The accidental mixing of two crystalline white powders, table salt (sodium chloride) and table sugar (sucrose), presents a classic chemical separation challenge. Salt is an ionic compound composed of charged ions. Sugar is a large, polar covalent molecule. The distinct physical and chemical differences between these two substances, particularly their behavior when exposed to different liquids, provides the foundation for their separation.
The Principle of Selective Dissolution
Separating a mixture of salt and sugar relies on exploiting the principle of selective dissolution, which leverages the adage “like dissolves like.” Water is a highly polar solvent, making it an excellent solvent for both the ionic salt and the polar covalent sugar. However, solubility varies significantly between different solvents based on the molecular forces involved. A specialized separation requires a solvent that dissolves one component significantly more than the other, creating a substantial difference in their solubility.
Separating the Mixture Using Water
The most accessible method involves using water as the solvent, which dissolves both components. The mixed powder is added to warm water and stirred until the solids are entirely dissolved, creating a homogeneous solution. Using warm water increases solubility and speeds up the process.
To recover the solids, the solution must first undergo filtration to remove any insoluble impurities, such as dust. Although the dissolved salt and sugar pass through the filter, this step ensures the final recovered materials are clean.
The final step is evaporation, which involves gently heating the clear, filtered solution to drive off the water vapor. As the water evaporates, the concentration of dissolved solids increases until the solution becomes supersaturated. The salt and sugar will then begin to crystallize out of the solution. Since both compounds were dissolved in the same solvent, this method ultimately yields a mixture of the two compounds once again.
Using Non-Aqueous Solvents for Purity
While water leads to a mixed recovery, using a non-aqueous solvent like ethanol provides a pathway to higher purity separation. Ethanol (rubbing alcohol) is a less polar solvent than water, which changes the solubility dynamics dramatically. Sucrose is moderately soluble in hot ethanol, but sodium chloride is nearly insoluble.
The process begins by adding the mixed powder to hot ethanol, which dissolves the sugar while leaving the salt largely undissolved as a solid residue. The ionic forces holding the salt crystal together are too strong for the less polar ethanol molecules to overcome, causing the salt to remain solid. This solid salt is then easily separated from the sugar-ethanol solution using filtration, leaving the purified salt behind.
The ethanol solution, now containing the dissolved sugar, is then heated gently to evaporate the solvent. Ethanol has a much lower boiling point than water, meaning evaporation happens faster and requires less heat. The rapid removal of the solvent leaves behind the solid, recovered sugar crystals, resulting in two separate, relatively pure compounds.
Assessing the Success of Separation
Assessing the success and purity of the recovered materials is an important final step. The most immediate check is visual inspection, looking for distinct crystal shapes—salt crystals are typically cubic, while sugar crystals are monoclinic hexagons. A simple taste test can also quickly indicate cross-contamination. For a more rigorous assessment, one can calculate the percentage yield by comparing the mass of the recovered materials to the initial mass of the mixture; loss in mass during transfer often reduces the yield. Checking the melting point of the recovered sugar can also reveal purity, as the presence of salt would alter its decomposition profile.