Which of the following salts in solution could be used to differentiate between NaHCO3 and Na2CO3 solutions by forming a precipitate when mixed with Na2CO3 aqueous, but not when mixed with NaHCO3 aqueous? (A) Ca(NO3)2, (B) LiNO3, (C) NaNO3, (D) KNO3, or (E) NH4NO3.
In this question, we’re being tasked with determining how we could differentiate two different salt solutions, one containing NaHCO3 or sodium hydrogen carbonate and the other containing Na2CO3 or sodium carbonate. We want to add a salt to both of these solutions that will form a precipitate when added to the Na2CO3 solution, but not the NaHCO3 solution. A precipitate is an insoluble product of a reaction. So we want to figure out which salts of the ones we’re given will form a solid product when reacted with Na2CO3.
When these two salts react, they’ll react according to a type of reaction called a double displacement reaction, which is where the cations and the anions of the two different salts swap. For example, if we were to add the salt lithium chloride to both of these salt solutions, it would form sodium chloride and lithium hydrogen carbonate when mixed with sodium hydrogen carbonate. And when the lithium chloride was mixed with sodium carbonate, it would form sodium chloride and lithium carbonate.
As we can see from this example, because sodium hydrogen carbonate and sodium carbonate both contain sodium, one of the products of the reaction when we mix our salt with both of these solutions will be the same. And since this product will be the same in both of our solutions, it won’t be useful for differentiating sodium hydrogen carbonate from sodium carbonate. That means we need to turn towards the second product of this reaction to figure out how to differentiate these two solutions. The second product will be the combination of the hydrogen carbonate anion or the carbonate anion and the cation from the salt that we mix in with the solution.
In other words, in order to differentiate these two salt solutions, we need to figure out which cation from the salts in our answer choices will be insoluble when it forms a product with the carbonate anion but soluble when it forms a product with the hydrogen carbonate anion. The hydrogen carbonate anion is generally soluble with all cations, but the carbonate anion is only soluble when it’s in a compound with group one cations or the ammonium cation. Otherwise, it’s insoluble.
If we look through our answer choices, (B), (C), and (D) are all salts that contain group one elements. So they would be soluble when they form products with both sodium hydrogen carbonate and sodium carbonate. The same is true for answer choice (E), which is a salt that contains ammonium. So it also would not form a precipitate with either of the salt solutions. This leaves answer choice (A) calcium nitrate. If we were to add calcium nitrate to both of our solutions, we would only form soluble products with the sodium hydrogen carbonate. But we would form solid calcium carbonate as the precipitate when mixed with sodium carbonate. So we would be able to add calcium nitrate to both sodium hydrogen carbonate and sodium carbonate solutions to differentiate them.