Salinity, Temperature and Density of Ocean Water: Composition, Factors and Distribution, ISC Class 11

What Salinity, Temperature and Density of Ocean Water is about

Weightage: CISCE gives no Class XI unit weightage for ISC Geography. Salinity, Temperature and Density of Ocean Water is topic 18 of the Class XI syllabus (The Realms of Water) and can be asked anywhere in the 70-mark Paper I (Theory, 3 hours). Class XI is examined by the school, so your school sets the paper and decides how many marks each topic carries.Source: CISCE ISC Geography (853) syllabus, examination year 2028, Class XI

Key concepts

Composition of sea water

Sea water is about 96.5 per cent water and 3.5 per cent dissolved substances, mainly salts. On average 1 kg of sea water contains about 35 g of salts.

Main salts (as a percentage of total dissolved salts, classic analysis)
1. Sodium chloride (common salt): about 77.8 per cent.
2. Magnesium chloride: about 10.9 per cent.
3. Magnesium sulphate: about 4.7 per cent.
4. Calcium sulphate: about 3.6 per cent.
5. Potassium sulphate: about 2.5 per cent.
6. Others (calcium carbonate, magnesium bromide and traces of many elements): the remainder.

In terms of ions, chloride and sodium together make up about 85 per cent of the dissolved material; sulphate, magnesium, calcium and potassium make up most of the rest.

Principle of constant proportions: although total salinity varies from place to place, the relative proportions of the main salts stay nearly the same everywhere in the open ocean, because the oceans are well mixed.

Sources of salts: weathering of rocks on land, carried by rivers; volcanic eruptions and hydrothermal vents on the sea floor (adding chlorine and other elements); and wind-blown dust. Sea water also contains dissolved gases (oxygen, carbon dioxide, nitrogen) vital for marine life.

Worked example

[4] Describe the composition of sea water and explain the principle of constant proportions. (Understanding)

Try it first, then show the solutionSolution

1. About 35 g salts per kg; mostly sodium chloride (~78 per cent) - 1 mark.
2. Magnesium chloride, magnesium sulphate, calcium and potassium sulphates - 1 mark.
3. Sources: rivers carrying weathered rock, volcanoes and vents - 1 mark.
4. Constant proportions: ratios of salts the same everywhere; total varies - 1 mark.

Answer: On average 1 kg of sea water contains about 35 g of dissolved salts, of which sodium chloride is about 78 per cent, followed by magnesium chloride, magnesium sulphate, calcium sulphate and potassium sulphate. The salts come from rocks weathered on land and carried by rivers, and from volcanoes and sea-floor vents. Total salinity varies, but the proportions of the main salts remain nearly constant everywhere.

Factors controlling salinity

Salinity rises when water is removed or salt is added, and falls when fresh water is added.

1. Evaporation: high evaporation (hot, dry, windy, clear skies) removes water and raises salinity, as in the subtropical oceans and the Red Sea.
2. Precipitation: heavy rain dilutes surface water and lowers salinity, as in the equatorial belt.
3. Fresh-water inflow from rivers: large rivers lower salinity near their mouths and in enclosed seas (the Bay of Bengal, fed by the Ganga and Brahmaputra; the Black Sea and Baltic Sea).
4. Freezing and melting of ice: when sea ice forms, salt is left behind, raising salinity of the water below; melting ice adds fresh water and lowers it (polar seas in summer).
5. Ocean currents and winds: warm currents can carry saline water into higher latitudes (the North Atlantic Drift keeps the North Sea relatively saline); winds move water and mix it.
6. Degree of enclosure and mixing: enclosed seas in hot, dry regions become very salty (Red Sea, Persian Gulf, Mediterranean), whereas enclosed seas in humid, cool regions with many rivers become fresh (Baltic).

Salinity, temperature and density are interrelated: warm water evaporates more (raising salinity), and both cold and salty water are denser.

Worked example

[4] Explain four factors that control the salinity of ocean water. (Understanding)

Try it first, then show the solutionSolution

1. Evaporation raises salinity (subtropics, Red Sea) - 1 mark.
2. Precipitation lowers salinity (equator) - 1 mark.
3. River inflow lowers salinity (Bay of Bengal, Baltic) - 1 mark.
4. Freezing raises, melting lowers; currents redistribute - 1 mark.

Answer: High evaporation in hot, dry areas removes water and raises salinity, as in the Red Sea; heavy rainfall dilutes the surface and lowers salinity near the equator; large rivers add fresh water and lower salinity, as in the Bay of Bengal and the Baltic Sea; and freezing of sea ice leaves salt behind, while melting adds fresh water. Ocean currents also carry saline or fresh water to other regions.

Distribution of salinity

Horizontal distribution
1. Open-ocean salinity generally ranges from about 33 to 37 o/oo, averaging about 35 o/oo.
2. Highest in the subtropics (about 20-30 degrees N and S), not at the equator: evaporation under clear skies and dry trade winds is high, and rainfall is low.
3. Slightly lower at the equator (about 35 o/oo) because of heavy daily rainfall, high humidity and calm air.
4. Lowest in polar seas, owing to low evaporation and melting ice.
5. Northern hemisphere averages are slightly higher than the southern because of large enclosed, saline seas.
6. Enclosed and partly enclosed seas: very high in hot, dry regions, for example the Red Sea (up to about 41 o/oo), the Persian Gulf and the Mediterranean; very low where rivers pour in and evaporation is weak, for example the Baltic Sea and the Black Sea.
7. In the Indian Ocean the Arabian Sea is more saline than the Bay of Bengal, because the Arabian Sea has higher evaporation and few large rivers, while the Bay receives huge volumes of river water.
8. Inland water bodies with no outlet can be extremely saline: Lake Van (Turkey), the Dead Sea and the Great Salt Lake (USA) have salinities of over 200 o/oo.

Vertical distribution
1. Salinity changes with depth, but the pattern depends on location.
2. In high latitudes, surface water diluted by melt water is fresher, so salinity increases with depth.
3. In the subtropics, the surface is very saline because of evaporation, so salinity decreases with depth.
4. A zone of rapid change of salinity with depth is called a halocline.
5. Below about 1,000 m, salinity is nearly uniform (about 34.5-35 o/oo).

Worked example

[4] Why is surface salinity higher in the subtropics than at the equator and the poles? (Analysis)

Try it first, then show the solutionSolution

1. Subtropics: clear skies, high temperature, dry trade winds - 1 mark.
2. High evaporation, low rainfall: salinity highest - 1 mark.
3. Equator: heavy daily rain, high humidity dilute surface water - 1 mark.
4. Poles: low evaporation, melting ice add fresh water - 1 mark.

Answer: In the subtropics, clear skies, high temperatures and dry trade winds cause strong evaporation while rainfall is low, so salinity is highest. At the equator, heavy daily rain and high humidity dilute the surface water, lowering salinity slightly. Near the poles, evaporation is weak and melting ice adds fresh water, so salinity is lowest.

Temperature of ocean water

Factors
1. Latitude: insolation decreases poleward, so surface temperature falls from the tropics towards the poles.
2. Unequal distribution of land and water: the northern hemisphere, with more land, receives more heat from adjoining continents, so its oceans are slightly warmer.
3. Prevailing winds: offshore winds push warm surface water away, allowing cold water to upwell (cooler coasts); onshore winds pile warm water against the coast.
4. Ocean currents: warm currents (Gulf Stream, North Atlantic Drift) raise temperature; cold currents (Labrador, Canary, Peru) lower it.
5. Enclosure: enclosed seas in low latitudes are warmer (Red Sea, Persian Gulf) than open oceans; those in high latitudes are colder.

Horizontal distribution
1. Average surface temperature is about 27 degrees C in the tropics and decreases poleward at roughly 0.5 degrees C per degree of latitude, to near 0 degrees C (or slightly below) near the poles.
2. The highest temperatures are found slightly north of the equator (the thermal equator).
3. The northern hemisphere oceans are warmer on average than the southern.

Vertical distribution
1. Maximum temperature is at the surface, which receives insolation directly; heat reaches deeper only by mixing.
2. In low and middle latitudes there are three layers: (a) a warm surface (mixed) layer, a few hundred metres thick, at about 20-25 degrees C; (b) the thermocline, a layer where temperature falls rapidly with depth; (c) a cold, deep layer extending to the floor, close to 0-4 degrees C, containing most of the ocean's water.
3. In polar regions the water is cold from top to bottom, so there is little change with depth (no thermocline).
4. Annual range of ocean temperature is small (water heats and cools slowly), which moderates coastal climates.

Worked example

[4] Describe the vertical distribution of temperature in tropical oceans. (Understanding)

Try it first, then show the solutionSolution

1. Maximum at the surface, heated by the sun - 1 mark.
2. Warm mixed layer, about 20-25 degrees C, a few hundred metres thick - 1 mark.
3. Thermocline: rapid fall in temperature with depth - 1 mark.
4. Cold deep layer near 0-4 degrees C to the floor; polar seas lack a thermocline - 1 mark.

Answer: In tropical oceans temperature is highest at the surface, which is heated by the sun and mixed by waves to form a warm layer a few hundred metres thick at about 20-25 degrees C. Below it lies the thermocline, where temperature falls rapidly with depth. Beneath this a cold, deep layer close to 0-4 degrees C extends to the floor. Polar seas are cold throughout and have no thermocline.

Density of ocean water

Density is mass per unit volume. Sea water is denser than fresh water because of its dissolved salts; surface sea water is usually about 1.025 to 1.028 g per cubic centimetre.

Factors
1. Temperature: colder water is denser (molecules are packed more closely). Unlike fresh water, which is densest at 4 degrees C, ordinary sea water keeps becoming denser as it cools down to its freezing point (about -1.9 degrees C).
2. Salinity: saltier water is denser.
3. Pressure: water at great depth is slightly compressed and so slightly denser.

Distribution
1. Horizontally, density is low in warm tropical surface waters and high in cold polar waters, especially where sea ice forms and leaves very cold, salty water behind.
2. Vertically, density increases with depth; a layer of rapid density increase (the pycnocline) usually coincides with the thermocline. This stable layering prevents surface and deep waters from mixing easily.

Importance
1. Density differences drive the thermohaline circulation: very cold, salty water sinks in the North Atlantic (near Greenland) and around Antarctica, spreading slowly along the ocean floor and returning to the surface elsewhere. This 'global conveyor belt' moves heat around the world and influences climate.
2. Density layering controls upwelling and the supply of nutrients to surface waters.
3. Ships float higher in dense, salty water than in fresh water; load lines (Plimsoll marks) on ships take this into account.

Worked example

[4] Explain the factors affecting the density of ocean water and its importance. (Understanding)

Try it first, then show the solutionSolution

1. Lower temperature increases density - 1 mark.
2. Higher salinity and greater pressure increase density - 1 mark.
3. Dense cold salty water sinks in polar regions - 1 mark.
4. Drives thermohaline circulation, moving heat and nutrients - 1 mark.

Answer: Ocean water becomes denser when it is colder, saltier or under greater pressure. Very cold, salty water formed in polar seas, especially where sea ice forms, therefore sinks and spreads along the ocean floor. These density differences drive the thermohaline circulation, a slow global conveyor that carries heat and nutrients around the oceans and influences climate.

Salinity, Temperature and Density of Ocean Water in real life

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

Topics in this chapter

• Composition and Salinity of Sea Water
• Ocean Temperature and Density

Salinity, Temperature and Density of Ocean Water: key facts

Salinity

$Salinity(o/oo)=gramsofsalt/gramsofseawaterx1,000$

Average about 35 o/oo.

Main salts

$NaCl77.8$

Constant proportions.

Mixing two water masses

$Salinityofmix=(S1xm1+S2xm2)/(m1+m2)$

Weighted average.

Temperature with latitude

About 0.5 degrees C fall per degree of latitude (surface)

From about 27 degrees C in the tropics.

Layers

Warm mixed layer -> thermocline -> cold deep layer

Halocline and pycnocline similar zones.

Density

$Colder+saltier+deeper=denser$

Drives thermohaline circulation.

How the school exam asks Salinity, Temperature and Density of Ocean Water

The question patterns to expect, and the method that works for each.

Tricks to remember Salinity, Temperature and Density of Ocean Water

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

• Salinity factors: 'EPRFC'

  Evaporation raises, Precipitation lowers, Rivers lower, Freezing raises (melting lowers), Currents move.

  Use it when: Factors of salinity.

• Salty seas

  Hot and enclosed = salty (Red Sea); cold and river-fed = fresh (Baltic).

  Use it when: Enclosed seas.

• Density: 'Cold, Salty, Sinks'

  Cold and salty water sinks to drive the deep conveyor.

  Use it when: Density and circulation.

Salinity, Temperature and Density of Ocean Water: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Salinity, Temperature and Density of Ocean Water

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

• ✗Mistake: Saying salinity is highest at the equator

  Assuming heat alone controls it.

  Wrong working

  Hottest water is saltiest, so the equator has maximum salinity.

  Right working

  Subtropics are saltiest; heavy rain lowers equatorial salinity.

  ✓Do this: Evaporation minus precipitation decides salinity.

  Usually costs: 1-2 marks.

• ✗Mistake: Salinity expressed in per cent

  Wrong unit.

  ✓Do this: Use parts per thousand (o/oo or ppt).

  Usually costs: 1 mark.

• ✗Mistake: Saying sea water is densest at 4 degrees C

  Fresh-water rule applied.

  ✓Do this: Sea water keeps getting denser down to its freezing point.

  Usually costs: 1 mark.

• ✗Mistake: No named seas

  Distribution needs examples.

  ✓Do this: Red Sea, Baltic, Black Sea, Mediterranean, Arabian Sea, Bay of Bengal.

  Usually costs: 1 mark.

• ✗Mistake: Mixing problems averaged without weighting

  Ignoring different masses.

  ✓Do this: Add the salt in each mass, then divide by total mass.

  Usually costs: 1-2 marks.

Practice questions on Salinity, Temperature and Density of Ocean Water

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

1. Q1 · 1 mark · Very short answerExam level

   Define salinity.

   Hint

   Grams per kg.

   Answer

   The total amount of dissolved salts in grams in 1,000 g of sea water, expressed in parts per thousand.

2. Q2 · 1 mark · Very short answerExam level

   What is the thermocline?

   Hint

   Rapid change.

   Answer

   The layer of ocean water in which temperature falls rapidly with depth.

3. Q3 · 1 mark · MCQExam level

   The most abundant salt in sea water is:
   (a) Magnesium chloride
   (b) Sodium chloride
   (c) Calcium sulphate
   (d) Potassium sulphate

   Hint

   Common salt.

   Answer

   (b) Sodium chloride.

4. Q4 · 1 mark · Very short answerHarder exam level

   Why does sea ice formation raise the salinity of the water below?

   Hint

   Salt left behind.

   Answer

   Ice forms mostly from fresh water, leaving its salt in the surrounding water.

5. Q5 · 4 marks · Short answerHarder exam level

   [4] Explain the vertical distribution of salinity in high and low latitudes.

   Hint

   Surface fresher or saltier.

   Answer

   (1) In high latitudes melt water and low evaporation make the surface fresher. (2) So salinity increases with depth there. (3) In the subtropics evaporation makes the surface very saline, so salinity decreases with depth. (4) A zone of rapid change is the halocline; below about 1,000 m salinity is nearly uniform.

6. Q6 · 4 marks · Short answerHarder exam level

   [4] Why are the oceans of the northern hemisphere warmer than those of the southern hemisphere?

   Hint

   Land.

   Answer

   (1) The northern hemisphere has much more land. (2) Land heats quickly and passes heat to adjoining seas. (3) Many enclosed seas in low latitudes there (Red Sea, Persian Gulf, Mediterranean) are very warm. (4) The southern hemisphere is mostly open ocean, bordered by the cold Southern Ocean around Antarctica.

7. Q7 · 4 marks · Short answerHarder exam level

   [4] Distinguish between the effects of temperature and salinity on the density of sea water.

   Hint

   Both increase density, opposite directions of change.

   Answer

   (1) Lowering temperature increases density as water contracts. (2) Raising salinity increases density by adding dissolved mass. (3) Warm, salty subtropical water can be less dense than cold, fresher polar water, since temperature often dominates. (4) The densest water forms where it is both cold and salty, as near Antarctica and Greenland.

8. Q8 · 4 marks · Short answerHOTS / competency

   [4] A sample of 2.5 kg of sea water leaves 92.5 g of salt on evaporation. Find its salinity. If 500 g of fresh water is added to the original sample, what is the new salinity?

   Hint

   Salt / water x 1,000.

   Answer

   (1) Salinity = 92.5 / 2,500 x 1,000 = 37 o/oo. (2) New mass = 2,500 + 500 = 3,000 g. (3) Salt unchanged = 92.5 g. (4) New salinity = 92.5 / 3,000 x 1,000 = about 30.8 o/oo.

9. Q9 · 4 marks · Short answerHOTS / competency

   [4] Explain why the highest ocean surface temperatures are found slightly north of the equator and not on it.

   Hint

   Land, thermal equator.

   Answer

   (1) The northern hemisphere has more land, which heats strongly and warms nearby seas. (2) The thermal equator therefore lies north of the geographical equator for most of the year. (3) Clouds and heavy rain at the equator cut insolation and cool the surface. (4) Warm enclosed seas north of the equator add to the effect.

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. 1.What is the average salinity of the oceans?Show answer

   About 35 parts per thousand.

2. 2.Which sea has very low salinity because of river inflow in northern Europe?Show answer

   The Baltic Sea.

3. 3.What is a halocline?Show answer

   A layer where salinity changes rapidly with depth.

4. 4.At about what temperature does ordinary sea water freeze?Show answer

   About -1.9 degrees C.

5. 5.Which is more saline: the Arabian Sea or the Bay of Bengal?Show answer

   The Arabian Sea.

6. 6.What is the thermohaline circulation?Show answer

   Deep ocean circulation driven by differences in temperature and salinity (density).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐35 g salt per kg; NaCl ~77.8%, MgCl2 ~10.9%, MgSO4, CaSO4, K2SO4; constant proportions.
• ☐Salinity up: evaporation, freezing; down: rain, rivers, melting; currents redistribute.
• ☐Highest in subtropics; lower at equator; lowest at poles; Red Sea high, Baltic and Black Sea low; Arabian Sea > Bay of Bengal.
• ☐Vertical: polar seas saltier with depth; subtropics fresher with depth; halocline.
• ☐Temperature: ~27 degrees C tropics, ~0.5 degrees C fall per degree latitude; warmest slightly north of equator.
• ☐Layers: warm mixed layer, thermocline, cold deep layer; polar seas uniform.
• ☐Density: colder, saltier, deeper = denser; pycnocline.
• ☐Cold salty water sinks (Greenland, Antarctica): thermohaline conveyor.

Salinity, Temperature and Density of Ocean Water: frequently asked questions