Question

Which statements is/are correct?
Statement-I : Soap does not form foam easily with hard water.
Statement-II : Hard soaps are sodium or potassium salt of sulphonic acids.
Statement-III : Deuterium oxide (\(\text{D}_2\text{O}\)) is called as hard water.

• A. Statement I only.
• B. Statement II and III only.
• C. Statement I and III only.
• D. All statements.

Hint:

Do not confuse "heavy water" (\(\text{D}_2\text{O}\)) with "hard water" (water containing \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) salts).
This is a very common trap in competitive chemistry exams.

Answer 1

The Correct Option is A

Step 1: Start with the easiest statement to verify, Statement III.
Deuterium oxide, $\text{D}_2\text{O}$, is the scientific name for heavy water, used in nuclear reactors, and has nothing to do with hardness of water, so Statement III is false.
Step 2: Check Statement II.
Hard soaps are actually sodium or potassium salts of long chain fatty acids like stearic or palmitic acid, not sulphonic acids. Sulphonic acid salts describe synthetic detergents instead, so Statement II is also false.
Step 3: Check Statement I with the chemistry of hard water.
Hard water carries dissolved calcium and magnesium ions, and soap reacts with them to form an insoluble scum: \[ 2\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \to (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca}\downarrow + 2\text{Na}^+ \] This scum formation uses up soap before it can produce lather, so soap genuinely struggles to foam in hard water, making Statement I true.
Step 4: Combine the results.
With only Statement I holding up under scrutiny, that is the sole correct statement. \[ \boxed{\text{Statement I only}} \]