Describe tests to identify the anions: (a) carbonate, $mathrm{CO}_3{ }^{2-}$, by reaction with dilute acid and then testing for carbon dioxide gas (b) chloride, $mathrm{Cl}^{-}$, bromide, $mathrm{Br}^{-}$, and iodide, $mathrm{I}^{-}$, by acidifying w

Experimental Techniques & Chemical Analysis – Identification of Ions & Gases

(a) Carbonate – \$\\mathrm{CO_3^{2-}}\$

Think of carbonate like a “bicarbonate balloon” that bursts when it meets acid.

  1. Take a small amount of the sample in a test tube.

  2. Add a few drops of dilute hydrochloric acid (\$\\mathrm{HCl}\$).

  3. Observe vigorous bubbling – that’s carbon dioxide gas (\$\\mathrm{CO_2}\$) escaping.

  4. Collect the gas in a test tube inverted over water; the tube will fill with bubbles.

  5. Optional: Pass the gas through a solution of alkaline silver nitrate – a pale yellow precipitate of silver carbonate (\$\\mathrm{Ag2CO3}\$) confirms the presence of \$\\mathrm{CO_2}\$.

⚠️ Exam tip: Remember the “bubbling” clue – if you see fizzing, you’re likely dealing with a carbonate.

(b) Halides – \$\\mathrm{Cl^-}\$, \$\\mathrm{Br^-}\$, \$\\mathrm{I^-}\$

Halides are like a “silver coin” that turns into a solid when they meet silver ions.

  1. Acidify the sample with dilute nitric acid (\$\\mathrm{HNO_3}\$).

  2. Add a few drops of aqua silver nitrate (\$\\mathrm{AgNO_3}\$).

  3. Observe the formation of a precipitate:

    • \$\\mathrm{AgCl}\$ – white, “chalky”

    • \$\\mathrm{AgBr}\$ – pale yellow

    • \$\\mathrm{AgI}\$ – yellowish‑brown

  4. Use the colour intensity to differentiate the halides.

📝 Exam tip: Write the equation for each halide: \$\\mathrm{X^- + Ag^+ \\rightarrow AgX(s)}\$ and note the colour change.

(c) Nitrate – \$\\mathrm{NO_3^-}\$

Nitrate is a “silent partner” – it doesn’t react until we give it a push.

  1. Reduce nitrate with a strip of aluminium foil in acetic acid (\$\\mathrm{CH3COOH}\$). The foil dissolves and releases ammonia gas (\$\\mathrm{NH3}\$).

  2. Collect the gas in a test tube inverted over water – you’ll see a greenish‑blue colour (due to the bromocresol green indicator).

  3. Alternatively, add aqua sodium hydroxide (\$\\mathrm{NaOH}\$) to the sample. Nitrate itself is inert, but the presence of ammonia gas indicates nitrate reduction.

🔍 Exam tip: Remember: “Aluminium + acid → NH₃” is the key clue for nitrate.

(d) Sulfate – \$\\mathrm{SO_4^{2-}}\$

Sulfate is like a “silent snowflake” – it only shows up when it meets barium.

  1. Acidify the sample with dilute nitric acid (\$\\mathrm{HNO_3}\$).

  2. Add a few drops of aqua barium nitrate (\$\\mathrm{Ba(NO3)2}\$).

  3. Observe a white precipitate of barium sulfate (\$\\mathrm{BaSO_4}\$).

  4. Confirm by adding a small amount of hydrochloric acid; the precipitate remains insoluble.

💡 Exam tip: Write the equation: \$\\mathrm{SO4^{2-} + Ba^{2+} \\rightarrow BaSO4(s)}\$ and note the insolubility.

(e) Sulfite – \$\\mathrm{SO_3^{2-}}\$

Sulfite is a “quick‑acting firestarter” that reacts with strong oxidisers.

  1. Acidify the sample with dilute nitric acid (\$\\mathrm{HNO_3}\$).

  2. Add a few drops of potassium manganate(VII) (\$\\mathrm{K2MnO4}\$) solution.

  3. Observe a rapid colour change from green to purple, indicating the formation of potassium permanganate (\$\\mathrm{KMnO_4}\$).

  4. Confirm by adding a few drops of hydrochloric acid; the purple colour fades.

🚨 Exam tip: Remember the colour shift – green to purple is the hallmark of sulfite reacting with manganate.

IonKey TestObservations
\$\\mathrm{CO_3^{2-}}\$Acid + \$\\mathrm{CO_2}\$Fizzy bubbles, pale yellow \$\\mathrm{Ag2CO3}\$
\$\\mathrm{Cl^-}\$, \$\\mathrm{Br^-}\$, \$\\mathrm{I^-}\$\$\\mathrm{AgNO_3}\$White (\$\\mathrm{AgCl}\$), pale yellow (\$\\mathrm{AgBr}\$), yellowish‑brown (\$\\mathrm{AgI}\$)
\$\\mathrm{NO_3^-}\$Aluminium + acid → \$\\mathrm{NH_3}\$Greenish‑blue gas, no precipitate
\$\\mathrm{SO_4^{2-}}\$\$\\mathrm{Ba(NO3)2}\$White insoluble \$\\mathrm{BaSO_4}\$
\$\\mathrm{SO_3^{2-}}\$\$\\mathrm{K2MnO4}\$ + acidGreen → purple, fades with HCl