Well, if that doesn't sound like a flashback in physics lessons! When we talk about mass per unit volume, we are talking about DENSITY. It is expressed in g/cm3 and is a material constant dependent on temperature and pressure. And what does that have to do with water treatment now? It's simple: In order to be able to differentiate cation and anion exchangers from one another, the DENSITY of the two components is crucial. If they are separated, the cation exchanger will settle first because it has a higher specific weight.