Acids, Bases and Salts - Identification of ions and gases

Flame tests are used to identify metal cations based on characteristic colors: $Li^{+}$ (red), $Na^{+}$ (yellow), $K^{+}$ (lilac), $Ca^{2+}$ (orange-red), $Ba^{2+}$ (light green), and $Cu^{2+}$ (blue-green).

Identification of aqueous cations using $NaOH_{(aq)}$ and $NH_{3(aq)}$: $Cu^{2+}$ forms a blue precipitate, $Fe^{2+}$ forms a dirty green precipitate, and $Fe^{3+}$ forms a reddish-brown precipitate.

Amphoteric hydroxides: $Al^{3+}$, $Zn^{2+}$, and $Cr^{3+}$ form white or green precipitates with $NaOH_{(aq)}$ that redissolve in excess reagent to form colorless or green solutions.

Anion testing for Halides: Acidify with dilute $HNO_3$ and add $AgNO_{3(aq)}$. $Cl^{-}$ gives a white precipitate ($AgCl$), $Br^{-}$ gives cream ($AgBr$), and $I^{-}$ gives yellow ($AgI$).

Testing for Carbonates ($CO_3^{2-}$): Add dilute acid (e.g., $HCl$); effervescence occurs as $CO_{2(g)}$ is evolved, which turns limewater ($Ca(OH)_{2(aq)}$) milky.

Testing for Sulfates ($SO_4^{2-}$): Acidify with dilute $HNO_3$ and add $Ba(NO_3)_{2(aq)}$ or $BaCl_{2(aq)}$. A white precipitate of $BaSO_4$ confirms the presence of sulfate ions.

Gas identification: $H_2$ is identified by a 'pop' with a lighted splint; $O_2$ relights a glowing splint; $NH_3$ turns damp red litmus paper blue; $Cl_2$ bleaches damp litmus paper.