Stage 1: An excess of the solid is added to the acid and allowed to react. Using an excess of the solid makes sure that all the acid is used up. If it is not used up at this stage, the acid would become more concentrated when the water is evaporated later (stage 3).

Stage 2: The excess solid is filtered out.

Stage 3: The filtrate is gently evaporated to concentrate the salt solution. This can be done on a heated water-bath or sand tray.

Stage 4: When crystals can be seen forming (crystallisation point), heating is stopped and the solution is left to crystallise.

Stage 5: The concentrated solution is cooled to let the crystals form. The crystals are filtered off and washed with a little distilled water. Then the crystals are dried carefully between filter papers.

titration: a method of quantitative analysis using solutions: one solution is slowly added to a known volume of another solution using a burette until an end-point is reached

burette: a piece of glass apparatus used for delivering a variable volume of liquid accurately

volumetric pipete: a pipette used to measure out a volume of solution accurately

end-point: the point in a titration when the indicator just changes color showing that the reaction is complete

Stage 1: The acid solution is poured into a burette. The burette is used to accurately measure the volume of solution added. A known volume of alkali solution is placed in a conical flask using a volumetric pipette. The pipette delivers a fixed volume accurately. A few drops of an indicator (e.g. thymolphthalein or methyl orange) are added to the flask.

Stage 2: The acid solution is run into the flask a few drops at a time from the burette until the indicator just changes color (Figure 12.12). The conical flask must also be swirled after each portion of acid to ensure everything is mixed and the reaction is complete. Having found the end-point for the reaction, the volume of acid (titre volume) added is noted. The experiment is then repeated without using the indicator. The same known volume of alkali is used in the flask. The same volume of acid as noted in the first part is then run into the flask. Alternatively, activated charcoal can be added to remove the colored indicator. The charcoal can then be filtered off.

Stage 3: The salt solution is evaporated and cooled to form crystals as described in method A.

Ba 2 (aq) SO4 2-(aq) → BaSO4(s)

Ag (aq) Cl-(aq) → AgCl(s)