Which of the following bonds are broken when ethanol is boiled? I. Covalent bonds II. Ionic bonds III. Hydrogen bonds

Analyze the question: Boiling is a physical change (liquid to gas). We need to identify which bonds or forces are overcome during this change.

Concept:

• Boiling only separates molecules from one another. It overcomes intermolecular forces, not the bonds inside the molecules.
• Ethanol (C₂H₅OH) is a molecular (covalent) compound. Its molecules are held together in the liquid by hydrogen bonds, which form between the O-H hydrogen of one molecule and the oxygen of another. Weaker van der Waals forces are also present.
• The covalent bonds within each molecule (C-C, C-H, C-O, O-H) are strong and stay intact. Breaking them would be a chemical change, such as decomposition or combustion.
• Ethanol has no ionic bonds, so II is out.

Solution:

• I. Covalent bonds: not broken, because they are intramolecular and the molecule remains C₂H₅OH as a gas.
• II. Ionic bonds: ethanol has none.
• III. Hydrogen bonds: broken, as they are the intermolecular attractions holding the liquid together.

Equation: C₂H₅OH(l) → C₂H₅OH(g). The molecules stay the same, and only the attractions between them are overcome.

Answer: Only hydrogen bonds are broken, so the answer is C. III only.

Tip: For any boiling or melting question on a molecular substance, remember that physical change means intermolecular forces break, while chemical change means intramolecular bonds break. Boiling water and boiling ethanol both break hydrogen bonds, not the bonds inside the molecules.