Acids, Bases and Salts - Identification of ions and gases (Qualitative analysis)

Qualitative analysis is used to identify the presence of specific ions (cations and anions) and gases in a sample using characteristic chemical reactions and physical observations.

Flame Tests for Cations: Used to identify metal ions by the color they impart to a Bunsen burner flame. $Li^+$ is red, $Na^+$ is yellow, $K^+$ is lilac, $Ca^{2+}$ is orange-red, $Cu^{2+}$ is blue-green, and $Ba^{2+}$ is light green.

Cation identification with Aqueous $NaOH$ and $NH_3$: Many metal ions form insoluble hydroxides. $Cu^{2+}$ forms a blue precipitate; $Fe^{2+}$ forms a green precipitate; $Fe^{3+}$ forms a reddish-brown precipitate. $Zn^{2+}$, $Al^{3+}$, and $Ca^{2+}$ all form white precipitates with $NaOH$, but only $Zn^{2+}$ and $Al^{3+}$ precipitates dissolve in excess $NaOH$. Only $Zn^{2+}$ precipitate dissolves in excess $NH_3$.

Anion Identification: Carbonates ($CO_3^{2-}$) react with dilute acids to produce $CO_2$. Halides ($Cl^-$, $Br^-$, $I^-$) are identified using silver nitrate ($AgNO_3$) in the presence of nitric acid ($HNO_3$), forming white ($AgCl$), cream ($AgBr$), and yellow ($AgI$) precipitates respectively. Sulfates ($SO_4^{2-}$) are identified using barium nitrate or barium chloride, forming a white precipitate of $BaSO_4$.

Identification of Gases: $H_2$ is identified by a 'squeaky pop' with a lighted splint; $O_2$ relights a glowing splint; $CO_2$ turns limewater ($Ca(OH)_2$) milky; $Cl_2$ bleaches damp litmus paper; $NH_3$ turns damp red litmus paper blue; $SO_2$ turns acidified aqueous potassium manganate(VII) from purple to colorless.