Ocean Water Chemistry, Salinity & Temperature: Sixth Grade Science
Ocean water is a complex chemical solution of dissolved mineral salts and gases whose temperature, salinity, and density structures drive global thermohaline circulation.
The Chemical Composition of Seawater
Ocean water is not pure H2O; it is a saline chemical solution containing an astonishing array of dissolved elements, minerals, organic molecules, and atmospheric gases. If you evaporated a 1,000-gram (1-kilogram) sample of typical seawater to dryness, you would be left with approximately 965 grams of pure liquid water and 35 grams of solid white mineral salts. Geochemists express this ratio as salinity: the total mass of dissolved mineral salts contained in a given mass of water.
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| SALINITY AND IONIC COMPOSITION |
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| Average Global Ocean Salinity = 3.5% = 35 parts per thousand (35 ppt) |
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| DISSOLVED SALTS BREAKDOWN: |
| - Chloride Ions (Cl-): 55.0% (Choking halogen in table salt) |
| - Sodium Ions (Na+): 30.6% (Alkali metal in table salt) |
| - Sulfate Ions (SO4 2-): 7.7% |
| - Magnesium Ions (Mg 2+): 3.7% |
| - Calcium Ions (Ca 2+): 1.2% (Crucial for coral and clam shells) |
| - Potassium Ions (K+): 1.1% |
| - All Other Trace Ions: 0.7% (Gold, iron, iodine, uranium) |
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The Origins of Ocean Salts
Where did the ocean get its salt? The saltiness of the sea is the result of billions of years of continuous geological processes:
- Chemical Weathering of Continents: Rainwater containing weak carbonic acid slowly weathers continental rocks, dissolving minerals into sodium, calcium, magnesium, and potassium ions. Rivers carry these dissolved ions across continents and deposit them into ocean basins.
- Undersea Volcanism and Hydrothermal Vents: Molten basalt eruptions and deep-sea hydrothermal vents spew massive volumes of dissolved chloride, sulfate, and hydrogen sulfide gases directly into seawater from Earth's mantle.
- Continuous Evaporation: While rivers deliver trace salts, evaporation removes only pure freshwater vapor, continually concentrating the dissolved salts in the oceans over billions of years.
The Principle of Constant Proportions
In 1884, chemist William Dittmar analyzed 77 seawater samples collected across the globe by the HMS Challenger oceanographic expedition. Dittmar discovered the Principle of Constant Proportions (Forchhammer's Principle): although total salinity varies from place to place based on evaporation and rain, the relative percentage ratio of major dissolved ions to one another remains strictly constant across all world oceans! Because the oceans have been thoroughly mixed by global currents over millions of years, measuring the concentration of just one major ion (typically chloride) allows oceanographers to calculate total salinity instantly.
Factors Influencing Regional Ocean Salinity
While the average ocean salinity is 35 parts per thousand (ppt), regional surface salinity fluctuates between 32 and 38 ppt based on the balance between processes that remove freshwater and processes that add freshwater:
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| FACTORS GOVERNING OCEAN SALINITY |
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| PROCESSES THAT INCREASE SALINITY (Remove Pure Water): |
| 1. High Evaporation Rates: Hot, dry tropical trade-wind belts |
| (e.g., Red Sea reaches 40 ppt; Mediterranean Sea reaches 38 ppt). |
| 2. Freezing of Sea Ice: When polar ocean water freezes, only pure |
| water molecules form ice, expelling salt into the liquid water |
| beneath in a process called BRINE REJECTION. |
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| PROCESSES THAT DECREASE SALINITY (Add Pure Water): |
| 1. Heavy Precipitation: Equatorial rain belts dilute surface sea. |
| 2. River Inflow: Major river mouths (Amazon, Mississippi) discharge |
| colossal volumes of freshwater into coastal waters. |
| 3. Glacial Melting: Melting icebergs and ice caps dump pure water. |
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Thermal and Density Structure of the Oceans
Ocean water is vertically structured into three distinct layers based on temperature and density:
- Surface Mixed Zone (Epipelagic): The warm, sunlit top layer (0 to 200 meters), mixed thoroughly by winds and surface waves. Holds roughly 2 percent of total ocean volume.
- The Thermocline: The transition zone (200 to 1,000 meters) where water temperature drops rapidly with depth—from roughly 20 degrees Celsius at the surface to near 4 degrees Celsius at the base. Concurrently, density increases sharply across this boundary (the pycnocline).
- The Deep Cold Zone: The vast, dark, freezing abyss (1,000 meters to ocean floor), containing roughly 80 percent of all ocean water. Temperatures remain uniformly freezing, hovering between 1 and 3 degrees Celsius year-round.
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| VERTICAL OCEAN THERMOCLINE |
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| Depth (m) |
| 0 +--------------------+ Warm Surface Zone (20 °C) |
| | | |
| 200 +--------------------+ <--- Start of THERMOCLINE |
| | \ | |
| | \ | (Temperature drops plunging downward) |
| | \ | |
| 1,000 +-------\------------+ <--- Base of Thermocline |
| | | | |
| | | | Cold Deep Zone (Uniform 2 °C to 4 °C) |
| 4,000 +--------+-----------+ |
| 0 10 20 Temperature (°C) |
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Exercises and Solutions
Exercise 1: Calculating Ocean Salt Mass in Municipal Aquariums. A municipal saltwater aquarium builds an artificial Indo-Pacific coral reef display tank holding 500,000 liters of water. To simulate natural seawater, marine biologists must add synthetic sea salt until the salinity reaches standard 35 parts per thousand (3.5% by mass). Assuming 1 liter of freshwater weighs exactly 1 kilogram (1,000 grams), calculate how many kilograms of salt crystals must be dissolved into the tank.
Solution 1: 500,000 liters of water has a mass of 500,000 kilograms. Salinity is 35 parts per thousand, which means 35 kilograms of salt per 1,000 kilograms of water (3.5%). Multiply total water mass by the salinity fraction: 500,000 kg x 0.035 = 17,500 kilograms (17.5 metric tons) of sea salt required to achieve natural oceanic salinity.
Exercise 2: Evaluating the Physical Mechanics of Deep-Water Formation via Brine Rejection. In the freezing waters off the coast of Antarctica, ocean temperatures drop to -1.9 degrees Celsius, and surface water begins freezing into sea ice. Explain the physical process of brine rejection and describe how it creates the densest water mass on planet Earth (Antarctic Bottom Water).
Solution 2: Water molecules (H2O) freeze into a rigid, highly organized hexagonal crystal lattice. Dissolved sodium and chloride ions cannot physically fit inside this crystalline ice matrix; therefore, as seawater freezes, the salt is systematically expelled from the newly forming ice into the remaining unfrozen liquid water directly beneath the ice pack—a process known as brine rejection. This leaves the underlying liquid water super-cold (near -1.9 degrees Celsius) and hyper-saline (salinity exceeding 37 ppt). Because cold water is naturally denser than warm water, and saline water is denser than freshwater, this combination creates Antarctic Bottom Water—the coldest, saltiest, and densest water mass on planet Earth. Under the pull of gravity, this ultra-dense water plunges off the Antarctic continental shelf down into the abyssal ocean trenches, flowing along the ocean floor across the globe as the engine of deep thermohaline circulation.