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.
Understanding Recrystallization and Purification Techniques
Choosing a solvent is really a test of molecular attractions. The solvent must pull apart particles of the target solid when heated, yet release them when the mixture cools. Polarity often gives a useful first clue.
Polar compounds tend to interact well with polar solvents, while nonpolar compounds tend to interact better with nonpolar solvents. This is only a starting point because crystal packing and hydrogen bonding matter too.
In a school lab, students may test tiny portions of a sample in several solvents before treating the full sample. A good choice gives little dissolution at room temperature, complete dissolution near the solvent boiling point, and crystals after cooling.
Crystal growth begins with nucleation. A few dissolved particles first form a stable tiny cluster, then more particles attach to it in a repeating arrangement. Cooling slowly limits the number of nuclei, so available material can build onto fewer growing crystals.
Rapid cooling often creates many small crystals. Small crystals have more surface area relative to their mass, so they can trap solution containing dissolved impurities. If no crystals appear after cooling, the solution may not be concentrated enough.
Some solvent can be removed by gentle heating, followed by another cooling period. Scratching the inside of the flask with a glass rod or adding a small seed crystal can provide a surface where nucleation starts.
Filtration steps need different conditions because the goal changes during the experiment. Before crystallization, the liquid and glassware should be warm. If the apparatus is cold, target material can crystallize in the filter funnel and be lost with the unwanted solid.
A stemless funnel is often preferred for this reason. After crystals have formed, they are collected under reduced pressure. The crystals should be washed with a very small amount of ice-cold solvent.
A warm wash would dissolve product, while a large cold wash can still carry away some product. Students should keep track of which material is supposed to pass through the filter at each stage. Confusing the filtrate with the solid residue is a common source of lost samples.
Recovery and purity do not always improve together. A very high mass of collected solid can mean that impurities or leftover solvent remain in the crystals. A low recovery can result from using too much solvent, transferring material between containers, filtering too early, or leaving product dissolved in the mother liquor.
Crystals must be dry before weighing and before measuring melting point. Residual solvent can make the measured mass too large and change the melting behavior. For a melting point test, pack a small amount of finely powdered sample into the capillary tube.
Heat slowly near the expected melting temperature. Fast heating makes the observed range less reliable because the sample and thermometer may not have the same temperature. Comparing recovery, crystal appearance, and melting range gives a more honest judgment than relying on only one result.