When concentrated iron(III) chloride is added to water at just below boiling point, a reaction occurs and produces Fe2O3, seen as a red colour in the water. This is a 'sol' of Fe2O3. A sol contains particles that are insoluble but do not form a precipitate. A student prepared a concentrated solution of iron(III) chloride by dissolving FeCl3.6H2O(s) in distilled water. [Figure showing U-tube with d.c. power supply, graphite electrodes, distilled water, and red Fe2O3 sol] Salt solutions can be added to sols to cause them to precipitate. This method is used in water purification. The student made up 100.0 cm³ of standard solutions containing 0.100 mol dm³ of the following ions. K+(aq) Mg2+(aq) Al3+(aq) Cl-(aq) SO42-(aq) PO43-(aq) The student carried out experiments to investigate how much of a particular salt solution was required to fully precipitate all the Fe2O3 sol in a 1000 cm³ sample. The salt solutions used were all of concentration 0.0100 mol dm³ with respect to the ion being investigated. [Figure table for Experiment 1 and Experiment 2 showing identity of salt solution, charge on anion/cation, and minimum amount required for complete precipitation]