Which of the following statements about functional groups in organic molecules is correct?

• A. The presence of a functional group does not affect the solubility of an organic compound.
• B. Functional groups are always composed solely of carbon $C$ and hydrogen $H$ atoms.
• C. Functional groups determine the chemical reactivity and physical properties of organic molecules.
• D. All organic molecules contain only one functional group.

Answer: (C)

Verified step by step guidance

1. Understand what a functional group is: a specific group of atoms within molecules that is responsible for the characteristic chemical reactions of those molecules.
2. Recall that functional groups often contain atoms other than carbon and hydrogen, such as oxygen, nitrogen, sulfur, or halogens, which influence the molecule's properties.
3. Recognize that functional groups significantly affect both the chemical reactivity and physical properties (like solubility, boiling point, and polarity) of organic compounds.
4. Note that organic molecules can contain zero, one, or multiple functional groups, so the statement that all organic molecules contain only one functional group is incorrect.
5. Conclude that the correct statement is the one indicating that functional groups determine the chemical reactivity and physical properties of organic molecules.