What is the correct sequence of the following salts in ascending order according to their presence in the sea water ? 1. Calcium Carbonate 2. Calcium Sulphate 3. Magnesium Chloride Select the answer using the code given below :

1. Ocean Salinity: Definition and Measurement (basic)

In simple terms, ocean salinity refers to the concentration of dissolved mineral salts in seawater. Imagine taking a kilogram of ocean water and evaporating it completely; the white crusty residue left behind is the 'salinity.' Formally, it is defined as the amount of salt (in grams) dissolved in 1,000 grams (1 kg) of seawater Fundamentals of Physical Geography, Geography Class XI (NCERT), Water (Oceans), p.104. This is expressed in parts per thousand (represented by the symbol ‰ or ppt). For example, the global average salinity of the oceans is approximately 35‰, meaning there are 35 grams of salt for every 1,000 grams of water GC Leong, Certificate Physical and Human Geography, The Oceans, p.107. While we often think of 'salt' as just table salt (Sodium Chloride), seawater is actually a complex chemical cocktail. Sodium Chloride (NaCl) is indeed the most dominant, making up about 77.7% of the total dissolved solids, followed by Magnesium Chloride (MgCl₂) at roughly 10.9% Physical Geography by PMF IAS, Ocean temperature and salinity, p.518. Other minerals like Magnesium Sulphate, Calcium Sulphate, and Calcium Carbonate are present in much smaller quantities. Oceanographers use a specific threshold of 24.7‰ as the upper limit to define 'brackish water'—water that is saltier than fresh water but less salty than true seawater Fundamentals of Physical Geography, Geography Class XI (NCERT), Water (Oceans), p.104. Salinity is not just a chemical fact; it is a physical driver of ocean dynamics. It influences the density of water (saltier water is heavier and sinks), its freezing point (salt water freezes at a lower temperature than fresh water), and even its evaporation rate. On maps, we represent these variations using isohalines—lines that connect points in the ocean having equal salinity GC Leong, Certificate Physical and Human Geography, The Oceans, p.107.

Sources: Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104; Certificate Physical and Human Geography, GC Leong (Oxford University press 3rd ed.), The Oceans, p.107; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Ocean temperature and salinity, p.518

2. Factors Controlling Ocean Salinity (basic)

To understand ocean salinity, think of the ocean as a giant soup. The 'saltiness' or salinity depends on how much salt is dissolved in a specific amount of water, usually measured in parts per thousand (‰). While the average salinity is about 35‰, it varies significantly across the globe because of a constant tug-of-war between processes that add fresh water (dilution) and processes that remove fresh water (concentration).

The primary drivers are Evaporation and Precipitation. In areas like the tropical trade wind belts (20°–30° N and S), high temperatures and dry air cause intense evaporation, leaving the salts behind and creating high-salinity zones GC Leong, Certificate Physical and Human Geography, p.107. Conversely, near the Equator, even though it is hot, heavy daily rainfall (precipitation) dilutes the water, keeping salinity lower than in the subtropics.

Another major factor is the influx of fresh water from rivers and melting ice. Massive rivers like the Amazon, Congo, and Ganges dump enormous volumes of fresh water into the sea, significantly lowering the salinity of coastal regions like the Bay of Bengal NCERT Class XI, Fundamentals of Physical Geography, p.105. In polar regions, the freezing and thawing of ice plays a dual role: when seawater freezes, salt is excluded (increasing the salinity of the remaining water), but when icebergs melt, they release fresh water (decreasing salinity). Finally, ocean currents act as a global mixing system; for example, the North Atlantic Drift carries saltier water into the higher latitudes of the North Sea, keeping it more saline than it would otherwise be PMF IAS, Physical Geography, p.518.

Process	Effect on Salinity	Example Region
High Evaporation	Increases Salinity	Mediterranean Sea, Red Sea
Heavy Precipitation	Decreases Salinity	Equatorial Belt
River Influx	Decreases Salinity	Bay of Bengal, Baltic Sea
Ice Melting	Decreases Salinity	Polar Regions (Summer)

Sources: Certificate Physical and Human Geography, GC Leong, The Oceans, p.107; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104-105; Physical Geography by PMF IAS, Ocean temperature and salinity, p.518-519

3. Spatial and Vertical Distribution of Salinity (intermediate)

Spatial (Horizontal) Distribution of salinity is primarily governed by the balance between evaporation and precipitation. While you might expect the Equator to be the saltiest due to intense heat, it actually records lower salinity (around 34.5‰) because of heavy daily rainfall and high humidity which limits evaporation. The highest surface salinity is typically found in the sub-tropical high-pressure belts (20°–30° N and S), where clear skies and high temperatures drive intense evaporation with very little rainfall. In polar regions, salinity drops significantly due to the melting of ice, which adds fresh water to the system FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104.

Regional variations are further influenced by landmasses and ocean currents. For instance, the North Sea maintains a surprisingly high salinity for its latitude because the North Atlantic Drift brings warmer, saltier water from the tropics FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.105. Conversely, enclosed seas like the Baltic Sea or Black Sea have very low salinity due to massive freshwater influx from rivers. On the extreme end of the spectrum, landlocked bodies like Lake Van in Turkey (330‰) and the Dead Sea (238‰) reach incredible salinity levels because they lack outlets and experience extreme evaporation Physical Geography by PMF IAS, Ocean temperature and salinity, p.520.

Region/Water Body	Salinity Level	Primary Reason
Equatorial Region	Lower (~34.5‰)	Heavy rainfall (dilution)
Tropical Region	Highest (~37‰)	High evaporation, low rainfall
Bay of Bengal	Low	Large river influx (Ganga, Brahmaputra)
Lake Van (Turkey)	Extreme (330‰)	Enclosed basin, zero outlet

Vertical Distribution refers to how salinity changes with depth. In the surface zone, salinity is highly variable due to weather, evaporation, and river discharge. However, as we go deeper, we encounter the Halocline—a distinct layer where salinity changes sharply with increasing depth. Because saltier water is denser, it tends to sink below fresher water. In the deep ocean, salinity becomes relatively constant, as it is insulated from the surface processes of evaporation and rain.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104-105; Physical Geography by PMF IAS, Ocean temperature and salinity, p.520

4. Ocean-Atmosphere Interaction and Density (intermediate)

At the heart of oceanography lies a dynamic 'handshake' between the atmosphere and the sea. This interaction is primarily governed by density, which acts as the steering wheel for ocean circulation. Ocean water density is determined by two main factors: temperature and salinity. As a rule of thumb, water becomes denser when it is either colder or saltier. When surface water becomes dense enough, it sinks, initiating a vertical movement that drives the global 'conveyor belt' of ocean currents, often referred to as thermohaline circulation Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.487.

The atmosphere influences this density through three primary mechanisms at the surface layer:

• Evaporation and Precipitation: Evaporation removes freshwater and leaves salts behind, increasing salinity and density (common in the subtropics). Conversely, heavy precipitation or freshwater inflow from rivers dilutes the salt content, decreasing density NCERT Class XI Fundamentals of Physical Geography, Water (Oceans), p.104.
• Temperature Exchange: The sun heats the surface water, causing it to expand and become less dense. In polar regions, the loss of heat to the cold atmosphere makes the water contract and become denser.
• Wind and Ice: Winds move surface waters, causing upwelling (bringing deep, cold water to the surface) or downwelling. Additionally, when sea ice forms, it excludes salt (brine rejection), making the surrounding water incredibly salty and dense Physical Geography by PMF IAS, Ocean temperature and salinity, p.518.

The interplay of these factors creates a complex map of ocean water. For instance, even if a region is very warm, high evaporation can make the water so salty that it becomes dense enough to sink. This delicate balance ensures that the ocean is not a stagnant pool but a moving system that redistributes heat across the planet.

Process	Effect on Salinity	Effect on Density
Heavy Precipitation	Decreases	Decreases (Lighter)
High Evaporation	Increases	Increases (Heavier)
Sea Ice Melting	Decreases	Decreases (Lighter)
Sea Ice Formation	Increases	Increases (Heavier)

Sources: Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.487; NCERT Class XI Fundamentals of Physical Geography, Water (Oceans), p.104; Physical Geography by PMF IAS, Ocean temperature and salinity, p.518

5. Marine Biogeochemical Cycles and Carbonates (intermediate)

The ocean is a massive chemical engine where the Marine Biogeochemical Cycle governs the movement of elements like carbon between the atmosphere, marine life, and the deep sea. At the heart of this process is the Biological Pump: phytoplankton in the surface waters absorb CO₂ through photosynthesis, converting it into organic matter. While most of this carbon is recycled near the surface when organisms decompose, a small but significant portion sinks to the deep ocean, effectively "locking" carbon away for centuries Environment, Shankar IAS Academy, Marine Organisms, p.208.

Beyond organic matter, carbon is stored as Calcium Carbonate (CaCO₃). Marine organisms like corals, molluscs, and certain plankton use carbonate ions to build their shells and skeletons in a process called calcification. However, this is highly sensitive to the ocean's chemistry. When atmospheric CO₂ increases, it dissolves in seawater to form carbonic acid, which then releases hydrogen ions. These ions react with carbonate ions to form bicarbonate. This chemical shift reduces the availability of "free" carbonate ions—the essential "bricks" organisms need for building shells Environment, Shankar IAS Academy, Ocean Acidification, p.264. Interestingly, not all CaCO₃ is the same. Marine life uses two primary forms:

Form of CaCO₃	Solubility	Commonly Found In
Calcite	Less soluble (more stable)	Planktonic algae, oysters, echinoderms
Aragonite	More soluble (vulnerable)	Most corals, pteropods (sea snails)

As these carbonate shells sink, they encounter two critical depth boundaries. The first is the Lysocline, the depth at which the rate of dissolution starts to increase significantly. Further down lies the Carbonate Compensation Depth (CCD). Below the CCD, the water is so undersaturated with carbonate that shells dissolve faster than they can accumulate. Consequently, no carbonate sediments are found on the very deep sea floor Environment, Shankar IAS Academy, Ocean Acidification, p.265. Today, rising CO₂ levels are making the oceans more acidic, causing the CCD to migrate upward and threatening the stability of marine ecosystems.

Sources: Environment, Shankar IAS Academy, Marine Organisms, p.208; Environment, Shankar IAS Academy, Ocean Acidification, p.263-265

6. Chemical Composition of Seawater: The Major Salts (exam-level)

To understand the oceans, we must look beyond the surface and into the 'chemical soup' that makes up seawater. Salinity is the term used to define the total content of dissolved salts in seawater. It is technically calculated as the amount of salt (in grams) dissolved in 1,000 grams (1 kg) of seawater, expressed as parts per thousand (ppt or ‰) NCERT Class XI Fundamentals of Physical Geography, Water (Oceans), p.104. While the average salinity of the oceans is approximately 35 ppt, this varies by location—from a low of 7 ppt in the Baltic Sea to as high as 39 ppt in the Red Sea due to evaporation and freshwater influx GC Leong, The Oceans, p.107.

While we often associate seawater primarily with common table salt (Sodium Chloride), it actually contains a variety of dissolved mineral salts. What is fascinating is that regardless of the total salinity, the relative proportions of these major salts remain remarkably constant throughout the global ocean. Sodium Chloride (NaCl) is the undisputed heavyweight, making up nearly 77.7% of the total dissolved solids. It is followed by Magnesium Chloride (MgCl₂), which contributes significantly to the bitter taste of seawater PMF IAS, Ocean temperature and salinity, p.518.

Understanding the hierarchy of these salts is crucial for exam preparation. After the chlorides of Sodium and Magnesium, we find sulphates. Here is the breakdown of the primary constituents:

Rank	Salt	Percentage Share (%)
1	Sodium Chloride (NaCl)	77.7%
2	Magnesium Chloride (MgCl₂)	10.9%
3	Magnesium Sulphate (MgSO₄)	4.7%
4	Calcium Sulphate (CaSO₄)	3.6%
5	Potassium Sulphate (K₂SO₄)	2.5%

Interestingly, Calcium Carbonate (CaCO₃) and Magnesium Carbonate are present in much smaller quantities compared to the salts listed above. This is partly because marine organisms constantly extract calcium carbonate from the water to build their shells and skeletons.

Sources: NCERT Class XI Fundamentals of Physical Geography, Water (Oceans), p.104; GC Leong Certificate Physical and Human Geography, The Oceans, p.107; Physical Geography by PMF IAS, Ocean temperature and salinity, p.518

7. Specific Ranking: Carbonates vs Sulphates vs Chlorides (exam-level)

To master the chemistry of the ocean, we must look beyond just "common salt." While seawater is famously salty, that salinity is actually a cocktail of various dissolved ions. The relative concentration of these salts follows a remarkably consistent hierarchy. Even though total salinity varies across different regions—for instance, the Arabian Sea is saltier than the Bay of Bengal due to higher evaporation and lower freshwater influx—the ratio of the major salts remains nearly constant Physical Geography by PMF IAS, Ocean temperature and salinity, p. 519.

The hierarchy of abundance is dominated by Chlorides, followed by Sulphates, and finally Carbonates. At the top, Sodium Chloride (NaCl) is the undisputed leader, accounting for approximately 77.7% of all dissolved solids. It is followed by Magnesium Chloride (MgCl₂), which adds another 10.9% to the mix. Together, these chlorides make up nearly 90% of the ocean's salt content Physical Geography by PMF IAS, Ocean temperature and salinity, p. 518.

Next in the sequence are the Sulphates. These include Magnesium Sulphate (4.7%), Calcium Sulphate (3.6%), and Potassium Sulphate (2.5%). While significant, their combined presence is still much lower than that of the Chlorides. Finally, we find the Carbonates (such as Calcium Carbonate, CaCO₃). You might find it surprising that Carbonates are at the bottom of the list, given how vital they are for marine life. However, because marine organisms like corals, oysters, and planktonic algae constantly extract dissolved carbonate to build their shells and skeletons (as calcite or aragonite), the concentration of dissolved carbonates in the water remains very low Environment, Shankar IAS Academy, Ocean Acidification, p. 263.

Salt Group	Primary Examples	Relative Abundance
Chlorides	Sodium Chloride, Magnesium Chloride	Highest (~88%+)
Sulphates	Magnesium Sulphate, Calcium Sulphate	Moderate (~10-11%)
Carbonates	Calcium Carbonate	Lowest (Traces)

Sources: Physical Geography by PMF IAS, Ocean temperature and salinity, p.518-519; Environment, Shankar IAS Academy, Ocean Acidification, p.263

8. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of ocean composition, this question tests your ability to apply the hierarchy of dissolved salts in seawater. You've learned that seawater isn't just "salty water" but a complex solution where Sodium Chloride (NaCl) reigns supreme, followed by Magnesium salts. In this specific scenario, you must rank three specific compounds: Calcium Carbonate, Calcium Sulphate, and Magnesium Chloride. The key is to remember that while Calcium is vital for marine life, it exists in much smaller concentrations as a salt compared to the dominant Magnesium and Chloride ions.

To arrive at the correct answer, follow this logical progression: First, identify the most abundant salt in the list. Since Chlorides are the most prevalent anions after Sodium, Magnesium Chloride (3) takes the top spot at approximately 10.9%. Next, consider the Sulphates; Calcium Sulphate (2) is a significant secondary salt, making up about 3.6% of the total salinity. Finally, Calcium Carbonate (1) is present in trace amounts because it is constantly being utilized by marine organisms to build shells and skeletons. Therefore, the ascending order (from smallest to largest) is 1, 2, and then 3. This leads us directly to Option (C).

A common trap in UPSC questions is the confusion between ascending and descending orders. Students often identify the most abundant salt first and instinctively look for an option that starts with it, which would lead to a descending sequence. Another pitfall is overestimating Calcium Carbonate due to its biological importance; however, as noted in Physical Geography by PMF IAS, its concentration as a dissolved salt remains the lowest among these three. Always ensure you differentiate between the abundance of ions and the biological utility of the salts to avoid these errors.