Chemistry Practice Test: Intermolecular Forces and Phase Changes

In water, hydrogen bonded to highly electronegative oxygen is attracted to oxygen on a neighboring molecule, forming hydrogen bonds. Ionic bonds hold ions together in compounds like NaCl, not separate water molecules.

Water molecules form hydrogen bonds, so water boils at 100 °C while H₂S, which cannot hydrogen bond, is a gas at room temperature. H₂S is heavier, but its weaker forces matter more than its mass.

CO₂ is linear, so its two polar bonds cancel and the molecule is nonpolar. HCl is polar (dipole-dipole forces), and NH₃ and CH₃OH can form hydrogen bonds.

Bigger electron clouds are easier to distort, so the temporary dipoles and the dispersion forces between atoms are stronger. Noble gas atoms do not form covalent or hydrogen bonds.

During a phase change the energy separates molecules from each other instead of raising their average kinetic energy, so the temperature stays flat. The water molecules themselves stay intact as H₂O in the steam.

A liquid boils when its vapor pressure equals the pressure above it; lower air pressure is matched at a lower temperature. Cold air would slow heating, and the forces between water molecules do not change with altitude.

Evaporation removes the highest-energy molecules, so the liquid left behind has a lower average kinetic energy and takes heat from your skin. "Cold" is not a substance that can be absorbed.

Sublimation is a solid becoming a gas without melting first. Deposition is the reverse, such as frost forming directly from water vapor.

At the triple point, all three phases are in equilibrium. The highest temperature at which a liquid can exist is the critical point, a different point on the diagram.

NaCl is an ionic solid held together by strong attractions between ions, so it melts at about 801 °C. The others are molecular substances held together only by weaker intermolecular forces.