Nucleation is the physical process where a system begins to change its phase or structure. The most common type of nucleation can be seen in water when it begins to boil or freeze with the help of impurities in it. Condensation of water vapour is another example where the process is initiated by several impurities in the cloud, acting as a nucleus or nucleation site. Based on the nature of the nucleation centre, the whole process can be classified into heterogeneous and homogeneous nucleation. If the nucleation is caused by impurities and other foreign objects at the boundaries of the system, the process is a heterogeneous process. If the nucleation happens inside the isolated system all by itself, driven by the temperature and pressure change, the process is a homogenous process. For example, when you increase the temperature of the pure water to the normal boiling point 100 °C, due to the absence of a nucleation site, the water will not boil and would continue superheating. The same happens when you lower the temperature of pure water to 0 °C, the water undergoes supercooling due to the lack of a nucleation site to initiate freezing. Nucleation is essential for DNA replication, transcription and is the first step in crystallization.
Frequently Asked Questions
What causes homogeneous nucleation?
Homogeneous nucleation is caused by the change in temperature or pressure normally. For example, if you supercool water below its normal freezing point of 0 °C, the water molecules would have less energy and because of the lack of a nucleus to begin nucleation and crystal growth, it will not crystallize into ice. On further reducing the temperature, the water molecules begin to assemble and self-organize into the very first nucleus. Upon reaching a definite radius, nucleation occurs abruptly and the ice crystals form. The process usually happens around −40 °C to −41 °C for pure water.