Video Transcript
What color precipitate is formed
when a few drops of aqueous ammonia are added to a salt or solution containing a
Cr3+ cation? (A) Orange precipitate, (B) yellow
precipitate, (C) gray-green precipitate, (D) white precipitate, or (E) pale-blue
precipitate.
This question features a test using
aqueous ammonia. A solution of ammonia is sometimes
added to unknown solutions to test for the identity of the cations present by
observing the resulting precipitate or solution. Ammonia gas, when in a solution
with water, can be written as ammonium hydroxide.
A test involving aqueous ammonia
involves the reaction between the metal cation in question and the hydroxide anion
to produce a distinctive metal hydroxide whose properties allow us to determine the
identity of the cations present. A solution or salts of chromium
three ions would often be a dark-green color. If a few drops of ammonia solution
were added, a gray-green precipitate of chromium(III) hydroxide would form.
Even in excess ammonia solution,
this precipitate would remain. This is worth noting, as sometimes
excess ammonia solution is used as a secondary test to distinguish between metal
hydroxides that are similar in appearance. Therefore, the color precipitate
that is formed when a few drops of aqueous ammonia are added to a salt or solution
containing chromium three plus ions is answer choice (C) a gray-green
precipitate.
