Define Action of heat on aluminium hydroxide.

Answer

### Step-by-Step Solution:

1. **Identify the Compound**: The compound in question is aluminum hydroxide, which has the chemical formula \( \text{Al(OH)}_3 \).

2. **Understand Thermal Decomposition**: When aluminum hydroxide is subjected to heat, it undergoes thermal decomposition. This means that it breaks down into simpler substances when heated.

3. **Write the Decomposition Reaction**: The thermal decomposition of aluminum hydroxide can be represented by the following chemical equation:
\[ 2 \text{Al(OH)}_3 \rightarrow \text{Al}_2\text{O}_3 + 3 \text{H}_2\text{O} \]
In this reaction, aluminum hydroxide decomposes to form aluminum oxide (\( \text{Al}_2\text{O}_3 \)) and water (\( \text{H}_2\text{O} \)).

4. **Balance the Reaction**: To ensure that the equation is balanced, we check the number of atoms of each element on both sides:
- On the left side, we have 2 aluminum (Al) atoms and 6 hydrogen (H) atoms, and 6 oxygen (O) atoms.
- On the right side, in \( \text{Al}_2\text{O}_3 \), there are 2 aluminum atoms and 3 oxygen atoms, and in \( 3 \text{H}_2\text{O} \), there are 3 hydrogen atoms and 3 oxygen atoms.
- Thus, the total on the right side is 2 Al, 6 H, and 6 O, confirming that the equation is balanced.

5. **Conclusion**: Therefore, the action of heat on aluminum hydroxide results in the formation of aluminum oxide and water.