Solid to Gas (Sublimation):
* Energy Absorption: The solid absorbs energy, typically in the form of heat.
* Increased Molecular Motion: The energy causes the molecules within the solid to vibrate more rapidly.
* Breaking Intermolecular Forces: The increased vibration weakens the forces holding the molecules in a fixed, rigid structure.
* Direct Transition: The molecules gain enough energy to overcome these forces entirely and escape into the gaseous phase without passing through a liquid state.
Gas to Solid (Deposition):
* Energy Release: The gas releases energy, typically as heat.
* Decreased Molecular Motion: The molecules slow down and lose kinetic energy.
* Formation of Intermolecular Forces: As the molecules slow, they can form attractive forces with one another.
* Direct Transition: The molecules lose enough energy to form a rigid structure, directly transitioning from gas to solid.
Key Points:
* Phase Changes: The key is that these changes involve a shift in the state of matter, but not a change in the chemical composition of the substance.
* Energy Exchange: Energy is either absorbed (solid to gas) or released (gas to solid) during these transformations.
* Molecular Arrangement: The arrangement of molecules changes dramatically between solids, liquids, and gases.
Examples:
* Sublimation: Dry ice (solid CO2) sublimates directly into CO2 gas.
* Deposition: Frost forms on cold surfaces when water vapor in the air deposits directly onto them as ice.
