Passage VI

A teacher showed 2 beakers to a chemistry class. One beaker contained mineral oil and the other contained H₂O. Solid iodine was added to each beaker. The solid iodine quickly dissolved in the mineral oil, which turned pink. The solid stayed at the bottom of the H₂O, which remained colorless. The teacher asked 3 students to explain these results.

Student 1

Mineral oil is made up of very large molecules, while H₂O is made up of very small molecules. Thus, the spaces between mineral oil molecules are much larger than the spaces between H₂O molecules (see figure).

For a solid to dissolve in a solvent, its molecules must squeeze in between solvent molecules. The molecular mass (the mass of 1 molecule in atomic mass units, amu) of a substance is a good indicator of molecular size, so if a solid has a molecular mass less than that of a solvent, it will most likely dissolve in the solvent. Therefore, the iodine dissolved in the mineral oil, but not in the H₂O.

Student 2

An H₂O molecule is polar because it has a region of positive charge and a region of negative charge. Nonpolar molecules, like those that make up mineral oil, do not have differently charged regions. Polar molecules are attracted to each other, and nonpolar molecules are attracted to each other, but polar molecules repel nonpolar molecules. Thus, polar solids dissolve in polar solvents, and nonpolar solids dissolve in nonpolar solvents. Therefore, the iodine dissolved in the mineral oil, but not in the H₂O. Molecular size is unrelated to solubility.

Student 3

Student 2 is correct, but with one exception. Nonpolar molecules are actually strongly attracted to polar molecules. The reason that nonpolar substances don' t dissolve in polar substances is that polar molecules are too strongly attracted to each other to allow nonpolar molecules to come between them. If a drop of mineral oil is placed on the surface of a pan full of H₂O, the drop will spread to form the thinnest layer possible in order to maximize contact with H₂O molecules.