A redox reaction always involves

Options

A. A change in oxidation number.
B. A change in phase
C. The transfer of protons
D. the formation of ions

Answer

To solve the question "A redox reaction always involves," we need to analyze the characteristics of redox reactions.

### Step-by-Step Solution:

1. **Understanding Redox Reactions**:
- A redox (reduction-oxidation) reaction is a type of chemical reaction that involves the transfer of electrons between two species.

2. **Identifying Key Components**:
- In a redox reaction, one species undergoes oxidation (loses electrons) and another undergoes reduction (gains electrons). This electron transfer is fundamental to the definition of redox reactions.

3. **Change in Oxidation States**:
- Since there is a transfer of electrons, the oxidation states of the reactants must change. This means that the oxidation number of at least one element in the reaction will increase (oxidation) and the oxidation number of another will decrease (reduction).

4. **Evaluating Other Options**:
- The other options provided in the question (change in phase, transfer of protons, and formation of ions) are not necessary for a redox reaction to occur.
- Change in phase is not a requirement since redox reactions can occur in the same phase (solid, liquid, or gas).
- The transfer of protons is not a defining characteristic of redox reactions; they specifically involve electron transfer.
- Formation of ions is also not a requirement, as redox reactions can occur without the formation of ions.

5. **Conclusion**:
- Therefore, the correct answer is that a redox reaction always involves a change in oxidation number due to the transfer of electrons.

### Final Answer:
A redox reaction always involves a change in oxidation number.

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