Sodium hydrogencarbonate is heated .

Explanation

**Step-by-Step Solution:**

1. **Identify the Compound**: Sodium hydrogencarbonate is also known as baking soda, and its chemical formula is NaHCO₃.

2. **Understand the Reaction**: When sodium hydrogencarbonate is heated, it undergoes a decomposition reaction. This means it breaks down into simpler substances.

3. **Write the Decomposition Reaction**: The decomposition of sodium hydrogencarbonate can be represented by the following chemical equation:
\[
...