Which substance cannot be broken down by a chemical reaction?

Options

A. Ammonia
B. Argon
C. Methane
D. Water

Answer

To determine which substance cannot be broken down by a chemical reaction, we need to analyze the options provided.

### Step-by-Step Solution:

1. **Understanding Chemical Reactions**: A chemical reaction involves the transformation of reactants into products. Some substances can be broken down into simpler substances through chemical reactions, while others cannot.

2. **Analyzing the Options**:
- **Option 1: Ammonia (NH₃)**: Ammonia can be broken down into nitrogen (N₂) and hydrogen (H₂) through a chemical reaction. Therefore, it is not the correct answer.
- **Option 2: Argon (Ar)**: Argon is a noble gas and is known for its chemical inertness. It does not undergo chemical reactions under normal conditions and cannot be broken down into simpler substances. This makes it a strong candidate for the answer.
- **Option 3: Methane (CH₄)**: Methane can be broken down into carbon (C) and hydrogen (H₂) through combustion or other chemical reactions. Thus, it is not the correct answer.
- **Option 4: Water (H₂O)**: Water can dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻) through a chemical reaction. Therefore, it is also not the correct answer.

3. **Conclusion**: After analyzing all the options, we find that Argon (Ar) is the only substance that cannot be broken down by a chemical reaction. Therefore, the correct answer is **Argon**.

### Final Answer:
**Argon (Ar)** cannot be broken down by a chemical reaction.

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