In which of the following substances , surface tension increases with increase in temperature ?

Options

A. Copper
B. Molten copper
C. Iron
D. Molten iron

Answer

To solve the question regarding which substance has an increase in surface tension with an increase in temperature, we can analyze the properties of various substances, particularly focusing on molten copper.

### Step-by-Step Solution:

1. **Understanding Surface Tension**:
- Surface tension is the property of a liquid that causes it to behave as if its surface is covered with a stretched elastic membrane. It is primarily influenced by the cohesive forces between molecules.

2. **Effect of Temperature on Surface Tension**:
- Generally, for most liquids, surface tension decreases with an increase in temperature. This is because as temperature rises, the kinetic energy of the molecules increases, which reduces the cohesive forces between them.

3. **Analyzing Molten Copper**:
- In the case of molten copper, the situation is different. As the temperature of molten copper increases, the amount of soluble impurities in the liquid also increases.
- These impurities can enhance the cohesive forces among the copper atoms, leading to an increase in surface tension.

4. **Comparison with Other Substances**:
- For other substances like molten iron, the cohesive forces may initially increase with temperature, but eventually, they decrease as the temperature continues to rise. This results in a net decrease in surface tension.
- Therefore, molten copper is unique in that its surface tension increases with temperature due to the effect of soluble impurities.

5. **Conclusion**:
- Based on the analysis, the substance in which surface tension increases with an increase in temperature is **molten copper**.