Astate change (or phase transition) is the transition of matter from one state to another, such as a solid state to a liquid state, a liquid state to a gas state, or a state of matter directly between a solid state and a gas state. These transitions occur without a temperature change and can be understood through kinetic particle theory and the internal energy. Internal energy is composed of kinetic energy (temperature- related) potential energy (particle positions and interactions) during phase transitions. During phase transitions, the temperature remains constant (and hence the kinetic energy), however, the potential energy significantly changes. This shift in potential energy causes the overcoming or generation of inter- particle forces during phase transitions.
1. Melting (Solid to Liquid)
When a solid melts, it becomes a liquid. In a solid, the particles are tightly packed in a stable, well- ordered state and vibrate in fixed positions. When energy (usually heat) is added to the solid, it causes the particles to vibrate more strongly. When energy is added to a solid at the melting point, it increases its potential energy. This weakens the forces that hold the particles in place without increasing their kinetic energy, so the temperature stays the same. When the energy is strong enough to overcome the forces holding the particles in place, the solid melts into a liquid where the particles are in a stable state but can move freely around one another.
2. Boiling (Liquid to Gas)
When a liquid boils, it turns into a gas.
According to the Kinetic Particle Theory in a liquid, particles are closely packed but can move past each other. As energy is added, particles move faster and eventually have enough energy to break free from the liquid's surface into the gas phase.
The added energy increases the potential energy of the particles, allowing them to overcome the attractive forces holding them in the liquid. This occurs at the boiling point, where the temperature remains constant as the liquid turns into gas.