Which of the following chemical equations correctly represents the formation of soap scum when soap reacts with calcium ions in hard water?

• A. 2C17H35COO^- (aq) + Ca^{2+} (aq) → (C17H35COO)_2Ca (s)
• B. C17H35COO^- (aq) + Na^+ (aq) → NaC17H35COO (s)
• C. C17H35COOH (aq) + Ca^{2+} (aq) → Ca(C17H35COOH)_2 (s)
• D. 2Na^+ (aq) + Ca^{2+} (aq) → CaNa_2 (s)

Answer: (A)

Verified step by step guidance

1. Identify the species involved in soap scum formation: soap molecules typically exist as carboxylate ions (RCOO^-) in aqueous solution, and hard water contains calcium ions (Ca^{2+}).
2. Recall that soap scum forms when calcium ions react with two soap anions to form an insoluble calcium soap salt, which precipitates out of solution.
3. Write the balanced chemical equation showing two soap anions (2 RCOO^-) reacting with one calcium ion (Ca^{2+}) to form the insoluble compound (RCOO)_2Ca (s).
4. Check the other options: sodium ions (Na^+) do not form insoluble precipitates with soap anions, and the neutral acid form (RCOOH) does not typically react with calcium ions to form soap scum.
5. Conclude that the correct equation is the one showing 2 C17H35COO^- (aq) + Ca^{2+} (aq) → (C17H35COO)_2Ca (s), representing the formation of insoluble soap scum.