What’s recrystallization?

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Recrystallization purifies substances by dissolving them in a solvent, heating the solution, filtering out impurities, and allowing the solution to cool and form pure crystals. The solvent must dissolve the target compound at a higher temperature and not react with it, while impurities should dissolve at room temperature or be insoluble at a higher temperature. Activated charcoal can be used to remove colored impurities.

Recrystallization is a process that is used to purify a substance. It can be used in various processes, such as making aspirin. This process is done by placing the impure compound in a solvent, heating the solution so that the compound dissolves, and filtering out the impurities. In some cases, it may be necessary to use charcoal to remove colored contaminants from the compound. The mixture is then cooled, allowing for the formation of pure crystals.

The main rationale behind recrystallization is the fact that substances usually become more soluble when the solvent is hot than when it is cold. For example, sugar dissolves better in hot water than it does in cold water, which is why it is often difficult to dissolve sugar in iced tea even if a person can mix it thoroughly. The difference in solubility with varying temperatures allows an impure substance to dissolve at a higher temperature and then slowly crystallize at a lower temperature without re-trapping the impurities.

To purify a substance using this process, a person must begin by choosing the appropriate solvent, sometimes through trial and error. The right solvent will not only dissolve the target compound at a higher temperature allowing it to crystallize at room temperature, it should also not react with the compound. The solvent also does not have to dissolve impurities at the same temperature as the target compound. The impurities should either dissolve at room temperature while the compound is insoluble or should be insoluble at a higher temperature to allow the impurities to filter out.

During recrystallization, a person would only need to use a small amount of solvent to dissolve the target compound. If too much is used, the compound may not recrystallize when the time comes. When the target has been completely dissolved, any insoluble impurities can be filtered out. The solution should then be allowed to cool slowly so that crystals can form. If the solution is cooled too quickly, the crystals can trap dissolved impurities.

If a white or clear compound should discolor while still in the hot solution, colored impurities may be present. If so, activated charcoal can be used to remove them. The charcoal will attract impurities and clear the solution, so both substances can be filtered out. Only a small amount of carbon should be used in this process because too much may start to react with the compound, reducing the final amount of the purified substance.

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