The classification of water as a pure substance or a mixture depends entirely on its chemical context and level of purity. Water is often called the “universal solvent” because of its ability to dissolve other materials, meaning its status changes based on what it is carrying. The water encountered daily is chemically different from the water studied in a laboratory. To understand water’s identity, one must first grasp the core principles scientists use to categorize all forms of matter.
Understanding Chemical Classification
All matter is broadly divided into two categories: pure substances and mixtures. A pure substance possesses a uniform and fixed composition, meaning its chemical and physical properties remain constant. Pure substances include elements (like gold) and compounds (like sodium chloride). A compound forms when two or more different elements are chemically bonded together in a specific, fixed ratio. In contrast, a mixture is a physical combination of substances where each component retains its chemical identity and can be separated using physical means.
When Water is a Pure Substance
Water (\(\text{H}_2\text{O}\)) is chemically a compound, and all compounds are defined as pure substances. It is composed of two hydrogen atoms chemically bonded to one oxygen atom, a fixed ratio that gives it consistent properties. Separating water into hydrogen and oxygen requires a chemical reaction, such as electrolysis, not a simple physical process. The only way to encounter water as a pure substance is to purify it extensively. Distilled water is created by boiling water and collecting the steam, which leaves behind non-volatile impurities. Deionized water is another highly pure form, produced by passing water through ion-exchange resins to remove electrically charged particles. Both forms are used in laboratories because they closely approach the theoretical purity of the \(\text{H}_2\text{O}\) compound.
When Water is Classified as a Mixture
The vast majority of water found on Earth, from the tap to the ocean, is classified as a mixture, specifically a solution. This is because water is capable of dissolving a wide range of solids, liquids, and gases. The presence of these dissolved materials means the water’s composition is no longer fixed, which immediately makes it a mixture.
Tap Water
Tap water is a common example of a homogeneous mixture containing various dissolved inorganic ions and solutes. Treatment facilities intentionally add chemicals like chlorine to kill microbes and fluoride for dental health. Tap water also contains naturally occurring minerals, such as calcium and magnesium, which contribute to water hardness and vary in concentration depending on the source.
Ocean Water
Ocean water represents an even more complex mixture. Seawater is primarily water and salts, along with dissolved gases and organic materials. The major dissolved components in seawater are six ions that account for about 99 percent of all sea salts:
- Chloride
- Sodium
- Sulfate
- Magnesium
- Calcium
- Potassium
The salt content can vary due to local evaporation or precipitation.