At pressures below 4.58 mmHg, the phase changes that may occur are sublimation and deposition, depending on the substance in question and the temperature conditions involved.
1. **Sublimation**: This is the process by which a solid transforms directly into a gas without passing through the liquid phase. An example of a substance that undergoes sublimation under certain conditions is dry ice (solid carbon dioxide), which can sublimate directly to its gaseous form at atmospheric pressure, well below 4.58 mmHg, especially in room temperature conditions.
2. **Deposition**: The reverse of sublimation, deposition is the process where a gas transforms directly into a solid without becoming a liquid first. Frost formation from water vapor in the air on cold surfaces is a common example of deposition.
Whether one of these processes occurs depends on the temperature in relation to the substance's phase diagram, which plots pressure against temperature and shows the areas corresponding to the solid, liquid, and gaseous states. Below 4.58 mmHg, if the temperature is also below the substance's sublimation point at that pressure, deposition could occur. Conversely, if the temperature is above the sublimation point, sublimation could occur.
These phase changes are not generic to all substances but are typical for those that have a sublimation point at or near the pressures under consideration. Phase diagrams are unique to each substance and must be consulted to determine the exact conditions under which these phase changes occur.
