Which of the following substances has the lowest boiling point?

Analyze the question: We need the substance with the lowest boiling point. That depends on the strength of the forces between its molecules.

Concept: Boiling point depends on intermolecular forces (and ions in solution):

• Hydrogen bonding is the strongest of the molecular forces, so it gives high boiling points.
• Van der Waals (London dispersion) forces are weak, so they give low boiling points.
• Dissolved solutes raise the boiling point of a solvent (boiling point elevation).

Step-by-step:

| Option | Substance | Main forces | Boiling point | |---|---|---|---| | A | Aqueous NaCl | Hydrogen bonding plus dissolved ions | Above 100 °C | | B | Ethanol | Hydrogen bonding (O-H) | about 78 °C | | C | Tetrachloromethane (CCl₄) | Only weak van der Waals forces (non-polar, symmetrical) | about 77 °C | | D | Water | Extensive hydrogen bonding | 100 °C |

Checking the close call: ethanol boils at about 78 °C and CCl₄ at about 76.7 °C. CCl₄ is slightly lower, so it has the lowest boiling point. Its heavy mass gives it some dispersion forces, but it has no hydrogen bonding, so it is still the lowest here. The reasoning on forces points the same way: it is the only substance with no hydrogen bonding.

Answer: C, Tetrachloromethane.

Exam tip: Rank boiling points by asking "Is there hydrogen bonding?" Substances with it (water, alcohols) boil higher than non-polar covalent compounds (CCl₄, hydrocarbons). Remember that solutions boil higher than pure solvents. Note that ethanol and CCl₄ are close in value, so rely on the type of intermolecular force rather than memorized numbers alone.