Impurities

Impurities are substances that are present in a material or compound which are not part of the desired chemical composition. They may be trace contaminants, degradation products, or deliberately added components that alter properties. Impurities are ubiquitous across chemistry, materials science, environmental science, and manufacturing, and their presence is described by terms such as purity, impurity profile, and contamination levels.

In chemical substances, impurities reduce purity and can influence reactivity, stability, color, and odor. They are quantified by methods such as chromatography, spectroscopy, or mass spectrometry, and their acceptable levels depend on the application. In metals and minerals, impurities are atoms other than the primary element that are incorporated into the crystal lattice or grain boundaries. They can improve or deteriorate properties: carbon in iron strengthens steel but excessive amounts cause brittleness; trace elements can affect corrosion resistance, electrical conductivity, and mechanical performance. In semiconductors, impurities are often introduced deliberately as dopants to control electrical conductivity; unintentional impurities can degrade device performance.

In water and environmental contexts, impurities include dissolved salts, organic compounds, microbes, and particulates. Regulations specify maximum allowable concentrations for safety and quality. Purification aims to reduce or remove impurities and may involve distillation, filtration, crystallization, precipitation, solvent extraction, or electrochemical methods. Higher purity is typically associated with higher cost and complexity, but is required for applications such as pharmaceuticals, optics, and electronics.