The so-called adsorption of the calcium ion by the hydroxides of iron and aluminum
Since the attention of scientists has been called to the phenomena of adsorption many chemical reactions have been attributed to adsorption which are not adsorption at all. This is not surprising since there is no general rule or law which specifies when adsorption will take place and when it will not. In other words adsorption is specific and the only way one can determine whether or not adsorption will take place is by actual experiment. By adsorption is meant an increase in the concentration of a solution or a gas upon the surface of a solid with which it is in contact. It is essentially a surface phenomenon. Since this is the case, a substance having great surface in contact with a solution might be expected to adsorb much of the solute from the solution. It must not be forgotten, however, that there are two kinds of adsorption, that is, positive and negative. In positive adsorption the solution in contact with the solid becomes less concentrated and in negative adsorption it becomes more concentrated. One of the oldest as well as one of the commonest examples of adsorption is the decolorizing of solutions by powdered charcoal. In view of the many factors that may cause precipitation of calcium along with the hydroxides of iron and aluminum it is not surprising that many chemists considered this loss from solution as adsorption. The writer believes that it has been shown by these experiments that it is not a case of adsorption at all but a case of precipitation.