Difficulty: Introductory lab technique | Prerequisites: General chemistry, basic lab safety, familiarity with solubility concepts
Recrystallization is one of the first purification techniques you will use in organic chemistry lab. It sits at the foundation of nearly every synthesis workflow: once you have made (or been given) a crude solid, you need a reliable way to strip out the impurities and confirm that what remains is pure. This experiment asks you to identify an unknown solid, purify it by recrystallization, and then use melting point data to judge how well the purification worked. If you are comfortable with solubility ("like dissolves like") and basic filtration, you have the background you need.
Recrystallization purifies a solid by dissolving it in a hot solvent, filtering off insoluble junk, then cooling the solution so the pure compound crystallises out. You collect the crystals, dry them, and measure the melting point to check purity. A sharp, narrow melting range means the sample is clean.
Recrystallization
A purification technique in which an impure solid is dissolved in a hot solvent, then allowed to re-form as crystals upon cooling. The slow, ordered crystal growth excludes most impurities.
In simple terms, you melt the solid into solution while it is hot, then let the pure stuff crystallise back out as the solution cools.
Solubility
The maximum amount of a solute that will dissolve in a given amount of solvent at a specific temperature.
Think of it as the solvent's capacity for that compound at that temperature. Recrystallization relies on a big gap between hot solubility (high) and cold solubility (low).
Melting point (mp)
The temperature at which a solid transitions to a liquid at standard pressure.
A pure compound has a sharp melting point (narrow range, typically 1 to 2 degrees C). Impurities lower and broaden the melting range, so melting point is your main purity check.
Melting point depression
The lowering and broadening of a substance's observed melting range caused by the presence of impurities.
If your recovered solid melts over a wide range or starts melting well below the literature value, impurities are still present.
Gravity filtration (hot filtration)
Filtration that relies solely on gravity to pull liquid through filter paper in a funnel. Used while the solution is hot to remove insoluble impurities (sand, sawdust) without losing dissolved product.
Vacuum filtration (cold filtration)
Filtration that uses a vacuum (reduced pressure beneath a Buchner funnel) to pull liquid through filter paper quickly. Used after cooling to collect the recrystallised solid.
Percent recovery
The mass of purified product divided by the mass of crude starting material, multiplied by 100.
It tells you how much of your original sample you managed to get back after purification.
Percent yield
The actual mass of product obtained divided by the theoretical (maximum possible) yield, multiplied by 100.
It measures how close your experiment came to the ideal outcome, accounting for losses at every step.
Crude solid / crude product
The impure material before purification. In this experiment, the unknown sample containing 97% target compound plus 2% sand and 1% sawdust.
Filtrate
The liquid that passes through the filter during filtration. In hot filtration, the filtrate contains your dissolved compound. In vacuum filtration, the filtrate is waste solvent.
Mel-Temp apparatus
A device used to measure the melting point of a small sample packed into a capillary tube. The standard instrument in undergraduate organic chemistry labs for purity assessment.
The solvent must meet two conditions at once:
The compound is only sparingly soluble (or insoluble) in the solvent when cold.
The compound dissolves freely in the solvent when hot.
That temperature-dependent solubility gap is what makes the technique work. A solvent that dissolves the compound at both temperatures is useless for recrystallization, and so is one that never dissolves it at all.
In the lab, you test candidate solvents by adding a small amount of the solid to a test tube with a few drops of cold solvent, observing whether it dissolves, then heating in a water bath to see if it dissolves when hot. A suitable solvent causes no dissolution when cold but full dissolution when hot. On cooling, the compound crystallises back out.
Recrystallization separates two kinds of impurity:
Insoluble impurities (sand, sawdust, glass fibres): these never dissolve in any solvent. They are removed by gravity (hot) filtration before crystallisation begins.
Soluble impurities: these dissolve in the hot solvent along with the target compound, but because they are present in much smaller amounts, they remain dissolved when the solution cools. They pass through the filter paper during vacuum filtration and are discarded with the filtrate.
The key insight is that the target compound is present in large excess relative to the impurities. When the solution cools, the target compound exceeds its cold-solubility limit and crystallises out, while the small quantity of soluble impurity stays in solution.
Melting point is the primary purity test in this experiment. A pure compound melts sharply over a narrow range (1 to 2 degrees C). The presence of impurities causes:
A lower onset temperature (the sample starts melting earlier than expected).
A wider range (the sample melts over 3 or more degrees rather than 1 to 2).
You compare your measured melting point and range against literature values for the candidate compounds. A close match to the literature value, with a narrow range, confirms identity and purity.
Getting the solvent volume right is critical:
Too much solvent: the solution is too dilute for crystals to form when cooled. The compound stays in solution and you lose product. Fix: gently boil off excess solvent to concentrate the solution, then cool again.
Too little solvent: the compound does not fully dissolve, so impurities remain trapped in undissolved solid. Fix: add more hot solvent, a small amount at a time, until dissolution is complete.
The practical rule is to add solvent drop by drop (roughly 1 mL at a time) to the hot mixture until the solid just dissolves, with no visible excess.
These are the candidate unknowns for this experiment. Knowing their melting points and solubility profiles lets you identify your unknown once purified.
Compound | Molar Mass (g/mol) | Melting Point | Soluble In | Not Soluble In |
|---|---|---|---|---|
Biphenyl | 154.21 | 69 degrees C | Ethanol, benzene, carbon tetrachloride, carbon disulfide, methanol | Water |
2,3-Dimethylphenol | 122.16 | 75 degrees C | Ethyl alcohol, ethyl ether, benzene, chloroform | Water |
Benzoic acid | 122.12 | 122.4 degrees C | Ethanol (slightly soluble in water) | Largely insoluble in cold water |
2-Methylbenzoic acid | 136.15 | 103.7 degrees C | Acetone | Water |
Benzene-1,3-diol (resorcinol) | 110.11 | 109.8 degrees C | Diethyl ether, ethanol, ethyl ether, carbon tetrachloride | Water |
4-Hydroxybenzaldehyde | 122.12 | 117 degrees C | Ethanol (slightly soluble in water) | Largely insoluble in cold water |
Solvent | Hazards |
|---|---|
Water | Non-hazardous |
Methanol | Flammable, acute toxic, health hazard |
Ethanol | Flammable |
Ethyl acetate | Flammable, irritant |
Acetone | Flammable, irritant |
n-Hexane | Flammable, irritant, health hazard, environmental hazard |
\text{Percent recovery} = \frac{\text{mass of purified product}}{\text{mass of crude starting material}} \times 100Example from this experiment: the crude sample is 4.8 g containing 97% unknown, 2% sand, and 1% sawdust. The maximum recoverable mass is 4.8 g x 0.97 = 4.656 g. If you recover 4.2 g of purified solid, your percent recovery is (4.2 / 4.8) x 100 = 87.5%.
\text{Percent yield} = \frac{\text{actual mass of product}}{\text{theoretical yield}} \times 100Theoretical yield is the maximum mass of product that could form based on stoichiometry. In a recrystallization (no chemical reaction), the theoretical yield equals the mass of pure compound in the starting material.
For recrystallization, percent recovery is the more appropriate metric. There is no chemical transformation, so "yield" in the synthetic sense does not apply. Percent recovery tells you directly how efficiently the purification worked: how much of the original substance you kept.
Recrystallization is the standard industrial method for purifying pharmaceutical intermediates and active ingredients. When a drug company synthesises a compound, the crude product almost always contains by-products, unreacted starting materials, and traces of catalyst. Recrystallization (often on a multi-kilogram scale) removes these before the compound is formulated into tablets or injectable solutions.
The same principle applies in food-grade chemical production. Table sugar, for instance, is purified by dissolving raw sugar in hot water and recrystallising it, which is why granulated sugar is so consistently white and pure.
Students often think a high percent recovery means the product is pure. It does not. You can recover a large mass of product that is still contaminated. Purity is assessed by melting point, not by mass recovered.
Students often confuse percent recovery and percent yield and use them interchangeably. They measure different things. Percent recovery compares purified mass to crude mass. Percent yield compares actual product to theoretical product from a reaction. For a recrystallization with no chemical reaction, percent recovery is the correct metric.
Students sometimes assume that if the solid dissolves in a hot solvent, that solvent is automatically suitable for recrystallization. That is only half the requirement. The compound must also be insoluble (or nearly so) in that solvent when cold, or nothing will crystallise out on cooling.
Students sometimes think that using more solvent will give a "cleaner" product. In practice, excess solvent means less product crystallises out on cooling, and you lose material to the filtrate. The goal is just enough solvent to dissolve the solid when hot.
Expect a question asking you to calculate percent recovery from a given crude mass and recovered mass. Know the formula cold.
Be ready to explain why melting point depression indicates impurity, not just that it does.
The difference between hot filtration and cold filtration, and why each is done at that temperature, is a common short-answer question.
Know which metric to use for recrystallization (percent recovery) and be able to justify why percent yield is less appropriate here.
Solvent selection criteria (insoluble cold, soluble hot) come up frequently. Be prepared to evaluate a solvent given solubility data for a compound.
True or false: A compound that dissolves in ethanol at both room temperature and at boiling point would be a good candidate for recrystallization from ethanol. (False: there is no solubility gap, so nothing would crystallise on cooling.)
Fill in the blank: Impurities cause the melting point to ______ and the melting range to ______. (decrease; broaden)
True or false: Percent yield and percent recovery are the same measurement. (False: percent recovery compares purified mass to crude mass; percent yield compares actual to theoretical.)
Fill in the blank: Insoluble impurities are removed by ______ filtration, and the purified crystals are collected by ______ filtration. (gravity/hot; vacuum/cold)
True or false: Adding excess solvent improves purity. (False: excess solvent reduces the amount of product that crystallises out.)
Q: You dissolve 4.8 g of crude solid (97% unknown, 2% sand, 1% sawdust) and recover 4.0 g of purified crystals. What is your percent recovery?
A: Percent recovery = (4.0 / 4.8) x 100 = 83.3%.
Q: Your purified sample melts at 60 to 66 degrees C. The literature melting point for biphenyl is 69 degrees C. What does this tell you?
A: The melting point is lower than the literature value and the range is broad (6 degrees), which indicates that impurities are still present. The sample needs further purification.
Q: Why is water unsuitable as a recrystallization solvent for biphenyl?
A: Biphenyl is insoluble in water at all temperatures. A suitable recrystallization solvent must dissolve the compound when hot.
Q: You accidentally add far too much ethanol to your recrystallization flask. What should you do?
A: Gently boil off the excess solvent to concentrate the solution, then allow it to cool slowly for crystallisation.
Q: Sucrose is a polar compound. If sucrose were the impurity in your sample, which solvents from the lab's solvent table would be unsuitable for recrystallization?
A: The polar solvents (water, methanol, ethanol) would dissolve sucrose along with the target compound, making separation by recrystallization ineffective. n-Hexane, being non-polar, would not dissolve sucrose and could allow its removal.
Q: Explain why you use hot solvent to rinse the filter paper during gravity filtration but cold solvent to wash crystals during vacuum filtration.
A: During gravity filtration, the goal is to keep the desired compound dissolved so it passes through the filter while insoluble impurities are retained. Hot solvent prevents premature crystallisation. During vacuum filtration, the goal is to keep the purified crystals solid on the filter while washing away soluble impurities. Cold solvent prevents the crystals from redissolving.
Solubility and "like dissolves like" connect directly to intermolecular forces (Chapter 2 in most organic chemistry texts). The polarity arguments you use to pick a recrystallization solvent are the same ones you will use later to predict extraction behaviour, chromatographic separation, and reaction solvent choice.
Melting point depression links to colligative properties from general chemistry. The same principle explains why salt lowers the freezing point of water on icy roads.
Filtration techniques (gravity and vacuum) appear again in nearly every synthesis experiment for the rest of the course, so the skills you build here transfer directly.
Recrystallization, crystallisation, purification of solids, melting point determination, melting point depression, percent recovery, percent recovery formula, percent yield vs percent recovery, solvent selection for recrystallization, gravity filtration, hot filtration, vacuum filtration, cold filtration, Buchner funnel, Mel-Temp, biphenyl, benzoic acid, 2,3-dimethylphenol, 2-methylbenzoic acid, resorcinol, benzene-1,3-diol, 4-hydroxybenzaldehyde, CHM 25502, organic chemistry lab, Purdue CHM 255, unknown solid identification, solubility and temperature