How do I get hard water spots off a bathroom mirror without wrecking it?

Why the usual answer works

Hard water spots are mineral deposits left behind when droplets evaporate on the glass, and an acid dissolves them. That is why every descaler and most glass sprays are acidic, and why vinegar is the household answer nearly everyone reaches for. On a plain pane of glass the reasoning is sound: soda-lime glass is not attacked by acetic acid at domestic strength.

Usually recommended: Acetic acid (vinegar), Citric acid.

Why it must not be used here

A mirror is not a pane of glass. It is glass with a reflective coating and a protective backing, and both are exposed at every edge. The glass industry's mirror division states that acid or alkali cleaners can attack the front surface, the edges and the backing, and names cleaners containing ammonia or vinegar specifically as ones not to use. The failure is called black edge, it starts where liquid runs into the perimeter, and it is degradation of the mirror rather than dirt on it.

This damage cannot be undone by cleaning. There is nothing left on the surface to remove.

More about mirrored glass and limescale.

What to do instead

Change the mechanism from dissolving to drying. The mirror division's own recommendation is clean warm water on a soft lint-free cloth, wrung out, with the mirror dried immediately afterwards — which works because the deposit is created at the moment of evaporation rather than while the water sits. Apply any liquid to the cloth and never spray it at the mirror, so nothing can run down into the edges, and finish by checking that the joints and edges are dry. Where a mark will not come off with water, the paper's stated method is 0000 oil-free steel wool, explicitly not a solvent, since solvents attack the edges and backing too. The prevention is the whole answer in a bathroom: ventilate, and wipe the mirror dry rather than letting the condensation evaporate on it.

• Water — The solvent almost all cleaning happens in, and on its own the right answer more often than it is given credit for.
• Microfibre cloth — Split synthetic fibres with a very large surface area, which lift and trap soil mechanically — often with water alone.
• Rinsing and drying — The step people skip, and the one that decides whether cleaning protected the surface or damaged it. Residue left behind is exposure that continues after you stop.
• Mild abrasion — Removing a deposit with particles softer than the surface beneath it. Safe or ruinous depending entirely on relative hardness.

The chemistry behind both halves

• Evaporative deposition — The mark is made when the water leaves, not while it sits. Everything dissolved in it stays behind on the surface.
• Water hardness — Dissolved calcium and magnesium picked up from the ground. It decides how much detergent you need, what your kettle looks like inside, and whether surfaces dry clean.
• Material compatibility — The reason CleaningHQ answers questions about surfaces rather than about products. The same agent is the right answer and the wrong answer depending on what it lands on.
• Solvents and polymers — A solvent cannot tell a soil from a surface made of similar stuff. Most modern surfaces are polymers, which is what makes this the commonest way to ruin one.