When a precipitation reaction occurs, the ions that do not take part in the reaction are called A. non participants
B. useless ions
C. independent variables
D. spectator ions

D. spectator ions.

Explanation:

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In this case, when going over net ionic equations by which precipitation reactions are analyzed, we can consider the example of lead (II) nitrate with potassium iodide to yield insoluble lead (II) iodide and soluble potassium nitrate according to:

Whereas aqueous species remain in solution:

It means that potassium and nitrate ions are spectator ions because they are not involved in the precipitation reaction, which is represented by the net ionic one:

Thus, the answer to this question is D. spectator ions.

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the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.