A pentane (C5H12) molecule is nonpolar while water (H2O) is polar. According to this information: a. Pentane has a higher melting point and boiling point than water b. Water has a higher melting point than pentane, but pentane has a higher boiling point than water c. Water has a higher melting point and boiling point than pentane d. Pentane has a higher melting point than water, but water has a higher boiling point than pentane
The Correct Answer and Explanation is:
Correct Answer: c. Water has a higher melting point and boiling point than pentane
Explanation:
The physical properties of a substance—like melting point and boiling point—are largely influenced by the types and strengths of intermolecular forces present. In this case, we are comparing pentane (C₅H₁₂), a nonpolar hydrocarbon, with water (H₂O), a polar molecule.
Water: Strong Intermolecular Forces
Water is a polar molecule due to the bent shape and the difference in electronegativity between oxygen and hydrogen. More importantly, water exhibits hydrogen bonding, one of the strongest types of intermolecular forces. These hydrogen bonds occur between the hydrogen atom of one water molecule and the oxygen atom of another. As a result, it takes a significant amount of energy to break these bonds for water to melt or boil.
- Melting Point of Water: 0°C
- Boiling Point of Water: 100°C
Pentane: Weak Intermolecular Forces
Pentane, on the other hand, is nonpolar, meaning it lacks partial charges that would create strong attractions between molecules. The only type of intermolecular force present in pentane is London dispersion forces, which are the weakest. These arise due to temporary shifts in electron clouds that induce momentary dipoles.
- Melting Point of Pentane: About -130°C
- Boiling Point of Pentane: About 36°C
Comparison
Because water has hydrogen bonds (strong) and pentane has only London dispersion forces (weak), water requires more energy to change states, leading to higher melting and boiling points. Both of water’s phase change points are significantly higher than those of pentane.
Conclusion
Therefore, the correct answer is (c): Water has a higher melting point and boiling point than pentane due to the strength of hydrogen bonding in water compared to the weak dispersion forces in pentane.