Reading path: Ocean depth and big questions →
Questions to keep separate
| Question or feature | Explanation | What it does not establish |
|---|---|---|
| Average salinity | About 35 g of dissolved salts per kg of seawater. [2] | An approximate average; estuaries and evaporative seas can differ. [1] |
| Salinity ingredients | Includes all dissolved salts; sodium and chloride are major ions. [2] [1] | A table-salt mixture does not duplicate seawater chemistry. [2] |
| Surface changes | Evaporation, precipitation, runoff and freezing or melting alter surface salinity. [3] | A value is tied to conditions and location; it is not uniform everywhere. |
| Modern measurement | Practical salinity is inferred from conductivity with temperature and pressure information. [3] | Conductivity is not a direct mass measurement of each dissolved substance. |
Where dissolved salts begin
Rainwater and flowing water slowly weather exposed rocks, carrying dissolved ions into streams and rivers. Rivers then deliver some of that material to the ocean. The seafloor adds another source: hydrothermal fluids interact with ocean crust and contribute chemicals to seawater. So ocean salt has more than one route in; it is not only washed off beaches or made by evaporation. [1]
The sea also loses dissolved material through biological and geological processes. Salinity reflects this ongoing exchange of inputs and removals, rather than a one-way process in which salt simply accumulates forever. [1]
Salinity is more than table salt
Salinity describes the dissolved salts in water as a whole. Sodium and chloride are major seawater ions, but seawater contains other dissolved substances too. A kitchen-salt demonstration can help show how dissolved material changes a mixture, but it does not reproduce seawater’s full chemistry. [2]
A useful approximate average is 35 grams of dissolved salts per kilogram of seawater. The denominator matters: this is a mass comparison, not 35 grams per litre. For a clearly simplified family model, combine 35 g of table salt with 965 g of fresh water. The resulting 1,000 g mixture has a 35-parts-per-thousand salt mass fraction. It illustrates a proportion; it is not real seawater. [2]
Why salinity changes from place to place
Salinity near the surface changes as water enters or leaves. Evaporation removes water and leaves dissolved salts behind, tending to concentrate them. Rain and river runoff add fresh water and tend to dilute them. When seawater freezes, salt is largely excluded from the forming ice, raising salinity in nearby water; melting ice adds fresher water and can lower it. [3]
That is why 35 g/kg is an approximate ocean average, not a value every family should expect at every coast. An estuary receiving river water can differ from an evaporative sea. A reading also describes a particular place and time: surface salinity can vary with weather, runoff and freezing or melting. [1] [3]
How scientists measure the saltiness
Scientists can define absolute salinity as a mass of dissolved salts relative to a mass of seawater. Modern instruments commonly infer practical salinity from electrical conductivity, using temperature and pressure information in the calculation. Conductivity is useful because dissolved ions carry electrical current, but it is not a direct weighing of all the salts in a sample. [3]
For a home demonstration, add water to the simplified salt mixture and observe that it becomes less concentrated; let water evaporate and the remaining mixture becomes more concentrated. Keep this as a model of dilution and concentration. Do not treat the model as a seawater sample, assume a litre equals a kilogram, or taste collected coastal water. [2]
Sources & further reading
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