Samples of benzaldehyde and acetaldehyde can be distinguished by using 

Concept:

• Chemical Reactions of Aldehydes: Aldehydes can undergo various chemical reactions that are characteristic of their functional group. These reactions can be utilized to distinguish them from other organic compounds.
• Tollen's Test: A chemical reaction used to identify aldehydes. In this test, an aldehyde reduces the diamminesilver(I) ion to metallic silver, producing a silver mirror on the test tube.
• Fehling's Test: A chemical test used to differentiate between aldehyde and ketone functional groups. Aldehydes are oxidized by Fehling's solution, resulting in a brick-red precipitate of copper(I) oxide, while ketones do not react.
• 2,4-Dinitrophenylhydrazine (DNPH) Test: Both aldehydes and ketones react with DNPH to form yellow, orange, or red precipitates of the corresponding hydrazones, making it a test to distinguish carbonyl compounds, but not specifically between aldehydes and ketones.

Explanation:

To distinguish between samples of benzaldehyde (an aromatic aldehyde) and acetaldehyde (an aliphatic aldehyde), we need a test that responds differently to these two types of aldehydes. Examining the given options:

• 2,4-Dinitrophenylhydrazine (Option 1): Both benzaldehyde and acetaldehyde react with DNPH to form their respective hydrazone derivatives. Therefore, this test cannot distinguish between them.
• Tollen's reagent (Option 2): While this test can distinguish aldehydes from ketones by identifying the reducing nature of aldehydes, it does not differentiate between types of aldehydes.
• Mixture of Fehling solution A + B (Option 3): This is the correct option because Fehling's test specifically reacts with aliphatic aldehydes, like acetaldehyde, to give a positive result (a brick-red precipitate), whereas aromatic aldehydes like benzaldehyde do not react due to the lack of an alpha hydrogen atom adjacent to the carbonyl group.
• Zn-Hg/HCl (Option 4): This reagent is used for reductions (Clemmensen reduction), not specifically for distinguishing between aldehydes.

Therefore, the correct answer is  Mixture of Fehling solution A + B.

Conclusion:

So,  benzaldehyde and acetaldehyde can be distinguished by using Mixture of Fehling solution A + B.