Passage VII

Partial freezing is a water purification technique based on the fact that as a salt solution freezes, salts concentrate in the liquid phase. Thus, the solid that forms has a lower salt concentration than the remaining liquid.

Students measured the conductivity (ease of flow of electrical current through a substance) of aqueous solutions to study the partial freezing.

Experiment 1

A 100 g solution was made by dissolving 0.5 g of NaCl in distilled H₂O. The conductivity of the solution was measured in millisiemens (mS) per cm at 13°C. The procedure was repeated for solutions with different NaCI concentrations (see Table 1).

Experiment 2

A beaker containing 20 mL of Solution 1 was stirred in a cold bath at -20°C. When approximately 25% of the solution was frozen, its liquid and solid portions were separated. The solid portion was then completely thawed, and both the solid and liquid portions were allowed to come to 13°C. The conductivity of each of the 2 portions was then measured. Additional trials were done by repeating this procedure, except that the liquid and solid portions were separated when the solution was approximately 50% and 75% frozen, respectively. These same procedures were then used for Solutions 2-6 (see Table 2).