Water spots are the minerals, not the water: when a drop evaporates, everything dissolved in it stays behind on the glass. Tap water carrying 50–300 ppm of calcium and magnesium leaves a white ring per drop. Purified water below 10 ppm has effectively nothing to leave, so it evaporates clean.
What a water spot actually is
Each spot is a mineral deposit — mostly calcium carbonate and magnesium salts — left at the edge of an evaporating droplet. The water leaves as vapour; the solids cannot. Harder water means denser deposits. Left long enough in sun, the deposits bond to the glass and need mechanical or chemical removal, not just another wash.
Why detergent makes it worse, not better
Soap solves a different problem: it lifts oils and dirt. It then becomes residue itself. A soap film dries into visible smears, and its sticky trace attracts fresh dust — which is why soaped windows look worse faster. The traditional fix is squeegeeing every drop off before it dries. The pure-water fix removes the need: nothing in the water, nothing on the glass.
The numbers that matter
| Water | Typical TDS | Drying result |
|---|---|---|
| SA tap water | 50–300 ppm | visible spotting |
| Rain water | 5–30 ppm | light marks possible |
| Distilled | ~0–5 ppm | spot-free, small volumes |
| RO water | 5–20 ppm | near spot-free |
| RO + DI water | 000 ppm | spot-free, any volume |
The full comparison across water types is in best water for cleaning windows; what the ppm number measures is in total dissolved solids (TDS).
Why pure water also cleans better on the way down
Deionised water is chemically hungry — stripped of ions, it dissolves mineral deposits and grabs dirt more aggressively than saturated tap water. The same purity that leaves nothing behind also lifts more on contact. That double effect is the entire basis of the water-fed pole method described in what does WFP stand for.
Making sub-10 ppm water
Confirm your starting point with a TDS meter (R260), then choose filtration in RO vs DI for window cleaning — or take a complete package from waterfed pole systems.