Question

Why do ionic solutions contain less particles than the number of ions originally added to the solution?
a.
Because some evaporate
c.
Ions tend to reassociate with each other
b.
Some are lost in the reaction
d.
Some ions will vaporize

Answer 1

They almost never do. This is a very strange question. To the extent it's ever true, it might be true in an electrolytic cell, where external energy is used to convert ions to less stable neutral forms. For example, molten NaCl (which consits of Na+ and Cl- ions) can be electrolyzed to sodium (Na) metal and chlorine (Cl2) gas, which destroys ions.

But aside from situations like this, which might be covered by "some are lost in the reaction" depending on what "the reaction" means, the number of ions is well-conserved.

One unfortunate fact might be that the person who wrote this question didn't understand that when ions associate to form an ionic solid, they are still ions. That is, NaCl itself is not charged, but it still contains intact Na+ and Cl- ions. Just an equal number of both, so the charge cancels out. I hope that's not the case, but you never know.

Answer 2

Hmm, now that I think about it, a much more common and reasonable case would be the reaction of a strong acid and base. If you have a solution of NaOH it will consist of Na+ cations, OH- anions, and water. If you also have a solution of HCl it will consist of H3O+ cations and Cl- anions. If you mix them in the right proportions, almost all of the H3O+ and OH- will react to form neutral H2O, leaving only the Na+ and Cl-, and greatly reducing the number of ions in the solution. This is a much more common situation than electrolysis, but is also covered under the "some are lost in the reaction."

Answer 3

This may also be a case of incomplete ionization, where you may expect 1 mole of MgCl2 to dissociate into 3 moles of ions, but in reality only get 2.7 - 2.8 moles of ions. It's not the same wording, but the intent might be similar

Answer 4

I suppose also if you are considering a precipitation reaction, such that ions that associate into an ionic solid are then considered to have left the solution, that works. I'm not entirely sure under which category that possibility would fit, because it has something to do with ions "reassociating" with each other. Bah.