Decomposition of ` CaCO_3` is a reversible reaction when carried out in an open kiln.

Answer

To determine whether the statement "Decomposition of CaCO₃ is a reversible reaction when carried out in an open kiln" is true or false, we can analyze the reaction step by step.

### Step 1: Understand the Decomposition Reaction
The decomposition of calcium carbonate (CaCO₃) can be represented by the following chemical equation:
\[ \text{CaCO}_3 (s) \rightarrow \text{CaO} (s) + \text{CO}_2 (g) \]

### Step 2: Identify the Nature of the Reaction
This reaction produces calcium oxide (CaO) as a solid and carbon dioxide (CO₂) as a gas.

### Step 3: Consider the Conditions of the Reaction
- **Open Kiln**: In an open kiln, the carbon dioxide gas produced during the decomposition can escape into the atmosphere.
- **Closed Vessel**: If the reaction were to occur in a closed vessel, the carbon dioxide would remain in the system.

### Step 4: Analyze Reversibility
- In a **closed vessel**, the products (CaO and CO₂) can react to form the reactant (CaCO₃) again, establishing an equilibrium:
\[ \text{CaO} (s) + \text{CO}_2 (g) \rightleftharpoons \text{CaCO}_3 (s) \]
This indicates that the reaction is reversible.

- In an **open kiln**, once CO₂ is produced, it escapes and is no longer available to react with CaO to form CaCO₃. Therefore, the reaction cannot reach equilibrium, and the reverse reaction cannot occur.

### Step 5: Conclusion
Since the reaction cannot achieve equilibrium in an open kiln due to the escape of CO₂, the statement is **false**. The decomposition of CaCO₃ is not a reversible reaction when carried out in an open kiln.

### Final Answer
The statement is **false**.

---