Impurity

Impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.

Impurities are either naturally occurring or added during synthesis of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance.

The levels of impurities in a material are generally defined in relative terms. Standards have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. Strictly speaking, then a material's level of purity can only be stated as being more or less pure than some other material.

Impurities can be destructive when they obstruct the working nature of the material. Examples include and debris in metals and leaf pieces in blank white papers. 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, some kinds of impurities can be removed by physical means. A mixture of water and salt can be separated by distillation, with water as the distillate and salt as the solid residue. Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids.

No matter what method is used, it is usually impossible to separate an impurity completely from a material. The reason that it is impossible to remove impurities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing the entropy of the system to zero. This would require an infinite amount of work and energy as predicted by the second law of thermodynamics. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible.

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