Learn English – ‘To’ or ‘For’? Usage of prepositions

For the most part, your book is correct, but you are right to be suspicious. The appropriate word can change depending on the intended meaning of the sentence in some cases. Here are a few examples:

• I have had a change of heart
  It is a change for the better

• What we have here is a failure to communicate
  This is a failure of epic proportions

• I have the ambition to succeed
  I have an ambition for greatness

Note: it is a good idea to talk to your instructor about questions like this. You may not be required to know all of the special cases right now. Special cases can make the learning process more complicated, so some classes will save them for later.

Wikipedia has this to say:

In ambiguous cases, there is not always a clear rule that dictates which adposition is appropriate, and different languages and regional dialects may have different conventions; the standard usage(s) of a given preposition can be idiomatic. Learning the conventionally preferred word is a matter of exposure to examples. For example, most dialects of American English have "to wait in line", but some have "to wait on line". It is for this reason that prepositions are one of the most difficult aspects of a language to learn for non-native speakers.

For reference, here is the description of adposition used in the same article:

In more technical language, an adposition is an element that ... indicates how that phrase should be interpreted in the surrounding context. Some linguists use the word preposition instead of adposition for all three cases.

The "charge" vs. "impurities" difference you actually seem to be asking about is nothing to do with the preposition as such; it's to do with "charge" being a "mass noun", so not calling for pluralization, and "impurity" being a "count noun", which it's appropriate to pluralize here.

This distinction is the same one as produces the "less vs. fewer" dichotomy.