Which statement about parts per million (ppm) is correct?

• A. ppm is used for solutions with relatively large concentrations of solute.
• B. ppm is a unit distinctly different from mg/L and has no relation.
• C. mg/L equals ppm in aqueous solutions.
• D. ppm is used only for solid samples.

Answer: (C)

Parts per million is a way to express very dilute concentrations. For solutions, especially water-based ones, the numeric values of ppm and mg/L line up closely because 1 liter of water weighs about 1,000,000 milligrams. That means 1 milligram of solute per liter of solution is roughly 1 part per million. So mg/L and ppm are effectively the same unit in aqueous solutions, which is why describing a solution as 5 mg/L is the same as saying 5 ppm.

Keep in mind this is most accurate for dilute aqueous solutions where the density is near that of water; for solutions with different densities or for gases, the exact relationship can vary. The other statements miss this relationship: ppm is not reserved for relatively large concentrations (that would be percent), and ppm is not a completely separate unit from mg/L in water-based solutions. It’s also not limited to solid samples; ppm is commonly used for liquids and gases as well.