The so-called "phase" refers to the state of matter. In the same phase, the chemical composition and physical properties are consistent. Take pure water as an example. It exists in liquid state of water, gas state of steam and solid state of ice, which are called liquid phase, gas phase and solid phase respectively. In a multicomponent system, one phase may contain multiple components, or multiple liquid phases may exist at the same time. Taking the oil-water system as an example, when half a cup of salad oil is poured into half a glass of water at room temperature, it can be seen that the oil floats on the water surface, forming an immiscible oil phase and a water phase with different compositions. However, if alcohol is poured into water, there is only one liquid phase due to mutual solubility, in which the chemical composition is uniform. In addition, solids can also be composed of multiple solid phases. For example, a stone often contains many small particles of different colors. These small particles may have different chemical compositions and crystal structures, but they belong to different solid phases, or they belong to the same phase but are dispersed in other phases. Under what conditions, in which phase or phases does a substance exist * * * is one of the important goals of thermodynamics.

At the same time, when the environmental conditions such as temperature and pressure change, the substance may change from the original phase to another phase, resulting in the so-called "phase transition", and the mechanism and rate of phase transition is the research goal of "nucleation theory". There are many kinds of phase changes, and the common ones can be divided into the following categories: from gas phase to liquid phase, which is called condensation; The change from liquid phase to gas phase is called boiling if it happens inside the liquid phase or at the interface between the liquid phase and the container, and evaporation if it happens at the gas-liquid interface; The change from liquid phase to solid phase is called solidification or crystallization; The change from solid phase to liquid phase is called melting; There are many other phase transitions, especially in multicomponent systems, and there are more possible types of phase transitions.

As for the initial nucleation of phase change, it can be divided into the following two types: homogeneous nucleation-the embryo nucleus of a new phase is directly generated from the original parent phase, for example, multiple water vapor molecules aggregate themselves to form the embryo nucleus of a water droplet; Heterogeneous nucleation-a new phase is formed by adding crystal nucleus or container surface as embryo nucleus. Generally speaking, heterogeneous nucleation is much easier than homogeneous nucleation when there are foreign embryo nuclei. Take water as an example. In the absence of any foreign embryo nuclei, water droplets with a diameter of one centimeter have to be cooled to MINUS 40 degrees Celsius to form ice beads, which is far below the general cognition that water can form ice at zero degrees Celsius. But generally speaking, the particles on the surface of the container or in the water can act as embryo nuclei, making the water freeze at about-65438 0 degrees Celsius.