A test strip is a plastic stick with small pads. Each pad holds a chemical that changes colour depending on how much of one substance is in the water. You dip the strip, wait the stated time, and match each pad to a colour chart. Real drinking-water labs use more precise instruments, but strips are the idea in miniature, and aquarium keepers use them every week.

pH tells you how acidic or basic water is. The scale runs from 0 to 14, with 7 neutral. It is a logarithmic scale: each step is a factor of 10 in the amount of hydrogen ions. Water at pH 6 has 10 times more hydrogen ions than water at pH 7, and pH 5 has 100 times more. (The Chemistry Lab explains why.) For drinking water the WHO gives no health-based limit for pH, but suppliers aim for roughly 6.5 to 8.5, because water outside that range corrodes pipes (and so can pick up metals) or leaves deposits, and tastes odd.