Well let me try.
Basics first! Ions are atoms or melecules that are either deficient or surplus of electrons. Puting it simply it means that the number of electrons and protons in an ions is not equal. When the electrons are more the ion is called an anion, and otherwise a cation. Since electrons are negatively charged this means that an anion is negatively charged like the chloride ion Cl- and cations are positively charged like the hydrogen ion H+.
Let us now turn to the second word
exchange, the cations and anions exist together, so as to maintain a neutral charge. But an anion can have any cation as its companion, much like young boys and girls. Some anions have a stronger attraction (call it preference) for certain cations, so when they are offered the choice, they would like to retain the cation they have stronger attraction for.
Now consider the scenario, a substance is made up of X cations and Y anions (x and Y are not numbers but names), now another substance is added to it which has Z cations and Q anions. The Y anions bind more strongly to Z cations then they do to X cations, so the mixing of the two substances results in Z cations being retained by the Y anions. The cations get
exchanged between Y and Q anions.
So now that you understand what the terms cation exchange means, try to figure out the capacity part of it,
Think, You cannot succeed without thinking
