Which measurement refers to the concentration of a substance in water?

The concentration of a substance in water is often expressed in terms of parts per million (ppm). This unit of measurement represents how many units of a substance are present in one million units of water. This is particularly useful in the field of wastewater treatment and environmental science, where understanding the minute concentrations of contaminants or chemicals is crucial for assessing water quality.

When using parts per million, it facilitates a clearer comparison of concentrations across various substances, allowing for better regulatory compliance and environmental monitoring. For example, if a water sample has a concentration of 10 ppm of a certain pollutant, it means there are 10 milligrams of that pollutant in every one liter of water, or equivalently, in one million milliliters.

Other units, like milligrams or pounds per gallon, do not specifically denote concentration in the same universally understood manner. While they can describe quantities, they do not directly relate to a standardized volume of water like ppm does, which makes ppm the most appropriate choice for expressing concentration in this context. Liters measure volume rather than concentration and thus would not apply here.