Domain(s)
Chemistry
Keywords
ionic salts non-polarised liquids polarised liquids Solubility
Age Group
11 12 13 14 15 16 17 18
Expected time
2 periods
Languages
en
Downloads
To dissolve or not dissolve.PDF
beaker symbol

To dissolve or not dissolve

What’s the mystery? 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.
Domain(s) Chemistry
Subdomain keywords Solubility, ionic salts, polarised and non-polarised liquids.
Age group 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).
Safety/supervision 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.
Learning objectives 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.
Resources There are a number of YouTube videos showing how the addition of salt causes the two liquids to separate out.

There are many animations of the solubility of salt in water.

The polarity of liquids can be shown using a charged plastic comb or pen and a stream of water or the other liquid.