Video Transcript
Which of the following metal
cations does not produce a white precipitate when a few drops of dilute aqueous
sodium hydroxide are added to a salt or solution of that metal cation? (A) Al3+, (B) Ca2+, (C) Zn2+, or
(D) Cr3+.
We are asked to determine which
cation does not when reacting with aqueous sodium hydroxide produce a white
precipitate. Aqueous sodium hydroxide is used to
test for the presence of cations by reacting an unknown solution or salt with
hydroxide ions and observing the resulting precipitate or solution. The general reaction expected is
between the metal cation present and the added hydroxide anions to produce a metal
hydroxide precipitate. Let’s have a look at each answer
choice to determine what observations would be made of the metal hydroxide they
produce.
Answer choice (A) aluminum cations
when reacted with hydroxide ions produce aluminum hydroxide. Aluminum hydroxide is a white
precipitate, so we can eliminate answer choice (A). Answer choice (B) calcium cations
when reacted with hydroxide ions produce calcium hydroxide, a white precipitate; we
can eliminate answer choice (B). Answer choice (C) zinc cations when
reacted with hydroxide ions produce zinc hydroxide, another white precipitate. We can also eliminate answer choice
(C). So, we are left with answer choice
(D), chromium(III) cations.
The other three solutions tested
were colorless, while a solution containing chromium(III) cations is usually dark
green. When chromium(III) ions are reacted
with hydroxide ions, they produce chromium(III) hydroxide. Chromium(III) hydroxide is a
gray-green precipitate. Therefore, the metal cation that
does not produce a white precipitate when a few drops of dilute aqueous sodium
hydroxide are added is answer choice (D) Cr3+.
