Impurity is a term used in chemistry, materials science, and other fields to describe chemical substances that are present inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Impurities can be either naturally occurring or added during the synthesis of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance. Impurities can be destructive when they obstruct the working nature of the material. For example, ash and debris in metals and leaf pieces in blank white papers can be considered impurities. The removal of impurities is usually done chemically. For example, in the manufacturing of iron, calcium carbonate is added to the blast furnace to remove silicon dioxide from the iron ore. Zone refining is an economically important method for the purification of semiconductors. However, it is usually impossible to separate an impurity completely from a material due to the thermodynamic nature of the process.

In the Bible, impurity refers to the condition of being defiled in some sense, and it can also refer to the contaminant itself: an unwanted substance that makes something unclean. The concepts of purity and impurity are important in the Bible’s presentation of holiness. Under the rituals of the Old Testament Law, the Israelites were often confronted with the ideas of ritual or ceremonial purity and impurity. Many things could make an Israelite ritually unclean or impure, such as menstruation, childbirth, nocturnal emissions, touching a corpse, and certain types of skin diseases. Eating unclean food would also make one impure. Impurity could be ceremonially passed to others.

In general, impurity can refer to any substance that ruins the uncontaminated nature of something. When water is pure, the only thing in that water is water. Add a contaminant, say salt or bleach, and you are introducing an impurity, something that turns the water impure. Impurities can also refer to substances that are present in small quantities in another substance and make it dirty or of an unacceptable quality. For example, the air in a factory is filtered to remove impurities. In electronics, impurities can refer to a small quantity of an element added to a pure semiconductor crystal to control its electrical conductivity.