Identification of Anions

Anions are negative ions. In the lab, simple tests help identify common anions. Watch for gas bubbles, colour changes, or solid precipitates. Acidifying means adding a few drops of dilute acid to remove interfering ions.

Carbonate, $\text{CO}_3^{2-}$

• Test: Add dilute acid (e.g. nitric acid). Collect gas and pass it into limewater.
• Observation: Effervescence (bubbles). Limewater turns milky due to $\text{CaCO}_3$.
• Key idea: $\text{CO}_3^{2-} + 2\text{H}^+ \to \text{CO}_2 + \text{H}_2\text{O}$; $\text{CO}_2$ turns limewater milky.

Halides: Chloride $\text{Cl}^-$, Bromide $\text{Br}^-$, Iodide $\text{I}^-$

• Test: Acidify with dilute nitric acid, then add aqueous silver nitrate, $\text{AgNO}_3(aq)$.
• Observation: White ppt = $\text{AgCl}$; cream ppt = $\text{AgBr}$; yellow ppt = $\text{AgI}$.
• Why acidify? Removes carbonates/sulfites that would form confusing precipitates with silver ions.

Nitrate, $\text{NO}_3^-$

• Test: Add aluminium foil and aqueous sodium hydroxide, then warm gently.
• Observation: Ammonia gas forms; damp red litmus turns blue (sharp smell).
• Idea: Nitrate is reduced to ammonia in alkaline conditions.

Sulfate, $\text{SO}_4^{2-}$

• Test: Acidify with dilute nitric acid, then add aqueous barium nitrate.
• Observation: White ppt of $\text{BaSO}_4$ (insoluble).

Sulfite, $\text{SO}_3^{2-}$

• Test: Add acidified aqueous potassium manganate(VII) (purple).
• Observation: Purple solution turns colourless (sulfite reduces $\text{MnO}_4^-$).

Common mistakes

• Skipping acidification before silver nitrate or barium nitrate tests.
• Confusing cream (Br−) with yellow (I−). View against white paper.
• Calling any fizz “carbonate” without confirming with limewater.

Real-world links: Carbonates are in limestone; sulfites are food preservatives; chlorides are in table salt. These tests help check what ions are present.