Question

Synthesis of soap from lipids lab

1. Propose a reasonable explanation for why soaps form scums with ions like Ca2+ but not with Na+.

2. Compare the previous results with the test with tap water. Do you believe you have hard or soft water in your area?

Answer 1

I have always thought one aspect contributing to this behavior is the fact that the divalent Ca²⁺ and Mg²⁺ bind two soap fatty acid molecules, whereas Na¹⁺ and K⁺ as monovalent ions can only strongly bind one molecule.

As a result, the alkaline earth cations have two fatty acid "tails" that can 'glom' around them, shielding them better from the water molecules and making the resulting complex more stable, as well as less soluble.

On the other hand, since the alkali cations only have one fatty acid "tail" they're strongly associated with, that one tail can't wrap around nearly as completely, and so there is significantly greater accessibility of the solvent to the cation.