When considering the purity of water, the question of its chemical classification often arises. Most water encountered daily is not chemically simple, leading to confusion about where distilled water fits into the categories of matter. Understanding the difference between pure substances and solutions provides the necessary framework to accurately classify water. This classification has practical implications for industrial, laboratory, and household uses.
Defining Pure Substances and Solutions
In chemistry, matter is broadly separated into pure substances and mixtures. A pure substance is defined as a form of matter that has a constant chemical composition and distinct properties throughout. It consists of only one type of element or one type of compound, such as gold or water (H₂O). Because the composition is fixed, a pure substance cannot be separated into other components by physical means.
A solution is a specific type of mixture that is physically blended but chemically homogeneous, meaning the components are uniformly distributed. All solutions consist of at least two components: a solute (the substance being dissolved) and a solvent (the medium doing the dissolving), such as salt dissolved in water. Unlike a pure substance, a solution’s composition is variable, and its components can be separated using physical processes like evaporation.
What Distilled Water Actually Is
Distilled water is produced through distillation, which involves heating water to convert it into steam and then cooling the steam to condense it back into a liquid state. This process exploits the difference in boiling points between water and most contaminants. As the water vaporizes, nearly all non-H₂O components, which have higher boiling points, are left behind.
The resulting condensed liquid is water that is virtually free of dissolved salts, minerals, organic materials, and non-volatile impurities. Due to this rigorous purification, the final composition of distilled water is overwhelmingly dominated by H₂O molecules. The process effectively removes more than 99.9% of dissolved solids, resulting in an extremely high-purity product.
The Classification of Distilled Water
Based on the chemical definitions, distilled water is classified as a pure substance, specifically a chemical compound. The goal of distillation is to isolate the compound water (H₂O) from all other dissolved or suspended materials. Since the product is almost entirely a single chemical entity with a fixed chemical formula, it meets the criteria of a pure substance.
Distilled water cannot be classified as a solution because it deliberately lacks a significant solute. Distillation removes the very things that would act as a solute, which a solution requires dissolved in a solvent. While no substance is ever 100% pure, the level of purity achieved means it functions chemically as a pure substance for almost all practical and scientific purposes.
Comparing Distilled Water to Common Water Types
The confusion over distilled water’s classification arises because most water people encounter is, in fact, a solution. Tap water, spring water, and mineral water all contain various dissolved minerals, salts, and gases, which act as solutes. For example, tap water contains trace minerals like calcium and magnesium, residual disinfectants like chlorine, and sometimes fluoride.
These common water types are solutions because they have dissolved solids uniformly distributed throughout the water solvent. The presence of these dissolved substances is why tap water and mineral water conduct electricity, as the solutes create mobile ions. Distilled water, lacking these dissolved ions, is a poor conductor of electricity, illustrating its purity compared to common water solutions.