Which of the following substances would you expect to have the lowest boiling point?

• A. NH3 (ammonia)
• B. CH4 (methane)
• C. H2O (water)
• D. HF (hydrogen fluoride)

Answer: (B)

Verified step by step guidance

1. Step 1: Understand that boiling point depends on the strength of intermolecular forces present in the substance. Stronger intermolecular forces lead to higher boiling points because more energy is required to separate the molecules.
2. Step 2: Identify the types of intermolecular forces in each substance: NH3, H2O, and HF all exhibit hydrogen bonding, which is a strong type of dipole-dipole interaction. CH4, being nonpolar, only exhibits weak London dispersion forces.
3. Step 3: Compare the strength of hydrogen bonding in NH3, H2O, and HF. Water (H2O) has the strongest hydrogen bonding due to two hydrogen atoms bonded to oxygen, allowing extensive hydrogen bonding networks. HF also has strong hydrogen bonding but less extensive than water. NH3 has hydrogen bonding but weaker than both H2O and HF.
4. Step 4: Recognize that CH4 (methane) has only London dispersion forces, which are much weaker than hydrogen bonds, so it requires less energy to boil and thus has a lower boiling point.
5. Step 5: Conclude that among the given substances, CH4 will have the lowest boiling point because it lacks hydrogen bonding and only has weak dispersion forces.