Question

Choose incorrect combination :

• A. Heavy water : \(\text{D}_2\text{O}\)
• B. Hard water : Salts of magnesium and calcium
• C. Soap : Sodium or potassium salt of sulphonic acids
• D. Scum : Insoluble precipitate

Hint:

Always remember the composition difference:
- Soap = Sodium/Potassium salt of carboxylic (fatty) acids.
- Detergent = Sodium salt of sulphonic acids.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the incorrect pair or definition from the given chemical combinations.

Step 2: Key Formula or Approach:
Analyze the precise scientific definition of each term listed in the choices.

Step 3: Detailed Explanation:

• (A) Heavy water : \(\text{D}_2\text{O}\): Correct. Deuterium oxide is indeed known as heavy water.

• (B) Hard water : Salts of magnesium and calcium: Correct. Hardness in water is caused by the presence of dissolved bicarbonates, chlorides, and sulfates of calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)).

• (C) Soap : Sodium or potassium salt of sulphonic acids: Incorrect. Soaps are sodium or potassium salts of long-chain fatty acids (carboxylic acids). Synthetic detergents are the ones composed of sodium salts of sulphonic acids.

• (D) Scum : Insoluble precipitate: Correct. Scum is the white, curdy insoluble precipitate formed when soap reacts with the calcium and magnesium ions present in hard water.


Step 4: Final Answer:
The incorrect combination is (C).