When magnesium ribbon burns in air to form magnesium oxide, it is a ………………… reaction.

Explanation

**Step-by-Step Solution:**

1. **Identify the Reactants and Products**:
- The reactant in this reaction is magnesium (Mg) in solid form, and the other reactant is oxygen (O₂) from the air, which is in gaseous form. The product formed is magnesium oxide (MgO), which is in solid form.

2. **Understand the Type of Reaction**:
- When magnesium burns in air, it reacts with oxygen to form magnesium oxide. This indicates that two substances (magnesium and oxygen) are combining to form a new substance (magnesium oxide).

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