Chemical analyses of natural water may be inadequate because of inadvertent human errors and sample aging. Characteristic deficiencies can be demonstrated by multiple chemical analyses of the same sample by different laboratories, multiple analyses of the same sample by one laboratory, repeated analyses of one sample as a function of time, and a simple laboratory experiment in rock‐water chemistry. Whenever possible, complete chemical analyses should be made. The most reliable analyses are those in which combining weights of the cation equal the combining weight of the anion and in which the sum of the individually measured constituents equals the observed residue on evaporation at 180°C (total dissolved solids).