At ordinary temperature `Cl_2` reacts with 

Options

A. `O_2`
B. `Na`
C. `He`
D. `Cu`

Answer

To solve the question "At ordinary temperature, Cl2 reacts with", we will analyze the reactivity of chlorine gas (Cl2) with the given substances: O2, Na, helium, and Cu.

### Step-by-Step Solution:

1. **Identify the Reactants**: The reactants we are considering are Cl2 (chlorine gas), O2 (oxygen gas), Na (sodium), helium, and Cu (copper).

2. **Reactivity of Cl2 with O2**: Chlorine gas does not react with oxygen at ordinary temperatures. The reaction between Cl2 and O2 requires higher temperatures or specific conditions to occur.

3. **Reactivity of Cl2 with Sodium (Na)**: Chlorine reacts vigorously with sodium at ordinary temperatures. The reaction can be represented as:
\[ \text{Cl}_2 + 2 \text{Na} \rightarrow 2 \text{NaCl} \]
This reaction produces sodium chloride (NaCl).

4. **Reactivity of Cl2 with Helium**: Helium is an inert gas and does not react with chlorine under ordinary conditions. Therefore, no reaction occurs.

5. **Reactivity of Cl2 with Copper (Cu)**: Chlorine does not react with copper at ordinary temperatures. Copper can react with chlorine, but this typically requires elevated temperatures.

6. **Conclusion**: Among the given options, Cl2 reacts with sodium (Na) at ordinary temperatures to form sodium chloride (NaCl). Therefore, the correct answer is sodium (Na).

### Final Answer:
At ordinary temperature, Cl2 reacts with sodium (Na) to form NaCl.

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