Correct Option
The correct option is Calcium and magnesium salts form insoluble scum with soap.
Explanation
Soaps are sodium or potassium salts of long-chain fatty acids (carboxylic acids). Hard water is characterized by the presence of dissolved calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions, typically as bicarbonates, chlorides, or sulfates.
Analysis of the Reaction
The requirement for a larger quantity of soap in hard water is due to a specific chemical precipitation reaction:
- Formation of Insoluble Salts (Scum): When soap is added to hard water, the calcium and magnesium ions react with the soap molecules. They displace the sodium or potassium ions to form insoluble calcium or magnesium salts of the fatty acids. This insoluble precipitate is known as scum.
- Wastage of Soap: The soap molecules are consumed in this reaction to form scum rather than reducing surface tension to form lather.
- Delayed Lathering: Lather can only be produced once all the calcium and magnesium ions present in the water have been precipitated out. Consequently, a significant portion of the soap is "wasted" in softening the water before any cleansing action or lathering can begin.
Options Soap evaporates faster in hard water, Soap dissolves too rapidly in hard water, and Hard water always has higher pH than soaps are incorrect because the primary hindrance to lather formation is the chemical formation of insoluble precipitates, not evaporation, dissolution speed, or pH differences.
Key Takeaway: Hard water consumes soap by reacting with it to form insoluble calcium and magnesium salts (scum), preventing the formation of lather until the interfering ions are removed.