Practice Problem 1

Elemental phosphorus is often stored under water because it doesn't dissolve in water. Elemental phosphorus is very soluble in carbon disulfide, however. Explain why P₄ is soluble in CS₂ but not in water.

Answer

The structure of the P₄ molecule is shown below.

This molecule is a perfect example of a nonpolar solute. It is therefore more likely to be soluble in nonpolar solvents than in polar solvents such as water.

The Lewis structure of CS₂ suggests that this molecule is linear.

Thus, even it there is some separation of charge in the C=S double bond, the molecule would have no net dipole moment, because of its symmetry. The electronegativities of carbon (EN = 2.55) and sulfur (EN = 2.58), however, suggest that the C=S double bonds are almost perfectly covalent. CS₂ is therefore a nonpolar solvent, which should readily dissolve P₄.