Abstract The effects of temperature on the ion product of water Kw, and the aqueous solution acidity are discussed. The fuzzy understanding that may occur on Kw and pH criteria for the aqueous solution acidity in teaching and textbooks are analyzed. Kw is reported to significantly increase with increasing temperatures, resulting in less than and greater than pH 7.0 for neutral aqueous solution at temperatures greater than and less than 25 ℃, respectively.
