Science Buddies: "Ask an Expert"

Hi,

For cooling crystals is better to cool them slowly right? To exclude impurities and prevent loss of product. But how does cooling slowly cause these two (exclude impurities and prevent loss of product) to happen?

Thanks!

Crystallization is, as the word denotes, the act of forming crystals. Consequently, when molecules or atoms come together to form their unique structure (ie. lattice, sheets, etc.) to form their crystalline structure, a slower process is better than a fast one. In a slow crystallization process, you are allowing the spontaneous formation of crystals to adhere to the most stable energetic considerations in crystal formation (this involves Gibb's Free Energy). Whereas, in a fast crystallization process, there is no time to let the system satisfy the most stable Gibb's Free energy conformation-you are left with what you are able to glean from a fast crystallization process. Simply put, if you allow contaminants in your crystal structure, you are not allowing the strongest structure possible (ie. Imagine building a log cabin by stacking giant logs for each side of the cabin and then placing a giant stone in one of the sides of the cabin, preventing neat and tight fitting of logs to sit on top of each other). Slow crystallization allows the environment to satisfy the thermodynamic requirements for clean, pure crystals. Fast crystallization satisfies the kinetic requirement, which are usually dirty crystals.

Hope that helps,

James