Difficulty: Intermediate | Prerequisites: Basic solubility concepts, familiarity with lab filtration equipment.
Recrystallization is one of the most common purification techniques in organic chemistry. It exploits the difference in solubility of a compound in a hot versus cold solvent to separate it from impurities. This lab asks you to purify an unknown solid contaminated with sand and sawdust, then confirm its identity by melting point. If you are comfortable with the idea that most solids dissolve more readily in hot solvent than cold, you have the foundation you need.
You dissolve an impure solid in a hot solvent, filter out insoluble junk, cool the solution so pure crystals form, collect those crystals by vacuum filtration, dry them, and measure the melting point to check purity. The narrower the melting range, the purer the compound.
Recrystallization
A purification technique in which a solid is dissolved in a hot solvent and then allowed to re-form as crystals upon cooling, leaving impurities behind in solution. Think of it as: washing a solid from the inside out by dissolving and re-growing it.
Solubility
The ability of a substance to dissolve in a given solvent. In recrystallization, you need a solvent in which the compound is poorly soluble when cold but freely soluble when hot.
Crude compound
The impure solid before purification. In this experiment, the crude sample is 97% unknown compound, 2% sand, and 1% sawdust.
Filtrate
The liquid that passes through a filter. During gravity (hot) filtration, the filtrate contains your dissolved compound. During vacuum (cold) filtration, the filtrate is waste solvent.
Gravity filtration
Filtering a hot solution through filter paper using only the force of gravity. Used to remove insoluble impurities (sand, sawdust) while keeping the desired compound dissolved in the hot filtrate.
Vacuum filtration
Filtering a cold slurry through a Buchner funnel under reduced pressure. Used to collect recrystallized product quickly and leave it relatively dry.
Percent recovery
Mass of purified product divided by mass of crude material, times 100. In simple terms, this tells you how much of your starting material you managed to get back after purification.
Percent yield
Mass of purified product divided by the theoretical yield (the maximum possible product from the starting material), times 100. This measures how close your result is to the ideal outcome, not just how much you recovered.
Melting point / melting point range
The temperature (or narrow range of temperatures) at which a solid becomes a liquid. A pure compound melts sharply over 1 to 2 degrees Celsius. A broad or depressed range signals impurities.
Mel-Temp apparatus
A benchtop device used to measure melting points. You pack a small capillary tube with your dried solid and heat it while watching through a magnifying lens.
Buchner funnel
A flat-bottomed porcelain funnel with small holes, used with filter paper and a vacuum flask for vacuum filtration.
The technique relies on the fact that solubility increases with temperature for most solids.
You choose a solvent in which the compound is insoluble (or nearly so) at room temperature but dissolves readily when heated.
Dissolving the crude solid in hot solvent separates the desired compound (now in solution) from insoluble impurities (sand, sawdust), which are removed by gravity filtration.
Cooling the filtered solution causes the purified compound to crystallise out, while soluble impurities remain dissolved in the cold solvent (their concentration is too low to crystallise).
The pure crystals are collected by vacuum filtration, washed with a small amount of cold solvent, and dried.
The compound must be insoluble or sparingly soluble in the solvent when cold.
The compound must dissolve fully in the solvent when hot.
When the hot solution cools, crystals should re-form. If they do not, the solvent is not suitable.
To test: add a small amount of solid to a test tube with a few drops of cold solvent. If it does not dissolve, heat the tube in a hot water bath. If it dissolves hot and recrystallises on cooling, the solvent works.
Too much solvent: The solution is too dilute for crystals to form on cooling. You lose product because it stays dissolved. Fix: gently evaporate excess solvent by heating.
Too little solvent: The solid does not fully dissolve, so impurities remain mixed in. Fix: add more hot solvent, 1 mL at a time, until the solid just dissolves.
Best practice: add solvent slowly, in small portions, until the solid barely dissolves. This minimises the volume of solvent and maximises crystal yield on cooling.
Percent recovery = (mass of purified product / mass of crude starting material) x 100. This is the metric you use for recrystallization because there is no chemical reaction, only purification.
Percent yield = (mass of purified product / theoretical yield) x 100. This is used when a chemical reaction has taken place and you are comparing actual product to the maximum predicted by stoichiometry.
For this experiment, use percent recovery. Percent yield is not directly applicable because recrystallization is a physical separation, not a reaction.
Given: 4.8 g crude sample, composition 97% unknown, 2% sand, 1% sawdust.
Maximum recoverable unknown = 4.8 g x 0.97 = 4.656 g.
In practice you will recover less than 4.656 g because some compound remains dissolved in the cold solvent.
Sand and sawdust are insoluble in all common recrystallization solvents. They are removed during gravity (hot) filtration and never reach the final product.
Sucrose (table sugar) is a different story. It dissolves in polar solvents (water, methanol, ethanol, acetone, ethyl acetate) but is insoluble in non-polar solvents like n-hexane.
If sucrose were the impurity, you would need to avoid polar solvents for your recrystallization, or it would dissolve alongside your compound and contaminate the crystals.
These are the six possible unknowns. Matching your measured melting point to one of these values is how you identify your compound.
Compound | Molar Mass (g/mol) | Melting Point (°C) | Appearance | Hazards |
|---|---|---|---|---|
Biphenyl | 154.21 | 69 | White solid | Irritant, environmental hazard |
2,3-Dimethylphenol | 122.16 | 75 | Clear to brown chunky solid | Toxic, corrosive, environmental hazard |
Benzoic acid | 122.12 | 122.4 | White crystals | Health hazard, corrosive |
2-Methylbenzoic acid | 136.15 | 103.7 | Light yellow / off-white crystals | Irritant |
Benzene-1,3-diol (resorcinol) | 110.11 | 109.8 | White to pink (darkens with light) | Environmental hazard, irritant |
Hydroxybenzaldehyde | 122.12 | 117 | Light brown solid | Corrosive, irritant |
Know which solvents are flammable before you heat them on a hot plate. All organic solvents should be used in the fume hood.
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 |
Students often confuse percent recovery with percent yield. Percent recovery compares purified product to crude starting material. Percent yield compares product to a theoretical maximum from a chemical reaction. Recrystallization is not a reaction, so percent recovery is the correct metric here.
Students sometimes think adding more solvent will improve purity. Excess solvent does help dissolve the compound, but it also keeps more of it dissolved during cooling, reducing the mass of crystals you collect.
A common mistake is assuming that a compound which does not dissolve in cold solvent is insoluble in that solvent altogether. Temperature-dependent solubility is the entire basis of recrystallization: the compound dissolves hot but not cold.
Students often assume any impurity can be removed by gravity filtration. That only works for insoluble impurities (like sand). A soluble impurity such as sucrose would pass through the filter with the compound and remain as a contaminant.
⚠️ A narrow melting point range (1 to 2 °C) indicates a pure compound. A broad or depressed range means impurities are present. Expect a question asking you to interpret melting point data.
⚠️ Know when to use percent recovery versus percent yield. For any purification (no reaction), the answer is percent recovery.
⚠️ Be able to explain why a particular solvent is or is not suitable for recrystallizing a given compound. The criteria: insoluble cold, soluble hot, crystals form on cooling.
⚠️ Understand why sucrose as an impurity changes your solvent choice. Polar solvents dissolve sucrose, so it would not be removed by filtration. Non-polar solvents (n-hexane) leave sucrose undissolved.
⚠️ Know the maximum theoretical recovery calculation. For a 4.8 g sample that is 97% compound, the maximum is 4.656 g.
True or False: A good recrystallization solvent dissolves the compound well at both high and low temperatures. (False. It should dissolve the compound when hot but not when cold.)
Fill in the blank: The metric used to evaluate a recrystallization (not a reaction) is percent __________. (recovery)
True or False: Sand and sawdust dissolve in hot water and pass through the filter during gravity filtration. (False. They are insoluble and are trapped by the filter.)
Fill in the blank: A pure compound typically melts over a range of __________ degrees Celsius. (1 to 2)
True or False: Using too much solvent during recrystallization will increase your crystal yield. (False. Excess solvent keeps more compound dissolved, reducing yield.)
Q: What three criteria must a solvent meet to be suitable for recrystallization of a given compound?
A: The compound must be insoluble (or sparingly soluble) in the cold solvent, freely soluble in the hot solvent, and it must form crystals when the hot solution cools.
Q: You start with 5.0 g of crude solid that is 97% pure. What is the maximum mass of pure compound you can recover?
A: 5.0 g x 0.97 = 4.85 g.
Q: Explain why percent recovery, not percent yield, is the appropriate metric for evaluating a recrystallization.
A: Recrystallization is a physical purification, not a chemical reaction. There is no theoretical yield based on stoichiometry. Percent recovery compares the mass of purified product directly to the mass of crude starting material, which is the relevant comparison.
Q: Your unknown has a measured melting point range of 100 to 108 °C. What does this tell you about purity?
A: The broad range (8 °C) indicates the sample still contains impurities. A pure compound would melt over 1 to 2 °C.
Q: If sucrose were the impurity instead of sand, which solvents from the table would be unsuitable for recrystallization, and why?
A: Water, methanol, ethanol, ethyl acetate, and acetone would be unsuitable because sucrose dissolves in polar solvents. It would not be removed by filtration and would contaminate the crystals. Only n-hexane (non-polar) would leave sucrose undissolved.
Q: You accidentally added too much solvent and your solution is very dilute. How do you fix this without starting over?
A: Gently heat the solution to evaporate excess solvent until the volume is reduced, then cool as normal to induce crystallisation.
Recrystallization connects directly to solubility and intermolecular forces: the reason a compound dissolves better in hot solvent is that increased kinetic energy overcomes the lattice energy holding the crystal together. This is the same thermodynamic reasoning you will revisit in discussions of dissolution, entropy, and Gibbs free energy.
Melting point determination ties into the concept of eutectic mixtures and phase diagrams. An impure solid melts at a lower temperature and over a wider range because the impurity disrupts the crystal lattice, which is the same principle behind freezing point depression in solutions.
The choice of polar versus non-polar solvent for removing different impurities is an application of the "like dissolves like" principle covered in general chemistry and relevant throughout organic synthesis.
Recrystallization, purification of solids, solvent selection, hot filtration, cold filtration, gravity filtration, vacuum filtration, Buchner funnel, melting point determination, Mel-Temp, percent recovery, percent yield, crude compound, filtrate, solubility, like dissolves like, CHM 25502, Purdue organic chemistry lab, biphenyl, benzoic acid, 2-methylbenzoic acid, 2,3-dimethylphenol, resorcinol, hydroxybenzaldehyde, sand and sawdust impurities, sucrose impurity, crystal purity, melting point range, ice bath crystallisation