Commercial soaps often contain additives such as fragrances, dyes, and antimicrobial agents; however, these do not improve the cleansing power of the soap, only the appearance and consumer appeal. Some bars of soap float. This is achieved by incorporating air bubbles into the bar as it is molded.
The cleansing power of soap is determined by its solubility. To clean effectively, soap must form suds readily in water. Soaps made from fatty acids with short carbon chains are more water soluble than those with long ones.
You may have learned by experience that soap does not clean well in hard water. Water is considered hard when it contains high concentrations of magnesium, calcium, and iron ions. These cations replace the sodium or potassium ions in soap and form insoluble precipitates commonly referred to as soap scum.
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The formation of soap scum removes available soap molecules from the solution, decreasing its ability to clean. In addition, soap scum can form films of residue in bathtubs and shower stalls. Below is the reaction for the formation of soap scum, where ‘R’ is shorthand for the long carbon chain of the soap molecule.
Many commercial soaps contain additional chelating agents that form complexes with metal ions in hard water and increase the cleansing efficiency of a product. However, chelating agents are accessible, so experienced soap-makers can add them to the formula and produce effective self-made soaps useful with hard water.