To remove soap scum and hard water deposits from a tile wall in a bathroom, mix one part muriatic acid with five parts water. Since muriatic acid is a harsh chemical, mix it outside.
Follow the warning and instructions on the bottle, and be sure to wear rubber gloves and eye protection. Apply the mixture with a nylon scouring pad in small circles over the tile. After rinsing the solution off with cold water, the tile will look as good as new.