Assertion : Acetaldehyde undergoes aldol condensation with dil. NaOH.
Reason Aldehydes which donot contain a-hydrogen undergo aldol condensation.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understanding the Assertion
The assertion states that acetaldehyde undergoes aldol condensation with dilute NaOH.

**Explanation**:
- Acetaldehyde (CH3CHO) is an aldehyde that contains an alpha-hydrogen atom (the hydrogen atom attached to the carbon adjacent to the carbonyl group).
- Aldol condensation involves the reaction of two aldehydes or ketones that have alpha-hydrogens, leading to the formation of a β-hydroxy aldehyde or ketone, which can further dehydrate to form an α,β-unsaturated carbonyl compound.

### Step 2: Understanding the Reason
The reason states that aldehydes which do not contain alpha-hydrogens undergo aldol condensation.

**Explanation**:
- This statement is incorrect. Aldol condensation typically occurs in aldehydes and ketones that do have alpha-hydrogens. Aldehydes without alpha-hydrogens (like formaldehyde) cannot undergo aldol condensation because they lack the necessary hydrogen to form the enolate ion, which is essential for the reaction.

### Conclusion
- The assertion is true: Acetaldehyde does undergo aldol condensation because it has alpha-hydrogens.
- The reason is false: Aldehydes without alpha-hydrogens do not undergo aldol condensation.

### Final Answer
- The assertion is true, but the reason is false.