Soaps and detergents

The governing concept is the reaction of soap with hard-water ions. Hard water contains dissolved calcium and magnesium ions. These ions react with the soap’s fatty-acid anions to form insoluble calcium or magnesium salts, which appear as a deposit called scum; therefore, option A is correct. Lather is the useful foam produced by soap, while an ester is an organic compound and a detergent is a different cleansing agent.

Saponification is the alkaline hydrolysis of an ester. When an ester reacts with sodium hydroxide, its alkoxy group becomes an alcohol and the acid part forms the sodium salt of a carboxylic acid. For a fat or oil, this sodium salt is soap. Hence option A is correct; carbon dioxide, hydrogen, diamond, and graphite are not the normal products of alkaline ester hydrolysis.

The governing concept is the amphiphilic structure of soap. A soap molecule has a long non-polar hydrocarbon tail that is hydrophobic, or attracted to oil and grease, and an ionic or polar head that is hydrophilic, or attracted to water. The tail embeds in oily dirt while the head remains in water, allowing rubbing and rinsing to remove the dirt. Thus option A is correct; the other choices incorrectly describe the whole molecule.

The governing concept is the reaction of soap with hardness-producing ions. Hard water contains dissolved calcium (Ca2+) and magnesium (Mg2+) ions. These ions react with the fatty-acid anions of soap to form insoluble calcium or magnesium salts, commonly called scum. Because part of the soap is consumed in this precipitate, less soap remains available to form lather, so foam decreases. Hence option A is correct.

The governing concept is resistance to hardness ions. In hard water, calcium and magnesium ions readily form insoluble salts with soap’s fatty-acid ions, producing scum and reducing lather. Synthetic detergents generally contain sulfonate or sulfate groups whose calcium and magnesium salts remain relatively soluble, so little precipitate forms. More detergent stays in solution to make micelles and remove grease. Therefore option A is correct; the other choices do not explain cleaning.

The governing concept is saponification, the alkaline hydrolysis of fats or oils. In this process, triglycerides react with a strong base such as sodium hydroxide or potassium hydroxide, producing glycerol and the salts of long-chain fatty acids; these salts are soap. Hydrogenation adds hydrogen to unsaturated compounds, fermentation uses microorganisms, and oxidation is a different chemical process. Hence, option A is correct.

Soap produces less lather in hard water because hard water contains dissolved calcium and magnesium ions. These ions react with the fatty-acid salts of soap and form an insoluble substance called scum, which is not useful for cleaning. Soft, distilled, and usually rainwater contain far fewer such ions, so soap can form lather more easily. Therefore, option A is correct.