Central question:
What is really present in a glass of water?
The big idea:
Water is an unusually effective solvent for many ionic and polar substances. As it moves through air, soil, rock, pipes, and treatment systems, it can acquire dissolved gases, ions, and other substances. Perfectly pure H₂O is difficult to maintain outside carefully controlled conditions. Clear drinking water is therefore not chemically empty: it is overwhelmingly H₂O, with much smaller quantities of other dissolved materials.
Many dissolved substances are harmless or useful at ordinary concentrations. Others become concerning above particular concentrations, or under particular conditions. A positive test alone does not establish that water is unsafe; students must distinguish detection, concentration, and health significance.
