Which of the following pure substances has the highest boiling point?

• A. CH\(_4\)
• B. H\(_2\)O
• C. CO\(_2\)
• D. O\(_2\)

Answer: (B)

Verified step by step guidance

1. Step 1: Understand that the boiling point of a pure substance depends primarily on the strength and type of intermolecular forces present between its molecules.
2. Step 2: Identify the types of intermolecular forces for each substance: CH\(_4\) (methane) has London dispersion forces; CO\(_2\) (carbon dioxide) also has London dispersion forces; O\(_2\) (oxygen) has London dispersion forces; H\(_2\)O (water) has hydrogen bonding, which is a much stronger intermolecular force than dispersion forces.
3. Step 3: Recall that hydrogen bonding occurs when hydrogen is directly bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine, leading to significantly higher boiling points compared to substances with only dispersion forces.
4. Step 4: Compare the boiling points qualitatively: substances with only London dispersion forces generally have lower boiling points, while substances with hydrogen bonding have higher boiling points due to stronger intermolecular attractions.
5. Step 5: Conclude that H\(_2\)O has the highest boiling point among the given substances because of its strong hydrogen bonding, which requires more energy (higher temperature) to break during boiling.