What will happen to the angle of contact on adding soluble impurity to a liquid?

Answer

To determine what happens to the angle of contact when soluble impurities are added to a liquid, we can follow these steps:

### Step-by-step Solution:

1. **Understanding Angle of Contact**:
- The angle of contact (or contact angle) is the angle formed between the tangent to the liquid surface and the solid surface at the point of contact. It is a measure of how well a liquid wets a solid surface.

2. **Identifying Forces at Play**:
- When a solid is immersed in a liquid, there are cohesive forces (attraction between liquid molecules) and adhesive forces (attraction between liquid molecules and solid surface) acting on the molecules at the interface.

3. **Effect of Soluble Impurities**:
- When soluble impurities are added to the liquid, they affect the surface tension of the liquid. Surface tension is the property of the liquid that causes it to behave as if its surface is covered with a stretched elastic membrane.

4. **Change in Surface Tension**:
- The addition of soluble impurities generally decreases the surface tension of the liquid. This is because the impurities disrupt the cohesive forces between the liquid molecules.

5. **Relation between Surface Tension and Contact Angle**:
- The contact angle is influenced by the balance between cohesive forces and adhesive forces. A higher surface tension leads to a larger contact angle, while a lower surface tension results in a smaller contact angle.

6. **Conclusion**:
- Since the addition of soluble impurities decreases the surface tension of the liquid, the angle of contact will also decrease. Thus, the contact angle will become smaller when soluble impurities are added.

### Final Answer:
The angle of contact will decrease upon adding soluble impurity to a liquid.

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