A needle can float on clear water but sinks when some detergent is added to it. This happens because

Options

A. addition of detergent increases the density of water
B. addition of detergent reduces the density of water
C. addition of detergent reduces the surface tension of water
D. addition of detergent increases the surface tension of water.

Answer

To solve the question regarding why a needle can float on clear water but sinks when detergent is added, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Surface Tension**:
- Surface tension is the property of a liquid that allows it to resist an external force. It is caused by the cohesive forces between liquid molecules. In the case of water, these forces are quite strong.

2. **Behavior of the Needle on Water**:
- A needle can float on water due to surface tension. The surface tension of water creates a "skin" that supports the weight of the needle, allowing it to remain on the surface without sinking.

3. **Effect of Adding Detergent**:
- When detergent is added to water, it acts as a surfactant. Surfactants are substances that reduce the surface tension of liquids.

4. **Result of Reduced Surface Tension**:
- With the addition of detergent, the surface tension of the water decreases. This means that the "skin" that supports the needle becomes weaker.

5. **Conclusion**:
- As a result of the decreased surface tension, the needle can no longer be supported by the water's surface and sinks.

### Final Answer:
The correct explanation for why a needle sinks when detergent is added to water is: **Addition of detergent reduces the surface tension of water (Option C).**