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 regarding what a redox reaction always involves, we will analyze the definition and characteristics of redox reactions step by step.

### Step-by-Step Solution:

1. **Understanding Redox Reactions**:
- A redox (reduction-oxidation) reaction is defined as a chemical reaction in which both oxidation and reduction occur simultaneously.

2. **Defining Oxidation and Reduction**:
- **Oxidation**: This is the process where an element or compound loses electrons, resulting in an increase in oxidation state.
- **Reduction**: This is the process where an element or compound gains electrons, leading to a decrease in oxidation state.

3. **Change in Oxidation Number**:
- Since oxidation involves an increase in oxidation state and reduction involves a decrease in oxidation state, it is evident that there must be a change in oxidation numbers for both processes to occur. Therefore, a redox reaction always involves a change in oxidation number.

4. **Evaluating the Options**:
- **Option 1**: A change in oxidation number - This is correct because it is a fundamental characteristic of redox reactions.
- **Option 2**: A change in phase - This is not a requirement for redox reactions and can occur independently.
- **Option 3**: Transfer of protons - This is incorrect as redox reactions involve the transfer of electrons, not protons.
- **Option 4**: Formation of ions - While ions can form during redox reactions, it is not a compulsory condition as oxidation and reduction can occur without ion formation.

5. **Conclusion**:
- The correct answer is **Option 1: A change in oxidation number**.