Question

The diagram shows molecules in a fluid. Explain the sequence of events that happens when this fluid cools down. Be sure to focus on what happens to the molecules and to the fluid in general. Five events should be included in your answer.

Answer 1

1) Molecules lose kinetic energy. As the temperature drops each molecule’s average translational kinetic energy falls, so individual molecules move more slowly.

2) Molecular motion amplitude and collision energy decrease. Slower motion means less frequent and less energetic collisions; thermal motion becomes smaller in range.

3) Intermolecular attractions become relatively stronger and molecules move closer together. With lower kinetic energy the attractive forces pull molecules into nearer positions; transient clusters form and the fluid’s internal resistance to flow (viscosity) increases.

4) Macroscopic contraction and density change. On the bulk scale the fluid usually occupies less volume (contracts) so its density increases; diffusion and convective flow slow down and evaporation/vapor pressure fall.

5) Phase change to a solid (at the freezing point). Once the temperature reaches the freezing point molecules arrange into a regular (crystalline) structure and become fixed in place (only vibrational motion remains). Latent heat of fusion is released during this crystallization and the material stops behaving as a flowing fluid.