Why is formaldehyde soluble in water? Is it an ionic compound?
Well, it's not. It's a molecule. The intermolecular interaction it exhibits is dipole-dipole interactions.
Formaldehyde is a one-carbon aldehyde:
The oxygen is more electronegative than the carbon, so the oxygen holds the partial negative charge. What must the partial charge be on carbon? What does this tell you about the polarity of the molecule? Draw the dipole arrow that represents this.
The hydrogens are not directly bonded to oxygen, so this molecule does not exhibit hydrogen-bonding with itself. Can it hydrogen-bond with water? Why?