Wild Chemistry Ride 7:


Crystallisation

Crystallisation is based on the principle of variable solubilities of solutes in solvents at different temperatures, and it is the most common method used to purify soluble solids.

We did a practical where we attempted to purify copper (II) sulfate crystals. In summary, the solution of copper (II) sulfate dissolved in water is heated to form a hot, saturated solution. This heating allows for excess solvent to be removed via evaporation. As the hot saturated solution cooled, the dissolved solids appeared as crystals as the solubility of the solute decreases with decreasing temperatures. This means that as the temperature of the solution drops, less solute can be dissolved in the solution. The excess solute would then appear in the solution in the form of crystals. 

At first, I was confused as to why we had to stop heating the solution before all the solvent evaporated. However, later on I realised that leaving some solvent behind prevents decomposing of copper (II) sulfate into copper (II) oxide. Another reason was that soluble impurities will be left behind if all the solvent evaporated, because some solvent is required to keep teh soluble impurities dissolved, so taht it can be separated from the crystals by filtration. 

From this experiment, I not only learnt how to crystallise using the seeding method, but I also learnt that by controlling the variables such as the evaporation rate, the size and shape of the crystals can be controlled. For example, crystals obtained by rapid cooling are small, many in number and have ill-defined shapes. On the other hand, crystals obtained by slow cooling are large, small in number and have a more refined shape, like the shape of a snowflake.