The following statements describe the tests for aluminum, zinc, and calcium ions that can be performed through the addition of a dilute ammonia solution, NH4OH aqueous. Which of these statements is correct? (A) Adding the excess ammonia solution to calcium hydroxide and zinc hydroxide has no effect. (B) Aluminum, calcium, and zinc ions all form white precipitates with the ammonia solution. (C) No changes are observed when a few drops of the ammonia solution are added to aluminum ions. (D) The calcium hydroxide formed reacts with the excess ammonia solution to form a dark-blue solution. Or (E) the zinc hydroxide formed dissolves in the excess ammonia solution.
An ammonia solution, which can also be referred to as ammonium hydroxide, can be used to test for the presence of cations based on the resulting precipitate or solutions formed. Metal cations can be identified by their reactions with the hydroxide anions from ammonium hydroxide and the distinctive metal hydroxide formed. To identify which of these statements is correct, let’s review how aluminum, zinc, and calcium ions react during the tests with the ammonia solution.
When a few drops of dilute ammonia solution are added to each of these metal cations, the following observations can be made. Aluminum ions react to form aluminum hydroxide, which is a white precipitate. Zinc ions react to form zinc hydroxide, which is also a white precipitate. Calcium ions do not react, and no change to the solution can be observed. If a secondary test is conducted by adding excess ammonia solution, the white aluminum hydroxide precipitate remains, while the white zinc hydroxide precipitate dissolves. A solution containing calcium ions would still see no change. Let’s now have a look at our answer choices.
Answer choice (A) states that excess ammonia solution would have no effect on zinc hydroxide or calcium hydroxide. While it is true there is still no change in the calcium solution, the excess ammonia solution does have an effect on zinc hydroxide. We can eliminate answer choice (A).
Answer choice (B) states that all three ions form white precipitates. While both aluminum and zinc ions produce white precipitates with the ammonia solution, calcium ions do not. We can eliminate answer choice (B).
Answer choice (C) states that no changes occur when the ammonia solution is added to aluminum ions. Even just a few drops of dilute ammonia solution would produce a white precipitate with aluminum ions. So, we can eliminate answer choice (C).
Answer choice (D) states that calcium hydroxide in excess ammonia solution forms a dark-blue solution. Even in excess ammonia solution, there is no change involving calcium ions. We can eliminate answer choice (D).
Answer choice (E) states that zinc hydroxide dissolves in excess ammonia solution. Zinc hydroxide, the precipitate formed by zinc ions in dilute ammonia solution, does indeed dissolve when in excess ammonia solution. Therefore, the statement that is correct is answer choice (E). The zinc hydroxide formed dissolves in the excess ammonia solution.