Seawater is not a pure substance but a complex combination of various components. Understanding this involves examining the fundamental differences between pure substances and mixtures, clarifying the nature of ocean water and its composition.
Defining Pure Substances
A pure substance consists of only one type of matter with a uniform and constant composition. Every sample has identical properties and a fixed chemical makeup. Elements like gold or oxygen are examples, as they are made of one type of atom. Compounds, such as distilled water (H₂O) or table salt (sodium chloride), also qualify; they involve two or more elements chemically bonded in a fixed ratio, resulting in a new substance with distinct properties. Pure substances cannot be separated into simpler components by physical methods like filtration or evaporation.
Defining Mixtures
Mixtures are formed when two or more chemical substances combine without a chemical reaction. In a mixture, each original substance retains its individual chemical properties and identity. The components of a mixture are not chemically bonded and can be present in variable proportions. Mixtures are either homogeneous (solutions with uniform composition, like sugar dissolved in water) or heterogeneous (non-uniform, like sand mixed with water). Their constituents can be separated by physical methods.
Seawater: A Complex Mixture
Seawater is a complex mixture, not a pure substance, comprising numerous non-chemically bonded components; water accounts for approximately 96.5% of seawater, with the remaining 3.5% consisting of various dissolved materials. The most abundant dissolved substances are salts, primarily sodium chloride, which makes up about 85% of the dissolved minerals. Other major ions include chloride, sodium, sulfate, magnesium, calcium, and potassium, which together constitute over 99% of the total dissolved salts. Beyond salts, seawater contains dissolved atmospheric gases such as nitrogen, oxygen, and carbon dioxide, which are crucial for marine life. Trace elements like iron, zinc, and copper are also present in very small concentrations, alongside various dissolved inorganic and organic materials like carbohydrates and amino acids. The composition of seawater can vary slightly depending on geographical location, influenced by factors like freshwater input from rivers or evaporation rates. Because its components are not chemically bound and can be separated through physical processes like distillation or evaporation, seawater is definitively classified as a mixture.