Solubility-Based Methods
High-Yield Summary
- Separation/purification breaks into three families: solubility-based methods (this article), distillation (boiling point), and chromatography (interaction with a stationary surface).
- Extraction transfers a dissolved compound from one solvent into another it's more soluble in, using two immiscible liquids — a polar aqueous phase and a nonpolar organic phase — separated with a separatory funnel via density.
- Multiple small-volume extractions recover more compound than one large extraction using the same total solvent volume — a key MCAT misconception to avoid.
- Filtration separates solid (residue) from liquid (filtrate): gravity filtration when the filtrate is wanted, vacuum filtration when the solid is wanted.
- Recrystallization purifies solids: dissolve in minimal hot solvent, cool so the desired compound crystallizes out while more-soluble impurities stay dissolved.
Extraction Workflow
- 1Add sample + two immiscible solvents (polar aqueous, nonpolar organic) to a separatory funnel.
- 2Shake to mix, then let layers separate by density — denser layer settles at the bottom.
- 3Open the stopcock to drain the bottom layer selectively.
- 4Use a rotary evaporator (rotovap) to remove the solvent and isolate the compound of interest.
Gravity vs. Vacuum Filtration
| Type | Use when |
|---|---|
| Gravity filtration | Desired product is dissolved in the liquid — you want the filtrate |
| Vacuum filtration | Desired product is the solid — you want it fast, left on the filter as residue |
Key Terms
- Aqueous phase
- The polar solvent (typically water) in an extraction.
- Organic phase
- The nonpolar solvent in an extraction.
- Residue
- The solid trapped on filter paper during filtration.
- Filtrate
- The liquid that passes through the filter paper during filtration.
- Rotary evaporator (rotovap)
- Instrument used to remove solvent and isolate a dissolved compound after extraction.
Common MCAT Trap
- One large extraction is NOT more effective than multiple small ones with the same total solvent — each pass only pulls out a portion of what remains, so repeated smaller extractions recover more overall.
- Recrystallization only works well when the compound's solubility differs meaningfully from the impurities' — don't assume it purifies any solid regardless of solubility profile.
- Don't mix up which filtration type keeps which fraction: gravity keeps the filtrate (dissolved product), vacuum keeps the residue (solid product) — reversing this is a common mix-up.
Quick Recall
You have 30 mL of extraction solvent. Is it better to do one 30 mL extraction or three 10 mL extractions?
Your desired product is a solid, and you want to isolate it quickly. Which filtration method?
What condition makes recrystallization an effective purification method for a given solid?