Slag is a product formed during smelting by combination of flux and impurities.

Answer

### Step-by-Step Text Solution:

1. **Understanding Slag Formation**: Slag is a byproduct formed during the smelting process, which involves the extraction of metal from its ore. It is created when flux combines with impurities present in the ore.

2. **Definition of Key Terms**:
- **Flux**: A substance added to the ore to facilitate the removal of impurities. It helps in the formation of slag.
- **Gangue**: The impurities that are present in the ore. These can be acidic or basic in nature.

3. **Process of Slag Formation**:
- When ore is heated during smelting, the flux interacts with the gangue.
- If the gangue is acidic (for example, silica), a basic flux (like calcium oxide) is added. The reaction between these substances leads to the formation of slag, which in this case would be calcium silicate.

4. **Examples of Flux and Gangue**:
- **Acidic Gangue**: Silica (SiO₂)
- **Basic Flux**: Calcium oxide (CaO)
- The combination of these two results in the formation of slag (calcium silicate, CaSiO₃).

5. **Nature of Flux and Gangue**:
- The flux must have an opposite nature to the gangue for effective slag formation. If the gangue is acidic, a basic flux is used, and vice versa.

6. **Conclusion**: Slag is essentially a combination of the flux and the gangue, which helps in purifying the metal during the smelting process.