Impurities are substances that are present in 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. Examples include ash 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. It is usually impossible to separate an impurity completely from a material due to the thermodynamic nature of the process, which is predicted by the second law of thermodynamics.
