + m 2) ∣ W e i g h t e d a v er a g e$
Salinity control: Raised by evaporation, freezing | Lowered by rain, rivers, melting

How to answer Composition and Salinity of Sea Water questions

Questions are 3 to 4 marks on factors and distribution, plus short salinity sums.

1Define salinity with the o/oo formula.
2List factors with a place example each.
3Describe distribution horizontally then vertically.
4In sums, write masses in the same unit and show the weighted average.

Composition and Salinity of Sea Water in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Salt in a tank

Suppose 5,000 kg of sea water has a salinity of 35 o/oo.

Find the mass of salt and of sodium chloride.

Try it first, then show the solutionSolution

Salt = 5,000 x 35/1,000 = 175 kg.
NaCl = 77.8% of 175 = 136.15 kg.

Answer: 175 kg of salts; about 136 kg of sodium chloride.

Memory hook: Per thousand, so divide by 1,000.

Salt pan

Suppose 1,000 g of sea water at 35 o/oo loses 100 g of water by evaporation.

Find the new salinity.

Try it first, then show the solutionSolution

Salt stays 35 g; mass left = 900 g.
Salinity = 35/900 x 1,000 = 38.9 o/oo.

Answer: About 38.9 o/oo.

Memory hook: Same salt, less water.

Tricks to remember Composition and Salinity of Sea Water

Short memory aids. Say each one aloud once, then use it on a question the same day.

Hot dry salty, wet cool fresh
High evaporation raises salinity; rain and rivers dilute.
Use it when: Factors.
Subtropics peak
Salinity peaks near 20-30 degrees, not at the equator.
Use it when: Distribution.

Composition and Salinity of Sea Water: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Salt · 1 mark · MCQBoard level

The most abundant salt in sea water is:
(a) magnesium chloride
(b) sodium chloride
(c) calcium sulphate
(d) potassium sulphate

Try it first, then show the solutionSolution

About 77.8 per cent.

Answer: (b) Sodium chloride.

Example 2: Definition · 2 marks · Short answerBoard level

Define salinity and state the average salinity of the open ocean.

Try it first, then show the solutionSolution

Grams of dissolved salt in 1,000 g of sea water.
About 35 o/oo.

Answer: Grams of salt per 1,000 g of sea water; about 35 o/oo.

Example 3: Mixing sums · 4 marks · Long answerHarder board level

200 kg of water at 36 o/oo is mixed with 300 kg at 32 o/oo. Find the salinity of the mixture and the mass of salt in it.

Try it first, then show the solutionSolution

Salt in first = 200 x 36/1,000 = 7.2 kg; in second = 300 x 32/1,000 = 9.6 kg.
Total salt = 16.8 kg; total mass = 500 kg.
Salinity = 16.8/500 x 1,000 = 33.6 o/oo.
Check: weighted average (7,200 + 9,600)/500 = 33.6.

Answer: 33.6 o/oo; 16.8 kg of salt.

Example 4: Bay of Bengal · 4 marks · Long answerHarder board level

Why is the Arabian Sea more saline than the Bay of Bengal?

Try it first, then show the solutionSolution

The Arabian Sea has high evaporation.
It receives few large rivers.
The Bay of Bengal receives huge volumes of river water from the Ganga and Brahmaputra.
So the Bay is diluted and less saline.

Answer: High evaporation and few rivers in the Arabian Sea; large river inflow dilutes the Bay.

Example 5: Distribution case · 5 marks · Case-basedHOTS / competency

Case: A sailor measures 41 o/oo in a hot narrow sea, 31 o/oo in a polar sea and 35 o/oo near the equator. (a) Identify the likely sea for 41. (b) Explain the polar reading. (c) Explain why the equatorial reading is lower than that of the subtropics.

Try it first, then show the solutionSolution

(a) The Red Sea (enclosed, hot, dry).
(b) Low evaporation and melting ice dilute the surface.
(c) Heavy daily rain, humidity and calm air at the equator reduce salinity compared with the subtropics.

Answer: (a) Red Sea (b) low evaporation and melt water (c) equatorial rain.

Where students lose marks in Composition and Salinity of Sea Water

The slips examiners see most often, and what to write instead.

Mistake: Using percentage instead of o/oo
Per hundred is used.
Wrong working
35 x 5,000/100
Right working
35 x 5,000/1,000 = 175 kg
Do this: Salinity is per thousand: divide by 1,000.
Usually costs: 2 marks.
Mistake: Saying the equator is the saltiest
Distribution misread.
Do this: Highest salinity is in the subtropics.
Usually costs: 1 mark.
Mistake: Forgetting enclosure
Seas treated alike.
Do this: Mention hot dry enclosed seas and cool river-fed seas.
Usually costs: 1 mark.

Exam tips

Quote salinity as o/oo.
Match each factor with a sea.
Show unit conversion in sums.
Include both horizontal and vertical distribution.

Practice questions on Composition and Salinity of Sea Water

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which sea has very low salinity because rivers pour in and evaporation is weak?
(a) Red Sea
(b) Persian Gulf
(c) Baltic Sea
(d) Mediterranean
Hint
Cool, river-fed.
Answer
(c) Baltic Sea.
Q2 · 3 marks · Short answerHarder board level
How much salt is present in 800 kg of sea water of 35 o/oo?
Hint
Per thousand.
Answer
800 x 35/1,000 = 28 kg.
Q3 · 3 marks · Short answerHarder board level
Explain why salinity is highest in the subtropics.
Hint
Clear skies, dry winds.
Answer
Under clear skies and dry trade winds evaporation is high and rainfall is low, so water loses vapour but keeps its salt.
Q4 · 4 marks · Long answerHarder board level
Describe the vertical distribution of salinity in high latitudes and in the subtropics.
Hint
Surface fresher or saltier.
Answer
In high latitudes the surface is diluted by melt water, so salinity increases with depth; in the subtropics the surface is very saline, so salinity decreases with depth; below 1,000 m it is nearly uniform.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A trader has 500 kg of sea water at 38 o/oo and wants to dilute it to 35 o/oo with fresh water. (a) Find the salt mass. (b) Find the total mass needed. (c) Find the fresh water to add.
Hint
Salt mass is fixed.
Answer
(a) Salt = 500 x 38/1,000 = 19 kg. (b) Total mass = 19/35 x 1,000 = 542.9 kg. (c) Fresh water = 542.9 - 500 = 42.9 kg.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Average salinity?Show answer
About 35 o/oo.
2.Most abundant salt?Show answer
Sodium chloride.
3.Salinity zone of rapid vertical change?Show answer
Halocline.
4.Highest salinity latitudes?Show answer
Subtropics.
5.Red Sea maximum?Show answer
Up to about 41 o/oo.

Composition and Salinity of Sea Water: frequently asked questions

Do proportions of salts change?

No, they remain nearly constant in the open ocean.

Why does freezing raise salinity?

Salt is left behind in the water.

Where is salinity over 200 o/oo?

In inland waters like the Dead Sea.