Module 10: Laboratory Techniques and Chemical Separation
Lab Techniques & Chemical Separation
This module covers high-yield experimental techniques that appear on the MCAT, especially in the context of experimental passages. You’ll learn how to interpret setups, predict outcomes, and distinguish between different types of separation based on molecular properties.
Experimental Design and Measurement Tools
Common Lab Equipment:
| Tool | Function |
|---|---|
| Beaker / Erlenmeyer | Contain, mix, or heat solutions |
| Graduated cylinder | Measure liquid volumes with moderate precision |
| Buret | Deliver precise volumes during titrations |
| Pipette | Transfer exact volumes of liquid |
| Volumetric flask | Prepare solutions of known concentration |
| Analytical balance | Accurately measure mass (to 0.0001 g) |
| pH meter | Measure pH electronically |
| Spectrophotometer | Measures absorbance → used to determine concentration |
MCAT Tip: Be ready to recognize which instrument is most appropriate for a given task. Don’t use a beaker when a pipette is needed!
Chemical Separation Methods
Based on Physical Properties:
| Separation Method | Basis of Separation | When to Use It |
|---|---|---|
| Filtration | Particle size / phase | Separate solid from liquid |
| Distillation | Boiling point | Separate miscible liquids |
| • Simple Distillation | Large BP difference (>25°C) | Clean water from saltwater |
| • Fractional Distillation | Small BP difference (<25°C) | Ethanol from water |
| Extraction | Solubility / polarity | Separate compounds between aqueous & organic layer |
| Chromatography | Polarity / size / charge | Analyze/separate mixtures (TLC, column, HPLC) |
| Centrifugation | Density | Isolate components like organelles or precipitate |
| Crystallization | Solubility (temperature-dependent) | Purify solid product |
| Decanting | Density | Pour off liquid from settled solid |
MCAT Strategy: Focus on what’s being separated and why—recognize how properties like boiling point, solubility, or charge influence separation.
1. Distillation — Separation by Boiling Point
What It Does:
Distillation separates components of a liquid mixture based on differences in boiling point (BP). The more volatile component (lower BP) vaporizes first and is collected via condensation.
Types:
| Type | When to Use |
|---|---|
| Simple Distillation | When BP difference is >25°C (e.g. water from salt) |
| Fractional Distillation | When BP difference is <25°C (e.g. ethanol from water) |
| Vacuum Distillation | For very high BP compounds that might decompose at normal pressure |
Setup:
- Heat mixture → vapor rises
- Enters condenser → cools → liquid collected
- First fraction = lowest BP component
MCAT Tip: In fractional distillation, the fractionating column provides multiple condensation–vaporization cycles → improves separation of similar BPs.
2. Chromatography — Separation by Polarity, Size, or Charge
What It Does:
Chromatography separates compounds based on differential affinities to a mobile phase and a stationary phase.
Types:
| Type | Mobile Phase | Stationary Phase | Used For |
|---|---|---|---|
| TLC (Thin Layer) | Organic solvent | Polar silica plate | Qualitative polarity comparison |
| Column Chrom. | Organic solvent | Polar silica gel | Isolating/purifying compounds |
| Gas Chrom. (GC) | Inert gas (e.g. He) | Coated capillary column | Analyzing volatile compounds |
| HPLC | Liquid solvent | Varies (reverse-phase = NP) | High-precision compound separation |
Key Idea:
- Nonpolar compound → moves farther in normal-phase TLC
- Polar compound → sticks more to polar stationary phase
Rf value (TLC):
$$
R_f = \frac{\text{Distance traveled by compound}}{\text{Distance traveled by solvent front}}
$$
Values between 0 and 1; higher = less polar.
3. Liquid–Liquid Extraction — Separation by Solubility
What It Does:
Uses two immiscible liquids (typically water + organic solvent) to selectively isolate compounds based on acid–base chemistry or polarity.
Acid–Base Extraction Strategy:
| Compound Type | How to Extract It |
|---|---|
| Carboxylic acid | Add NaHCO₃ → forms water-soluble salt |
| Phenol | Add strong base (NaOH) → deprotonates phenol |
| Amine (basic) | Add HCl → forms water-soluble ammonium salt |
Setup:
- Shake separatory funnel
- Layers separate based on density (organic layer often on bottom)
- Drain one layer → isolate target compound
MCAT Tip: Know which functional groups are acid/base and how they behave with aqueous acid/base washes. Also, denser layer (not always aqueous) is at the bottom.