U3.3 · Solids, Liquids, and GasesPractice · 固体、液体与气体

Vapor: V = nRT/P = 1.00 × 0.08206 × 373.15 ÷ 1.00 = 30.62 L. Liquid: V = m/ρ = 18.02 g ÷ 0.958 g/mL = 18.81 mL = 0.01881 L. Ratio ≈ 30.62 ÷ 0.01881 = 1.63 × 10³. For the same number of molecules, the spaces between gas molecules are far larger than the molecules themselves, while in a liquid the molecules touch.

Molecules in a liquid have a range of kinetic energies. Raising the temperature raises the average, so a larger fraction of molecules can break free of their neighbors’ attraction (mostly hydrogen bonds for water). More molecules escape, there are more in the vapor at equilibrium, and the vapor pressure is higher: from 15 °C to 30 °C it grows about 2.5 times. Evaporation doesn’t break O–H bonds, and the strength of the intermolecular forces doesn’t change with temperature.

In a liquid the molecules are in contact (so the volume is essentially fixed and hard to compress), but they have no fixed positions and can slide past one another, so the liquid flows and takes the shape of its container. Spreading out to fill the container describes a gas; molecules fixed in place describes a solid.

Above its boiling point, a substance is a gas at 1 atm. Only butane’s boiling point (−0.5 °C) is below 25 °C, so it is a gas; the others are liquids. Longer chains have more electrons, stronger dispersion forces, and higher boiling points.

A constant temperature means the average kinetic energy is constant. The heat absorbed during boiling goes into overcoming the attractions between molecules (mostly hydrogen bonds for water) so they can separate; it raises their potential energy. Steam is still $\ce{H2O}$ molecules, so no O–H bonds break.

Compare at one temperature: at 40 °C, X, Y, and Z have vapor pressures of about 909, 151, 62 mmHg. Stronger intermolecular forces make it harder for molecules to escape, so the vapor pressure is lower and the liquid must be heated further before it reaches 760 mmHg (its normal boiling point): about 100 °C for Z, the highest. X reaches 760 mmHg first because its molecules escape most easily, so its forces are the weakest. Between 20 °C and 60 °C, Z’s vapor pressure rises only about 136 mmHg, less than Y’s (about 313 mmHg).