Recrystallization is a purification method used to separate a desired solid compound from soluble and insoluble impurities. Students need this cheat sheet because the technique combines solubility, careful heating, filtration, and crystal formation in one lab process. A clear reference helps connect each lab step to the chemistry behind it.
It also helps students calculate recovery and judge whether a purified sample is truly cleaner.
The main idea is that most solids are more soluble in hot solvent than in cold solvent. An impure solid is dissolved in a minimum amount of hot solvent, insoluble impurities are removed, and the desired compound crystallizes as the solution cools. Purity is often checked using melting point range, where a pure solid has a sharp melting range.
Percent recovery, calculated as , measures how much material was recovered.
Key Facts
- Recrystallization works best when the target compound is very soluble in hot solvent but only slightly soluble in cold solvent.
- Use the minimum amount of hot solvent needed to dissolve the crude solid so the solution becomes saturated as it cools.
- Percent recovery is calculated with .
- A saturated solution contains the maximum dissolved solute at a given temperature, so cooling can cause excess solute to crystallize.
- Hot gravity filtration removes insoluble impurities before crystals form, while vacuum filtration collects crystals after cooling.
- Slow cooling usually forms larger, purer crystals because molecules arrange into an ordered lattice more carefully.
- A pure compound usually has a narrow melting point range, often about to , while impurities lower and broaden the range.
- Activated charcoal can remove colored impurities, but too much charcoal can adsorb the desired compound and lower recovery.
Vocabulary
- Recrystallization
- A purification technique in which an impure solid is dissolved in hot solvent and then crystallized from the cooled solution.
- Solubility
- The amount of solute that can dissolve in a given amount of solvent at a specific temperature.
- Saturated solution
- A solution that contains the maximum amount of dissolved solute possible at a given temperature.
- Mother liquor
- The liquid left behind after crystals form, which often contains dissolved impurities and some remaining product.
- Hot gravity filtration
- A filtration method used to remove insoluble impurities from a hot solution before crystallization begins.
- Melting point range
- The temperature interval over which a solid sample melts, used to help evaluate purity.
Common Mistakes to Avoid
- Adding too much solvent, because an overly dilute solution may not become saturated when cooled and will give few crystals.
- Cooling the solution too quickly, because rapid crystallization can trap impurities inside the crystal lattice.
- Using vacuum filtration while the solution is still hot, because the desired compound may remain dissolved and pass through with the filtrate.
- Skipping the melting point test, because percent recovery alone does not prove that the recovered solid is pure.
- Scraping wet crystals from the filter paper too soon, because leftover solvent increases the measured mass and gives an artificially high .
Practice Questions
- 1 A student starts with of crude benzoic acid and obtains of dry purified crystals. Calculate .
- 2 A compound has a solubility of in hot water and in cold water. If of saturated hot solution is cooled, what mass of crystals can form?
- 3 A sample melts from to , while the pure reference melts from to . What does this suggest about the sample purity?
- 4 Explain why a solvent that dissolves the compound equally well at hot and cold temperatures is a poor choice for recrystallization.