(iii) Substances which convert infusible impurities present in ores into fusible substances during smeting are called slag.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Terms:**
- In metallurgy, ores are naturally occurring solid materials from which a metal or valuable mineral can be extracted. However, these ores often contain impurities known as gangue.

2. **Identifying Impurities:**
- Gangue refers to the unwanted materials or impurities that are present in the ore. These impurities are typically infusible, meaning they do not melt easily.

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