Video Transcript
When barium nitrate is added to a solution of an unknown salt, no precipitate is observed. However, the addition of magnesium nitrate causes a white precipitate to form. Which additional material or reagent could be used to confirm the identity of the anion in the solution? (A) Concentrated ammonia solution, (B) blue litmus paper, (C) sodium hydroxide, (D) silver nitrate, (E) red litmus paper.
In this question, we’re starting with an unknown salt solution. The unknown salt solution will contain positive ions, also known as cations. The solution will also contain dissolved negative ions, known as anions. To answer this question, we need to identify the anion that could possibly be present in this solution.
We can see that barium nitrate has been added to our unknown salt solution first of all. Barium nitrate has the formula Ba(NO3)2. When dissolved in aqueous solution, barium nitrate dissociates into barium ions and nitrate ions. Barium ions have the formula Ba2+. Being a positively charged ion or cation, barium ions can react with negatively charged ions or anions. Nitrate ions have the formula NO3−. Since nitrate ions are negatively charged, they can react with positive ions or cations. When the positive ions in our unknown salt solution react with the nitrate ions, it’s unlikely we’ll see a precipitate forming. This is because, with just a few exceptions that are very uncommon, all nitrate salts are highly soluble. This is not the case for all barium salts though.
Although barium nitrate is highly soluble, other barium salts are less so. Barium ions will easily react with sulfate ions, sulfite ions, and carbonate ions. In each case, barium sulfate, barium sulfite, and barium carbonate are formed. The formation of barium sulfate, barium sulfite, or barium carbonate will be accompanied by the appearance of a heavy white precipitate. When barium nitrate was added to an unknown salt solution in our question, no precipitate was observed. We can therefore rule out the presence of sulfate, sulfite, and carbonate ions in our unknown solution. The presence of these negative ions or anions would’ve caused a white precipitate to form in the first instance.
We need to explore the question further to establish which negative ion is present in our unknown salt. In the next procedure, magnesium nitrate is added to the solution. Magnesium nitrate has the formula Mg(NO3)2. As a nitrate, it’s a highly soluble salt. When magnesium nitrate is added to the solution, it will release positive magnesium ions, Mg2+, and negative nitrate ions, NO3−. Only the positive magnesium ions will react with the anions or negative ions in our unknown solution. When the magnesium nitrate was added to our unknown salt solution, a white precipitate was seen. The white precipitate was formed because the magnesium two plus ions reacted with an unknown anion in the solution and formed an insoluble compound.
On the basis that the white precipitate is an insoluble magnesium compound, we can begin to identify the anion in the unknown salt in this question. Many magnesium salts are highly soluble, but two exceptions which are commonly found are magnesium hydroxide and magnesium carbonate. Since we already know that the unknown salt does not contain carbonate ions, we can rule out the formation of magnesium carbonate. It is therefore likely that the white precipitate that formed upon the addition of magnesium nitrate to our unknown salt is in fact magnesium hydroxide. This means that the negative ion or anion in the solution of our unknown salt is most likely to be the hydroxide ion.
To answer the question, we need to come up with a confirmatory test for the hydroxide ion. The hydroxide ion is a strong base. In solution, it will yield a high pH. Concentrated ammonia solution contains ammonia molecules, which is in fact a weak base. Ammonia molecules won’t easily react with hydroxide ions, and there’d be nothing to see if we added concentrated ammonia solution to our unknown salt. It’s not the correct answer.
Blue litmus paper will turn red in the presence of an acidic solution. Blue litmus paper won’t tell us if hydroxide ions are present and it’s not the correct answer. Adding sodium hydroxide to a solution that already contains hydroxide ions is not gonna produce a precipitate or any observable change at all. Sodium hydroxide is not the correct answer.
Silver nitrate solution contains aqueous silver ions or Ag+ ions. Since silver ions form precipitates with many negative ions, it wouldn’t be a definitive test for the presence of hydroxide ions. Silver nitrate is not the correct answer either.
Red litmus paper will immediately turn blue when placed in a basic solution with a high pH due to the presence of hydroxide ions. Red litmus paper is a good choice of material to use as a simple test for hydroxide ions. Red litmus paper is the correct answer.