Inorganic unknowns: ions, gases and independent evidence

What you’ll learn

• Connect observations to ions using balanced ionic equations.
• Choose independent tests and avoid contamination or unsupported conclusions.

Understand the idea

Core practical 8 asks you to combine observations into an identification. Record an observation before giving an inference: a white precipitate is an observation; sulfate ions are a possible conclusion under a suitable test. Carbonate or hydrogencarbonate ions release carbon dioxide with acid, and carbon dioxide turns limewater cloudy. Ammonium ions release ammonia with hydroxide in the appropriate supervised test; the gas changes damp red litmus to blue. These gas results must be interpreted with the reaction context.

Acidified barium chloride gives a white sulfate precipitate. For halides, silver nitrate after appropriate nitric-acid acidification gives white silver chloride, cream silver bromide or yellow silver iodide. Chloride dissolves in dilute ammonia; bromide requires concentrated ammonia; iodide remains insoluble in the usual ammonia test. The acid choice matters: adding hydrochloric acid before testing for chloride introduces the very ion being sought. Use separate aliquots for independent tests to prevent one reagent from changing the next result.

Flame colours can support identification of a metal ion but should be combined with anion evidence and charge balance. Gas and ammonia observations are supplied analytical evidence here; gases should not be identified by directly inhaling them. School procedures specify ventilation and appropriate collection or indicator methods, and metal-containing test residues require the prescribed waste route.

Key formulae

$$\mathrm{CO_3^{2-}}+2\mathrm{H^+}\rightarrow\mathrm{CO_2}+\mathrm{H_2O}$$

$$\mathrm{Ba^{2+}(aq)}+\mathrm{SO_4^{2-}(aq)}\rightarrow\mathrm{BaSO_4(s)}$$

$$\mathrm{NH_4^+}+\mathrm{OH^-}\rightarrow\mathrm{NH_3}+\mathrm{H_2O}$$

$$\mathrm{Ag^+(aq)}+\mathrm{Cl^-(aq)}\rightarrow\mathrm{AgCl(s)}$$

Worked example

Separate portions of a soluble salt give a lilac flame and a white precipitate with acidified barium chloride. In the supplied reference, potassium gives a lilac flame and sulfate gives this barium test. Assuming one cation and one anion are present, identify the ions and write the salt formula.

1. The lilac flame supports the presence of $\mathrm{K^+}$.
2. The barium test supports $\mathrm{SO_4^{2-}}$. The separate portions keep the evidence independent.
3. Two $\mathrm{K^+}$ ions balance one $\mathrm{SO_4^{2-}}$ ion, giving $\mathrm{K_2SO_4}$.

Answer: Potassium ions and sulfate ions; $\mathrm{K_2SO_4}$.