Which of the following substances does not form a curdy precipitate when it is added to hard water?

Options

A. potassium salt of fatty acid
B. sodium salt of fatty acid
C. sodium lauryl sulphate
D. none of the above

Explanation

To determine which substance does not form a curdy precipitate (also known as scum) when added to hard water, we need to analyze the options provided:

1. **Understanding Hard Water**: Hard water contains high levels of calcium and magnesium ions. When soap is added to hard water, it reacts with these ions to form an insoluble precipitate known as scum or curdy precipitate.

2. **Identifying the Options**:
- **Option A: Potassium salt of fatty acid** - This is a type of soap. Soaps react with calcium and magnesium ions in hard water and form scum.
- **Option B: Sodium salt of fatty acid** - This is also a type of soap. Like potassium salt, it will also react with hard water to form scum.
- **Option C: Sodium lauryl sulfate (SLS)** - This is a detergent. Detergents are designed to work effectively in hard water and do not form scum.
- **Option D: None of these** - This option suggests that all the substances listed form scum, which we need to evaluate.

3. **Evaluating Each Option**:
- **Potassium salt of fatty acid**: Forms scum in hard water.
- **Sodium salt of fatty acid**: Forms scum in hard water.
- **Sodium lauryl sulfate**: Does NOT form scum in hard water as it is a detergent.

4. **Conclusion**: The substance that does not form a curdy precipitate when added to hard water is **Sodium lauryl sulfate (Option C)**.

### Final Answer:
The correct answer is **Option C: Sodium lauryl sulfate**.