Unit 14
Precipitation and complex ions
Hospitals let patients swallow barium — normally a poison — because barium sulfate barely dissolves; ammonia dissolves silver chloride but not silver iodide; a single drop of reagent can pick one ion out of several. This unit ties precipitation, dissolution, separation and identification into one calculable system of solubility products and formation constants, and ends with you identifying an unknown on a virtual bench.
- 1 The solubility product Even “insoluble” salts dissolve a little. The solubility product Ksp turns that little into a number, and the ion product Q tells you whether mixing two solutions will give a precipitate. 25 min
- 2 The common-ion effect and pH Add a common ion and solubility plunges; protonate the anion and it soars. Limestone caves, tooth decay and ocean acidification are all stories about these two effects. 25 min
- 3 Activities and the salt effect Add potassium nitrate, which has nothing in common with barium sulfate, and the sulfate dissolves more. Ions screen one another: the ionic atmosphere, Debye–Hückel theory and activities put numbers on it, and ion pairs explain why gypsum dissolves three times better than its Ksp predicts. 30 min
- 4 Selective precipitation and separating ions The more their solubility products differ, the more cleanly ions can be precipitated one at a time. pH control separates Fe³⁺ from Cu²⁺, and metered Ag⁺ makes a chloride titration its own indicator. 30 min
- 5 Complex ions and formation constants Ammonia molecules bind to Cu²⁺ one after another, turning the solution from pale to deep blue. Complexation dissolves silver chloride, fixes photographs, and lets aluminium hydroxide dissolve in both acid and base. 30 min
- 6 Qualitative analysis of cations String every equilibrium so far into one flowchart: hydrochloric acid brings down silver, lead and mercury(I); sodium hydroxide and ammonia then sort copper, iron, aluminium and zinc for confirmation. 35 min