Despite the fact that the molecular formula for water is H2O, even absolutely pure water contains other combinations of oxygen and hydrogen. Chemical bonds are continually being formed and broken in matter, and water is no exception. It tends to “dissociate” to a slight extent, with a hydrogen occasionally breaking off from one molecule and rebonding to a nearby intact water molecule. This leaves one negatively charged OH combination, and a positively charged H3O. Under normal conditions, a very small number of molecules exist in the dissociated state, something on the order of two ten-millionths of a percent. This is a small number but a significant one, because the presence of relatively mobile hydrogen ions, which are the basic units of positive charge (protons), can have drastic effects on other molecules in solution. A structure that is stable with a few protons around may be unstable when many protons are in the vicinity. So significant is the proton concentration that humans have a specialized taste sensation to estimate it: sourness. Our term for the class of chemical compounds that release protons into solutions, acids, derives from the Latin acere, meaning to taste sour. We call the complementary chemical group that accepts protons and neutralizes them, bases or alkalis.