Everything in the universe is composed of matter, which exists in different physical forms. A phase change describes a physical process where matter transitions from one state to another without changing its chemical composition. These transitions occur when the energy within the substance is altered, typically by heating or cooling. The thermal energy dictates how quickly the particles of a substance move and interact with one another. When enough energy is added or removed, the substance reaches a specific temperature at which its physical structure rearranges itself.
The Foundation: Defining States of Matter
The most commonly observed states of matter are solid, liquid, and gas. A solid is characterized by particles held in fixed positions, vibrating only slightly, giving the substance a definite shape and a fixed volume. Intermolecular forces hold these particles tightly together, preventing significant movement and maintaining rigidity.
Liquids have particles that are close together but can slide past one another. They have a fixed volume but will take the shape of their container. The particles possess more kinetic energy than those in a solid, allowing for fluidity. This increased energy permits the molecules to overcome some of the attractive forces.
Gases have the highest energy, with particles moving rapidly and independently. This results in neither a fixed shape nor a fixed volume. They will expand to fill any container and are easily compressible because the distances between individual particles are large and intermolecular attraction is negligible.
Phase Changes That Require Energy Input
Phase changes that require an input of thermal energy from the surroundings are known as endothermic processes. This energy must be absorbed to increase the kinetic energy of the particles, allowing them to overcome the attractive forces that bind them together. The three changes that absorb heat are melting, vaporization, and sublimation.
The transition from a solid to a liquid is called melting. When a solid absorbs heat, the energy disrupts the rigid crystalline structure of the substance. This energy is used to break the intermolecular bonds, allowing the molecules to move freely as a liquid.
Vaporization describes the change from a liquid to a gas, which can occur through boiling or evaporation. Boiling happens when the vapor pressure of the liquid equals the surrounding atmospheric pressure, forming bubbles throughout the bulk of the liquid. A substantial energy input is needed to completely separate the liquid molecules into the gaseous state.
Evaporation is a surface phenomenon where higher-energy molecules escape the liquid phase below the boiling point. The latent heat of vaporization must be supplied to give the molecules enough energy to completely overcome the remaining intermolecular forces.
Sublimation is the direct transition from a solid to a gas without first passing through the liquid state. Dry ice, which is solid carbon dioxide, provides a common example of this process. The energy absorbed must be sufficient to bypass the liquid phase entirely, granting the particles enough kinetic energy to immediately become a gas. This requires breaking all intermolecular bonds in one step.
Phase Changes That Release Energy
Conversely, phase changes that release thermal energy into the surroundings are termed exothermic processes. These changes occur when the particles slow down and lose energy, allowing attractive intermolecular forces to pull them closer together and establish a more ordered structure. The three changes that release heat are freezing, condensation, and deposition.
The process of a liquid becoming a solid is freezing, which is the reverse of melting. As a liquid cools, its particles lose kinetic energy, causing them to move more slowly. The attractive forces between the molecules then become dominant, locking the particles into the fixed positions of the solid crystalline lattice, releasing latent heat in the process.
Condensation is the reverse of vaporization, where a gas turns into a liquid. When water vapor contacts a cold surface, the gas particles transfer heat energy to the surface and slow down. This loss of energy allows the intermolecular forces to pull the particles back into close proximity, resulting in liquid droplets like dew or fog. The energy released during condensation is equal to the energy absorbed during vaporization for the same substance.
Deposition is the direct transition from a gas to a solid, bypassing the liquid state, and is the opposite of sublimation. A common demonstration of deposition is the formation of frost on a cold morning. Water vapor in the air comes into contact with a surface that is below the freezing point, and the gas molecules immediately form ice crystals, releasing energy as they structure themselves into a solid.