A litre of water weighs a kilogram — a thousand grams, a million milligrams. So one milligram dissolved in one litre is one part in a million: $1\ \mathrm{mg/L} = 1\ \mathrm{ppm}$ — read aloud: one milligram per litre IS one part per million. That is not a coincidence to memorise; it is bookkeeping that water does for you. Water reports, aquarium kits and greenhouse feed charts all speak this dialect, and they are all saying mg/L.

This chapter also reads instruments. A conductivity meter reports in µS/cm — say micro-siemens per centimetre — and that is NOT a concentration: it measures how easily a current crosses the water, and turning it into dissolved salt is a later lesson's business. First job of a water chemist: look at a report line and know WHAT kind of number it is. The unit does the telling, every time.