The process of converting from one phase of matter to another, such as from solid to liquid or liquid to solid, is known as phase transition. It involves the absorption or release of energy as the arrangement of particles and their motion change.
Let's take the example of converting from a solid to a liquid, which is known as melting or fusion:
Solid phase: In the solid phase, the particles (atoms, molecules, or ions) are closely packed and have a fixed position. They vibrate around their equilibrium positions but do not have enough energy to overcome the attractive forces holding them together.
Heating: To convert the solid into a liquid, heat energy is added to the system. This energy increases the kinetic energy of the particles, causing them to vibrate more vigorously and gain enough energy to break free from their fixed positions.
Melting point: As the solid absorbs heat, its temperature gradually rises. At a specific temperature called the melting point, the solid phase begins to transition into the liquid phase. During this process, the particles become less ordered, and the solid's crystalline structure breaks down.
Liquid phase: In the liquid phase, the particles have more freedom of movement. They can slide past one another but still remain relatively close together due to intermolecular forces. The liquid takes the shape of its container.
The reverse process, from liquid to solid, is called freezing or solidification:
Liquid phase: In the liquid phase, the particles have sufficient energy to overcome the attractive forces between them, allowing them to move more freely.
Cooling: The liquid is cooled, which causes the particles to lose energy. As the temperature decreases, the particles' kinetic energy decreases, reducing their movement.
Freezing point: At a specific temperature called the freezing point (which is the same as the melting point for a substance), the liquid phase begins to transition into the solid phase. The particles slow down further, come closer together, and arrange themselves into a more ordered structure.
Solid phase: In the solid phase, the particles have minimal kinetic energy, and they settle into fixed positions. The substance regains its rigid structure, forming a solid with a defined shape.
It's important to note that the exact temperature at which phase transitions occur (melting point, freezing point) depends on the substance and the prevailing pressure. Different substances have different melting and freezing points, which is why different materials can exist in different phases at room temperature.