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Water boils 160 °C hotter than H₂S.
Same bent shape, lighter molecule, yet water boils at 100 °C and H₂S at about −60 °C. Oxygen makes the O–H bonds polar enough to hydrogen-bond; sulfur does not. Forces between molecules, not mass, set the boiling point.
Question 1 of 3from the course, Chem/Phys · passage “Phase behavior of carbon dioxide in decaffeination”
Dry ice sublimes at −78.5 °C and 1.00 atm, but water needs a far higher temperature to become a gas. What explains this?
Pick one. The red pen does the rest.
The deciding lineHydrogen bonding in water raises its sublimation and boiling points; weak dispersion forces give CO₂ a low sublimation point.
Why C: Strong intermolecular forces (hydrogen bonds in ice) need much more energy to overcome than the dispersion forces between nonpolar CO₂ molecules.
Why each choice fails
ARight effect, wrong mechanism: sublimation separates molecules and does not break covalent bonds.
BWrong comparison: CO₂ (44 g/mol) is heavier than H₂O (18 g/mol), and mass alone does not outweigh hydrogen bonding.
CHolds: the linear molecule has no net dipole, and no hydrogen bonds, so little energy is needed.
DDirection reversed: CO₂ forms no hydrogen bonds; stronger bonds would raise, not lower, the temperature.
Ethanol (bp 78.4 °C) and benzene (bp 80.1 °C) are injected into a gas chromatograph with a polar stationary phase. Which elutes first, and why?
Pick one. The red pen does the rest.
The deciding lineOn a polar stationary phase, analytes that interact more strongly with the phase, such as hydrogen-bonding polar compounds, are retained longer than less polar analytes of similar volatility.
Why C: With similar boiling points, the difference in interaction with the stationary phase decides: ethanol (polar, hydrogen-bonding) is retained longer, so benzene elutes first.
Why each choice fails
AWrong comparison: on a polar phase, ethanol’s hydrogen bonds to the stationary phase retain it longer, outweighing the 1.7 °C difference in boiling points.
BSays more than the data: boiling point is not the only factor; the polar phase separates them by polarity.
CHolds: benzene is nonpolar and has only weak attraction to the polar stationary phase, whereas ethanol hydrogen-bonds to it; benzene elutes first despite its slightly higher boiling point.
DDirection reversed: polar analytes are retained more, not less, on a polar stationary phase.
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