A cup of coffee can be weak or strong. That is concentration: the amount of solute dissolved in a given amount of solution. A concentrated solution has a lot of solute. A dilute solution has little. Concentration is usually expressed in molarity, moles of solute per liter of solution. It can also be expressed in mass percent, parts per million, or molality. The unit depends on the context.

Concentration matters because reactions depend on it. The rate of a reaction often increases with concentration because more particles collide. Equilibrium positions shift with concentration changes. Biological systems are sensitive to concentration. Enzymes work best at specific substrate concentrations. Cells maintain ion gradients across membranes. The human body regulates blood glucose, electrolyte levels, and oxygen concentration within narrow ranges.

Concentration is measured in several ways. A spectrophotometer measures how much light a solution absorbs, which relates to concentration. Titration uses a known concentration to determine an unknown one. Electrodes measure ion concentration directly. Each method has a range and a precision. Choosing the right method depends on the substance and the required accuracy.

Dilution is the process of lowering concentration by adding solvent. The dilution equation, M1V1 = M2V2, relates the initial and final concentrations and volumes. It is used in laboratories to prepare solutions of desired concentration from stock solutions.