To determine which statement is not correct among the given options, let's analyze each statement one by one.
### Step 1: Analyze the first statement
**Statement 1:** HNO2 can act both as a reducing agent as well as an oxidizing agent, but HNO3 can act only as an oxidizing agent.
- **Analysis:**
- HNO2 (Nitrous acid) can indeed act as both a reducing agent and an oxidizing agent because it can change its oxidation state from +3 to +5 (oxidation) and from +3 to -3 (reduction).
- HNO3 (Nitric acid) has nitrogen in the +5 oxidation state, which is its maximum oxidation state. Therefore, it cannot act as a reducing agent.
**Conclusion:** This statement is correct.
### Step 2: Analyze the second statement
**Statement 2:** The oxidation number of phosphorus can vary from -3 to +5.
- **Analysis:**
- Phosphorus can have oxidation states ranging from -3 (in phosphides) to +5 (in phosphoric acid).
**Conclusion:** This statement is correct.
### Step 3: Analyze the third statement
**Statement 3:** The reaction between NaOH and H2SO4 is a redox reaction.
- **Analysis:**
- The reaction between sodium hydroxide (NaOH) and sulfuric acid (H2SO4) is a neutralization reaction, producing sodium sulfate (Na2SO4) and water (H2O).
- In this reaction, there is no change in the oxidation states of the elements involved. Sodium remains +1, hydrogen remains +1, sulfur remains +6, and oxygen remains -2 throughout the reaction.
**Conclusion:** This statement is incorrect.
### Step 4: Analyze the fourth statement
**Statement 4:** Oxidation numbers can have positive, negative, zero, and fractional values.
- **Analysis:**
- Oxidation numbers can indeed be positive (e.g., +1 in Na+), negative (e.g., -2 in O2-), zero (e.g., in elemental forms like P4), and fractional (e.g., in compounds like Fe3O4 where the average oxidation state of iron is 8/3).
**Conclusion:** This statement is correct.
### Final Conclusion
The statement that is not correct is **Statement 3**, which claims that the reaction between NaOH and H2SO4 is a redox reaction.
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