Which of the following substances is expected to have the lowest boiling point at standard pressure?
Show the answer and explanation
Correct answer
A. CH4
Principle or equation
For similar hydrides in the same group, boiling points generally increase with molar mass due to stronger London dispersion forces. The lightest molecule has the weakest dispersion forces and thus the lowest boiling point.
Why this answer is correct
CH4, SiH4, GeH4, and SnH4 are all nonpolar tetrahydrides. Their intermolecular forces are primarily London dispersion forces, which increase with molecular size and polarizability. Since CH4 has the smallest molar mass and electron cloud, it has the weakest dispersion forces and the lowest boiling point.
Example
For the series CH4, SiH4, GeH4, SnH4, the boiling points are approximately -161°C, -112°C, -88°C, and -52°C, respectively, showing a steady increase with molar mass.
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