Seawater is the largest water body on Earth, covering over 70% of the planet’s surface. From a scientific perspective, seawater is definitively classified as a mixture. This classification is based on the physical state of its components and the nature of the bonds between them.
Defining Mixtures and Solutions
A mixture is a substance created by combining two or more different materials without causing a chemical reaction between them. In a mixture, each substance retains its unique chemical identity and properties. Because the components are not chemically bonded, they can be separated from one another through physical means.
Solutions are a specific type of mixture where one substance is completely dissolved into another at a molecular level, creating a uniform, homogeneous composition. Seawater fits this definition perfectly, as it consists of a solvent—the water—and various solutes that are uniformly dispersed throughout. The components of a solution, such as the dissolved salts in the ocean, cannot be seen separately, unlike the components in a heterogeneous mixture.
The Chemical Composition of Seawater
Seawater is primarily composed of water, making up approximately 96.5% of its mass. The remaining 3.5% consists of dissolved materials, largely inorganic salts and gases. These dissolved solids are mostly present as electrically charged atoms or molecules called ions.
The most abundant dissolved ions are chloride (Cl⁻) and sodium (Na⁺). Chloride accounts for about 55% of the total dissolved solids, while sodium makes up about 30.6%. Other major components include sulfate (SO₄²⁻), magnesium (Mg²⁺), calcium (Ca²⁺), and potassium (K⁺).
The Principle of Constant Proportions states that the ratio of the major dissolved ions remains constant throughout the global ocean. This is true even though the overall salt concentration, or salinity, can vary due to evaporation or freshwater input. Seawater also contains dissolved atmospheric gases, primarily nitrogen, oxygen, and carbon dioxide, which influence marine life and global climate cycles.
Physical Methods for Component Separation
The components of seawater can be easily separated using physical processes, confirming their non-chemically bonded nature. The simplest method for separating the water from the salt is evaporation, which leaves the solid salt crystals behind as the water turns into vapor. This process is used commercially to harvest sea salt.
More controlled methods, such as distillation, are employed when the goal is to recover the pure water. Distillation involves boiling the seawater and then capturing and condensing the steam back into liquid water, which separates it from the non-volatile dissolved salts. A modern technique called reverse osmosis achieves separation by forcing the seawater through a semi-permeable membrane at high pressure. This membrane allows the smaller water molecules to pass through while blocking the larger salt ions, effectively creating fresh water.