Two glasses are half full of clear liquid. When a teaspoon of salt is added to both and stirred, it dissolves in one but not in the other. When the two liquids are mixed together, they dissolve in each other to form a solution. When salt is added, the solution separates into two layers.
Solubility, ionic salts, polarised and non-polarised liquids.
Junior secondary (11 to 15 years) or senior secondary (16 to 18 years), depending on the level of explanation.
Expected time for the mystery
Approximate time for teacher preparation: 15 min.Approximate time in classroom: one double lesson (70 to 90 min).
The chemicals used are safe. One liquid is flammable, so there should be no naked flames. The solvent should be disposed of as an organic solvent. Eye protection should be worn.Disclaimer: the authors of this teaching material will not be held responsible for any injury or damage to persons or properties that might occur in its use.
Preparation and listof materials
2 x 250ml beakers, water (200ml). Propan-2-ol (isopropyl alcohol) (200ml) (also known as rubbing alcohol), sodium chloride (3 x 5g), stirrer, ,easuring spoon, a large 400ml beaker, access to other common ionic salts and solvents and laboratory equipment.
The importance of the nature of the solvent in solubility. The solubility of ionic salts like sodium chloride in water compared to its solubility in an organic solvent. What factors affect solubility of a salt in a solvent.
There are a number of YouTube videos showing how the addition of salt causes the two liquids to separate out.