Nucleation is a physical process where a substance changes state around nuclei. Heterogeneous nucleation occurs with pre-existing nuclei, while homogeneous nucleation occurs spontaneously. Nucleation is used in manufacturing processes and natural phenomena, such as cloud formation and freezing. Examples include the formation of ice crystals, supersaturated solutions, and the explosive reaction between mints and soda. Nucleation is also used in the production of catalysts, semiconductors, and candy sugar.
Nucleation is a physical process in which a change of state, for example from liquid to solid, occurs in a substance around certain focal points, known as nuclei. Common examples are the condensation of water vapor into droplets in the atmosphere, the formation of ice crystals when water freezes, and the appearance of gas bubbles in a liquid. Heterogeneous nucleation occurs where pre-existing nuclei are present, such as tiny dust particles suspended in a liquid or gas. Homogeneous nucleation occurs where no such contaminants are present and is much less frequent. This physical reaction underlies a variety of manufacturing processes and interesting natural phenomena.
Often, once nucleation begins to occur, it exhibits an exponential growth curve. For example, once crystals begin to form in a solution, their surface area increases as they grow, attracting more molecules and promoting growth at an ever-increasing rate, until the solution stabilizes and no more crystals can form. . This explains why it takes time for a river to freeze over in the winter, but once ice starts forming in the body of the river, it usually covers the river very quickly.
Heterogeneous nucleation
Water normally freezes at 32°F (0°C). This is because it contains numerous tiny solid particles of dust and organic matter around which freezing can occur – they can be thought of as “seeds” that start the process. Extremely pure water, which does not contain these particles, actually freezes at -43.6°F (-42°C). If pure water is cooled below its normal freezing point, but above its pure freezing point, it remains liquid and is said to be supercooled. The water can then be made to freeze very quickly by adding a single small crystal of ice, which serves as a core.
A similar phenomenon can be demonstrated using a supersaturated solution of sodium acetate. More of the compound will dissolve in hot water than in cold water, but a supersaturated solution can be made by adding it to very hot water until it no longer dissolves, then allowing it to cool undisturbed. When the water is cold, it will have more sodium acetate in solution than could have been dissolved by simply adding it to cold water. This is a supersaturated solution. If a single speck of the compound is now added, the crystals will form rapidly through nucleation and spread throughout the liquid, so that it appears to freeze.
Another fun example, this time involving the release of dissolved gas, is the famous explosive demonstration using popular brands of mints and sodas. The surface of the mint is covered in sugar, which forms many small nucleation centers. The carbonated drink contains a lot of dissolved carbon dioxide, which becomes a gas on contact with the mint, forming large quantities of bubbles and creating a pressure that pushes the liquid out of the container at high speed, forming a fountain or “geyser”.
Cloud formation in the atmosphere is an example of a gas condensing into a liquid through nucleation. It would be much more difficult for clouds to form without the presence of dust particles in the air, known as condensation nuclei. It is thought that smoke, soot and other particles produced by human activities may do this, increasing cloud cover in polluted areas. In some cases, tiny crystals are deliberately released into the atmosphere to act as condensation nuclei and promote cloud formation in very dry areas; this practice is known as “cloud seeding”.
Homogeneous nucleation
This occurs spontaneously in a substance that contains no impurities that can serve as pre-existing nuclei, and is therefore completely uniform. It usually happens in response to a change in temperature or pressure. For the process to take place, new nuclei must be created from the substance itself, through random fluctuations once the conditions are right. An example is freezing absolutely pure water at -43.6°F (-42°C).
Another is the formation of ice crystals in clouds or frozen fog at ground level. Water droplets suspended in the air can be cooled well below freezing without forming ice; for this reason clouds, even when the air temperature is below zero, are generally made up of supercooled water droplets. Freeze fog is formed by droplets of water that freeze instantly when they come into contact with a surface; frozen fog, made up of ice crystals, forms only at extremely low temperatures.
Applications
Nucleation affects and is used in many manufacturing processes. For example, it is employed in the production of catalysts for the chemical industry and many catalysts obtain their effects through this process. It is also used in the production of semiconductors for the electronics industry. In confectionery, the production of candy sugar is an example of nucleation which allows the formation of large crystals from a supersaturated sugar solution.
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