Identify the correct statements among the following. (a) Formaldehyde can react with both \(-\mathrm{NH}_{2}\) groups in semicarbazide. (b) In mixed Cannizzaro reaction involving HCHO, it is always the intermediate formed from HCHO which transfers its hydride to the other aldehyde. (c) Benzaldehyde reduces both Tollens' reagent and Fehling's solution. (d) In Fehling's solution, Cu(II) ions are kept in soluble form of a complex in alkaline medium.

Formaldehyde, or \(\mathrm{HCHO}\), can react with a variety of molecules containing nucleophilic sites, such as amine groups (\(-\mathrm{NH}_{2}\)). Semicarbazide contains two amine groups that can potentially react with formaldehyde. Let us evaluate if formaldehyde indeed can react with both these groups. Semicarbazide has the following structure: $$\mathrm{HN-C(=NH)-NH_2}$$ A reaction between formaldehyde and an amine group involves the formation of an imine or Schiff base. Analyzing the structure of semicarbazide, we observe that it can indeed form an imine with formaldehyde at both its amine sites. Therefore, statement (a) is correct.

The mixed Cannizzaro reaction involving HCHO (formaldehyde) refers to the reaction between formaldehyde and another aldehyde without an alpha hydrogen atom. The reaction occurs via hydride transfer between the two aldehyde molecules. Let's assess if it's true that HCHO is always the intermediate which transfers its hydride to the other aldehyde. Formaldehyde does not have an alpha hydrogen atom, and in the mixed Cannizzaro reaction, it typically serves as a hydride donor. As the reaction progresses, formaldehyde is oxidized into formic acid, transferring a hydride to the other aldehyde, reducing it to an alcohol. Therefore, statement (b) is correct.

Benzaldehyde is an aromatic aldehyde that can potentially participate in redox reactions. In this statement, we need to evaluate if benzaldehyde can reduce both Tollens' reagent and Fehling's solution. Tollens' reagent consists of a silver-ammonia complex which can oxidize aldehydes to carboxylic acids. Benzaldehyde, however, cannot be oxidized by Tollens' reagent because of the presence of an aromatic ring that prevents the standard oxidation process from taking place. Fehling's solution is a mixture of copper(II) complex and sodium hydroxide. The Cu(II) ion in the solution can oxidize aldehydes to carboxylic acids, with the Cu(II) ion being reduced to Cu(I) ion. Like with Tollens' reagent, benzaldehyde will not react with Fehling's solution because the aromatic ring prevents oxidation. Since both cases fail to oxidize benzaldehyde, statement (c) is incorrect.

Fehling's solution is a mixture of copper(II) ions as a copper(II) complex and an alkaline medium. We need to assess if the Cu(II) ions are kept in a soluble form as a complex. Recalling the composition of Fehling's solution, it consists of copper(II) sulfate, sodium potassium tartrate, and sodium hydroxide. The copper(II) ion forms a complex with the tartrate anion in the alkaline medium, maintaining its solubility. Consequently, statement (d) is correct. To summarize: - Statement (a) is correct. - Statement (b) is correct. - Statement (c) is incorrect. - Statement (d) is correct.