An example of a pure substance is

a) an element
b) a compound
c) h2o
d) carbon dioxide
e) all of these

e) all of these

Explanation:

Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Copper

Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: water  , carbon dioxide

Mixtures are not pure substances as they consist of elements and compounds combined physically and not in any fixed ratio. Example: Air

Thus all given substances are pure substances.

the increase of protons   hope this

the correct answer is:

a. formation of precipitation