concentration

Concentration is a measure of how much of a substance is present in a defined amount of another substance. In chemistry, concentration quantifies the amount of solute dissolved in a solvent or present in a solution. It is typically expressed as a ratio or percentage. The most common units are molarity (moles of solute per liter of solution) and molality (moles per kilogram of solvent). Other descriptors include mass fraction, volume fraction, percentage by mass, and mole fraction.

Concentrations can be determined by various methods. Titration uses a reagent of known concentration to react with the solute until the reaction reaches completion. Spectroscopic methods infer concentration from absorbance or emission. Gravimetric methods depend on isolating and weighing the solute. Calibration against standards and the use of standard curves are also common approaches.

Concentration also refers to mental focus—the ability to direct attention and sustain it on a task or thought. In psychology and education, cognitive concentration is influenced by fatigue, arousal, motivation, distractions, and environmental conditions. Poor concentration can impair performance, while high concentration can enhance accuracy and learning.

In scientific and practical contexts, concentration is a fundamental variable that influences processes such as reaction rates, equilibria, and the efficacy of solutions. Describing a system by its concentration or concentration changes helps compare conditions, predict outcomes, and inform experiments and applications.