A chemist is tasked with separating a complex mixture containing three solid components: insoluble fine sand (
\(\text{SiO}_2\)
), sodium chloride (
\(\text{NaCl}\)
), and copper(II) sulfate (
\(\text{CuSO}_4\)
). The solubilities in
\(100 \text{g}\)
of water are:
\(\text{NaCl}\)
,
\(\sim 36 \text{g}\)
at
\(0^{\circ}\text{C}\)
and
\(\sim 40 \text{g}\)
at
\(100^{\circ}\text{C}\)
;
\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\)
,
\(\sim 32 \text{g}\)
at
\(0^{\circ}\text{C}\)
and
\(\sim 200 \text{g}\)
at
\(100^{\circ}\text{C}\)
. The goal is to obtain pure, hydrated copper(II) sulfate crystals (
\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\)
) using the following proposed steps:
I. Dissolve the mixture in a moderate amount of hot distilled water.
II. Filter the resulting mixture to remove the sand.
III. Evaporate the filtrate to reduce the volume significantly, and then cool the concentrated solution slowly to obtain solid residue X.
IV. Recrystallize solid residue X to purify it.
Which modification to Step III would make the overall separation procedure most efficient for obtaining pure
\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\)
crystals, minimizing the contamination in residue X?