Barium sulfate is an insoluble white salt. A student plans to prepare a pure, dry sample of barium sulfate by mixing a solution of barium chloride with a so...

This tests preparing an insoluble salt by precipitation, then purifying and drying it. Two soluble solutions are mixed and the insoluble salt is filtered off.

(a) The reaction that forms the insoluble salt when the two solutions mix is precipitation. [1] The ionic equation is \[\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)\]

(b) The full method scores up to six marks: [6]

1. Measure out volumes of the two solutions (barium chloride and sodium sulfate). [1]
2. Mix them together in a beaker and stir. [1]
3. A white precipitate of barium sulfate forms. [1]
4. Filter the mixture so the precipitate is collected as the residue in the filter paper. [1]
5. Wash the residue with distilled water. [1]
6. Dry it in a warm oven or between sheets of filter paper. [1]

(c) The apparatus for each step: measuring out the solutions uses a measuring cylinder; [1] separating the solid uses a filter funnel and filter paper; [1] drying the solid uses a (warm) oven or desiccator. [1]

(d) The solid is washed with distilled water to wash away soluble impurities, chiefly the sodium chloride solution clinging to the precipitate. [1]

(e) To check it is dry, reweigh the sample [1] and keep reweighing until its mass stays constant (does not change on further drying). [1]

Exam tip: an insoluble salt is always made by precipitation, filtration, washing and drying, in that order.