# Question 83607

Sep 4, 2015

There are several possibilities for X. I'll nominate copper(II) chloride.

#### Explanation:

Copper(II) chloride is $C u C {l}_{2 \left(a q\right)}$. It is readily soluble. No precipitate would occur if chloride, sulfate or nitrate ions were added because these ions are all soluble with copper(II).

Copper(II) carbonate is a green, insoluble substance so a green precipitate would result if carbonate ions were added:

$C {u}_{\left(a q\right)}^{2 +} + C {O}_{3 \left(a q\right)}^{2 -} \rightarrow C u C {O}_{3 \left(s\right)}$

Sep 4, 2015

Another candidate is magnesium nitrate.

#### Explanation:

The idea here is that you're looking for a cation that will not form a precipitate when mixed with those three anions, chloride, sulfate, and nitrate, but that it will form one when mixed with a solution that contains carbonate anions.

My cation of choice is the magnesium cation, ${\text{Mg}}^{2 +}$. When mixed with carbonate anions it forms magnesium carbonate, a white insoluble solid that precipitates out of solution.

${\text{Mg"_text((aq])^(2+) + "CO"_text(3(aq])^(2-) -> "MgCO}}_{\textrm{3 \left(s\right]}} \downarrow$

Magnesium chloride, ${\text{MgCl}}_{2}$, magnesium sulfate, ${\text{MgSO}}_{4}$, and magnesium nitrate, "Mg"("NO"_3)_2#, are all soluble compounds, so they will not precipitate out of solution when solution $X$ is mixed with the chloride, sulfate, and nitrate solutions.

If you assume that solution $X$ can contain one of the anions with which it does not form a precipitate when mixed, then you can say that solution $X$ can be magnesium nitrate, magnesium sulfate, or magnesium chloride.