Which one of the following pairs of substances on reaction will not not evolve `H_2` gas?

Options

A. Iron and steam
B. Iron and `H_(2)SO_(4)` (aqueous)
C. Copper and HCl (aqueous)
D. Sodium and ethyl alcohol

Answer

To determine which pair of substances will not evolve hydrogen gas (H₂) upon reaction, we need to consider the reactivity of the metals involved and their reduction potentials.

### Step-by-Step Solution:

1. **Understand the Concept of Hydrogen Evolution**:
Hydrogen gas is evolved when a metal reacts with an acid. The ability of a metal to displace hydrogen from acids depends on its position in the electrochemical series.

2. **Review the Electrochemical Series**:
- Metals with a more negative reduction potential than hydrogen can displace hydrogen ions (H⁺) from acids, resulting in the evolution of H₂ gas.
- Conversely, metals with a positive reduction potential cannot displace hydrogen and will not evolve H₂ gas.

3. **Identify the Metals in the Question**:
- The metals mentioned are iron (Fe), sodium (Na), and copper (Cu).
- Their standard reduction potentials are approximately:
- Sodium (Na): -2.71 V
- Iron (Fe): -0.44 V
- Copper (Cu): +0.34 V

4. **Analyze Each Metal's Ability to Evolve H₂**:
- **Sodium (Na)**: With a reduction potential of -2.71 V, sodium can easily displace hydrogen from acids, thus evolving H₂ gas.
- **Iron (Fe)**: With a reduction potential of -0.44 V, iron can also displace hydrogen from acids, leading to H₂ evolution.
- **Copper (Cu)**: With a reduction potential of +0.34 V, copper cannot displace hydrogen from acids, meaning it will not evolve H₂ gas.

5. **Conclusion**:
Based on the analysis, the pair that will not evolve H₂ gas upon reaction is the one that includes copper. Therefore, the answer is the pair that contains copper.

### Final Answer:
The pair of substances that will not evolve H₂ gas is the one that includes copper.

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