What does the term "parts per million" (ppm) express?

The term "parts per million" (ppm) is a measurement that expresses the concentration of a substance within a solution or mixture. Specifically, it denotes very low concentrations, typically used when the amount of the substance is so minor that it is most conveniently described relative to a million units of the total mixture.

For instance, if we have one milligram of a substance in one liter (which is about a million milliliters) of water, this represents a concentration of one part per million. This method of measuring is particularly useful in contexts such as environmental science, where pollutants may exist in very small amounts and the effects need to be quantified.

The other options do not accurately reflect the meaning of ppm. High concentrations pertain to situations where a substance makes up a considerable part of the mixture, which is not what ppm signifies. Complete saturation refers to a state in which a solution cannot dissolve any more of a substance, and this term does not relate directly to ppm. The total weight of a solution generally considers all components rather than isolating a small fraction, which again is not characteristic of ppm. Thus, the designation of ppm highlights its utility in conveying low concentration levels effectively.