Core Chemistry 14 - 16 (Changes of state between solids, liquids and gases)

 

Understanding Changes of State: Solids, Liquids, and Gases

This page explores how particles behave during phase transitions—melting, freezing, boiling, condensing, and sublimation—and encourages viewing these changes through the lens of particle behavior, energy, and forces.

Changes of State between Solid and Liquid

Melting

When a solid is heated, its particles gain energy and vibrate more intensely. As the temperature rises, the vibrations eventually overcome the attractive forces holding the particles together in a fixed structure, causing the solid to melt into a liquid. The melting point varies depending on the strength of these forces; for instance, sulfur melts at 113°C while tungsten melts at 3410°C due to its stronger interparticle attractions.

Freezing

Freezing is the reverse process. As a liquid cools, its particles lose energy, moving more slowly. Eventually, these particles can’t overcome the attractive forces and become locked into a solid structure, thus freezing the liquid.

Changes of State between Liquid and Gas

Boiling

Heating a liquid increases the energy of its particles until they can overcome the intermolecular forces holding them in the liquid state. When this happens, the particles escape into the gas phase. Boiling is a vigorous process where bubbles form throughout the liquid as particles transition to gas.

Condensing

Condensation is the process where gas particles lose energy and transition back into a liquid. This occurs when the gas particles slow down enough that their intermolecular attractions can pull them together. For instance, helium, which condenses at -269°C, has very weak attractions between particles.

Evaporation

Evaporation differs from boiling in that it occurs at the surface of a liquid at any temperature below boiling. Surface particles gain enough energy to break free from intermolecular attractions and escape into the gas phase. This process cools the remaining liquid, as the higher-energy particles leave.

Changes of State between Solid and Gas

Sublimation

Sublimation is when a solid transitions directly into a gas without passing through a liquid phase. Examples include dry ice (solid carbon dioxide), which sublimates at -78.5°C, and iodine, which sublimates under certain conditions but can also melt and then boil.

Key Concepts

• Temperature and Energy: Temperature reflects the average kinetic energy of particles. Adding energy increases the temperature and, consequently, the movement of particles. The average energy remains constant at a fixed temperature.
• Dynamic Equilibrium: In a sealed container, evaporation and condensation occur at the same rate, establishing a dynamic equilibrium where the number of particles leaving the surface equals those returning to it.

Understanding these principles helps explain everyday phenomena and the behavior of different materials during phase transitions.