The demonstration is based on the traditional determination of water hardness in which EDTA is used as the titrant that complexes Mg²⁺ and Ca²⁺ ions. The color change that signals the end of the titration represents the successful removal of the Mg²⁺ ions from the metallochromic indicator Calmagite. This removal is possible because the value of the formation constant of the [Mg(EDTA)]^2- complex is about 3 orders of magnitude greater than that corresponding to the [Mg(Calmagite)]^- complex. The color change from wine-red (metal-bound indicator) to blue (unbound indicator) can be seen clearly in a large classroom with the use of overhead projection.