Analytical Chemistry

The One Thing

Aqueous alkalis identify ions in salt solutions by forming characteristic metal hydroxide precipitates. The precipitate’s colour, solubility in excess reagent, and changes on warming or exposure to air distinguish ions, while amphoteric hydroxides react with both acids and excess alkalis to form soluble complex salts.

Reactions, Processes and Experiments

Key Terms

• Alkali: A soluble base that produces hydroxide ions in water, such as sodium hydroxide or ammonium hydroxide.
• Salt solution: An aqueous solution containing positively charged metal or ammonium ions and negatively charged acid radicals.
• Precipitate: An insoluble solid formed when two solutions react.
• Double decomposition reaction: A reaction in which the ions of two compounds exchange partners to form new compounds.
• Metal hydroxide: A compound formed when a metal ion combines with hydroxide ions; it may be soluble or insoluble in water.
• Amphoteric hydroxide: A hydroxide that reacts with both acids and excess alkalis.
• Sodium hydroxide test: A qualitative test in which sodium hydroxide is added to a salt solution to observe the colour and solubility of the hydroxide precipitate.
• Ammonium hydroxide test: A qualitative test in which ammonium hydroxide is added to a salt solution; the precipitate may dissolve in excess reagent depending on the ion.
• Complex salt: A soluble compound containing a central metal ion bonded to additional ions or molecules, such as a hydroxo complex.
• Qualitative analysis: The identification of ions or substances by observing reactions such as precipitate formation, colour changes, and solubility.

Easily Confused

• Sodium hydroxide and ammonium hydroxide: Sodium hydroxide is a strong alkali, whereas ammonium hydroxide is a weak alkali; this can affect the amount and persistence of a precipitate.
• Initial precipitate formation and solubility in excess reagent: Sodium hydroxide is first added drop by drop to observe formation, then in excess to test whether the precipitate dissolves.
• Copper(II), iron(II) and iron(III) hydroxides versus zinc, aluminium and lead(II) hydroxides: Copper(II), iron(II) and iron(III) hydroxides generally remain insoluble in excess sodium hydroxide, whereas zinc hydroxide, aluminium hydroxide and lead(II) hydroxide dissolve in excess sodium hydroxide.
• Iron(II) hydroxide and iron(III) hydroxide: Iron(II) hydroxide is dirty green and gradually turns brown in air; iron(III) hydroxide is reddish-brown and is insoluble in excess sodium hydroxide and ammonium hydroxide.
• Amphoteric behaviour with acids and with excess alkalis: An amphoteric hydroxide forms salt and water with an acid, but forms a soluble complex salt with excess alkali.
• Precipitate colour and complete identification: Colour alone is not conclusive; the reagent used, solubility in excess reagent, and changes on warming or exposure to air must also be considered.
• Ammonium ions and metal ions: Ammonium ions do not form a precipitate with sodium hydroxide in the ordinary cation test; on warming, they release ammonia gas.

What Gets Asked

• Writing ionic equations for precipitation: Questions may require the equations for a $2+$ or $3+$ metal ion, such as M2+ + 2OH- -> M(OH)2(s). or M3+ + 3OH- -> M(OH)3(s). Marks are lost by using the wrong hydroxide ratio.
• Identifying ions from observations: Students may be given precipitate colours and solubility results for copper(II), iron(II), iron(III), zinc, lead(II) or aluminium ions. Marks are lost by relying on colour alone rather than the complete set of observations.
• Distinguishing amphoteric hydroxides: Questions may ask why zinc hydroxide or aluminium hydroxide dissolves in excess sodium hydroxide and acids. Marks are lost by stating only that the hydroxide dissolves without identifying its amphoteric behaviour or the soluble complex salt formed.
• Interpreting the ammonium-ion test: Students may need to describe the absence of a precipitate, warming, ammonia release, the pungent smell, the moist red litmus result, or white fumes with hydrogen chloride gas. Marks are lost by confusing ammonia gas formation with precipitate formation.
• Explaining the iron(II) observation: Questions may ask why dirty green iron(II) hydroxide gradually turns brown in air. Marks are lost by omitting that it is oxidised to iron(III) compounds.
• Describing the correct sodium hydroxide procedure: Students may be asked why sodium hydroxide is added drop by drop and then in excess. Marks are lost by reporting only the initial precipitate and failing to test its solubility in excess reagent.