Question Video: Identifying the Cation That Forms a Colored Precipitate Chemistry

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+.