The ocean’s vastness can lead to the assumption that seawater is a simple, single substance. While water itself is a fundamental compound, the liquid filling the world’s oceans is far more complex than simple \(text{H}_2text{O}\). Understanding seawater requires classifying it based on the rules of chemistry, which dictate whether a material is a pure substance or a mixture.
What Defines a Pure Substance
In chemistry, matter is categorized into pure substances and mixtures based on composition. A pure substance is any form of matter that has a fixed, uniform chemical composition and distinct properties throughout a sample. These substances are either elements, made of one type of atom like gold (Au), or compounds, which contain two or more elements chemically bonded in a fixed ratio, such as distilled water (\(text{H}_2text{O}\)).
Pure substances possess characteristic properties, such as a precise melting or boiling point, that remain constant. Mixtures are physical combinations of two or more pure substances where each component retains its individual chemical identity. A mixture’s composition is variable, and its physical properties, like boiling point, are not fixed. Mixtures are classified as either heterogeneous, where components are visibly separate like sand and water, or homogeneous, where components are uniformly intermingled and appear as a single phase, often called a solution.
Seawater’s Classification as a Mixture
Seawater is classified as a homogeneous mixture, or a solution, rather than a pure substance. Although the solvent—the component present in the largest amount—is the pure compound \(text{H}_2text{O}\), the presence of other dissolved materials prevents the overall liquid from being classified as pure. The uniformity of the dissolved components, such as salts, means that any sample taken from the ocean fulfills the criteria for a homogeneous mixture.
Seawater contains numerous dissolved solids, ions, and gases, most notably sodium chloride (\(text{NaCl}\)), or common table salt. The measure of these dissolved solids is known as salinity, which averages about 35 parts per thousand. This means 35 grams of dissolved material are found in every 1,000 grams of seawater. This 3.5% non-water content, which includes ions like chloride, sodium, sulfate, and magnesium, confirms that the chemical formula of seawater is not fixed, which is the defining characteristic of a mixture.
Methods for Component Separation
The ability to separate the components of seawater using physical processes provides evidence that it is a mixture, as pure substances cannot be broken down this way. One method of separation is distillation, which utilizes the different boiling points of water and salt.
During distillation, seawater is heated, causing the water to evaporate into steam while the non-volatile salts remain as residue. The steam is collected and condensed back into pure, liquid \(text{H}_2text{O}\). Another modern method used in desalination plants is reverse osmosis, which forces seawater through a semi-permeable membrane. This membrane allows small water molecules to pass through while blocking the larger dissolved salt ions, separating the mixture into fresh water and concentrated brine.