Magnesium reacts vigourously with hot water to form magnesium hydroxide.

Answer

To understand the reaction of magnesium with hot water to form magnesium hydroxide, we can break it down into a series of steps:

### Step-by-Step Solution:

1. **Identify the Reactants**:
- The reactants in this reaction are magnesium (Mg) and hot water (H₂O).

2. **Understand Magnesium's Properties**:
- Magnesium is a metal with the atomic number 12. Its electronic configuration is 1s² 2s² 2p⁶ 3s². This means it has two electrons in its outermost shell (the 3s orbital).

3. **Behavior with Cold Water**:
- When magnesium is placed in cold water, there is no significant reaction because the energy required to remove the two electrons from the 3s orbital is not provided.

4. **Effect of Heat**:
- When the water is heated (to boiling), the increased temperature provides enough energy for magnesium to react. The heat helps overcome the energy barrier for the removal of the two valence electrons.

5. **Chemical Reaction**:
- The reaction can be represented by the following chemical equation:
\[ \text{Mg (s)} + 2 \text{H}_2\text{O (l)} \rightarrow \text{Mg(OH)}_2 (aq) + \text{H}_2 (g) \]
- Here, magnesium reacts with water to form magnesium hydroxide (Mg(OH)₂) and hydrogen gas (H₂).

6. **Observation**:
- During the reaction, hydrogen gas is evolved, which can be observed as bubbles forming in the solution.

7. **Conclusion**:
- Thus, magnesium reacts vigorously with hot water to produce magnesium hydroxide and hydrogen gas.