When HCHO reacts with water, which substance does it form?

When formaldehyde (HCHO) reacts with water, it undergoes a hydration reaction to form methylene glycol, which is also known as formaldehyde hydrate. In this reaction, water molecules add to the carbonyl group of the aldehyde, resulting in the formation of a stable diol. Methylene glycol can exist in equilibrium with formaldehyde, especially in aqueous solutions, but when HCHO is dissolved in water, it predominantly exists as methylene glycol.

The other options do not represent the direct product of HCHO reacting with water. Ethanol is a different compound altogether, not related to the hydration of formaldehyde. An aldehyde refers to a broader class of compounds that includes formaldehyde itself, but it is not a product of the hydration reaction. Formic acid, while related to formaldehyde in terms of chemical structure, is produced through different reactions and is not formed directly by the addition of water to HCHO. Thus, the formation of methylene glycol is the correct outcome of this reaction.