Separation techniques — filtration, distillation, chromatography, centrifugation, sublimation

Core Principle: Exploit Physical Property Differences

Separation techniques work because mixture components differ in at least one physical property. We amplify that difference through a process, then collect components separately.

1. Filtration — Separate by Particle Size

When to use: Solid–liquid mixtures where the solid is insoluble and has significantly larger particles than the pore size.

How Filtration Works

1. Pour mixture onto filter paper in a funnel
2. Gravity or pressure pulls liquid through pores
3. Solid residue (called the residue or retentate) stays on filter
4. Liquid (called the filtrate) collects below

WHY does particle size matter? Pore diameter in filter paper: ~2–10 μm. Sand particles: ~50–2000 μm. Sand can't fit through. Dissolved salt ions: ~0.003 μm. They pass easily.

Filtration does not remove dissolved substances. Salt water filtered = stillalty water.

Step 1: Pour into filter funnel. Why this step? Gravity moves the mixture toward the filter, starting separation.

Step 2: Wait for liquid to drip through. Why this step? Water + dissolved NaCl molecules are small enough to pass through pores. Sand grains (200 μm) are 100× too large.

Step 3: Collect clear liquid (filtrate = salt water) below; scrape sand off filter paper (residue). Why this step? Physical collection completes the separation.

Result: Sand separated. Salt still dissolved in water—need another technique (evaporation or distillation) to separate salt from water.

2. Distillation — Separate by Boiling Point

When to use: Liquid–liquid mixtures (or solid dissolved in liquid) where components have different boiling points—ideally >25°C apart for simple distillation.

How Distillation Works

1. Heat the mixture in a flask
2. Component with lower BP vaporizes, rises as gas
3. Vapor enters condenser (cold water-cooled tube)
4. Vapor cools → condenses back to liquid
5. Distillate (purified liquid) collects in receiving flask
6. Higher-BP component remains in original flask

WHY does boiling point matter? At BP, vapor pressure = atmospheric pressure. Lower BP means molecules escape liquid phase at lower temperature. Ethanol (BP 78°C) vaporizes before water (BP 100°C) when you heat wine.

For a mixture, the component with lower BP reaches this condition first. Raoult's Law (ideal mixtures): $P_{\text{total}} = P_A^0 \chi_A + P_B^0 \chi_B$ where $P_A^0$ is pure-component vapor pressure, $\chi_A$ is mole fraction. The volatile (low-BP) component contributes more to total pressure, so it dominates the vapor phase initially.

Types of Distillation

Simple distillation: One vaporization–condensation cycle. Works when BP difference >25°C. Example: water (100°C) from salt (doesn't vaporize).

Fractional distillation: Multiple vaporization–condensation cycles in a fractionating column (packed with glass beads or rings). Each cycle enriches the more volatile component. Works when BP difference <25°C. Example: separating crude oil into gasoline (40–200°C), kerosene (150–300°C), diesel (250–350°C).

Step 1: Heat wine in distillation flask to ~80°C. Why this step? Ethanol boils; water mostly doesn't. Vapor is ~60% ethanol (not 100% because it's a mixture—see Raoult's Law).

Step 2: Vapor rises, enters condenser, cools to liquid. Why this step? Cooling below78°C condenses ethanol vapor. Cold water jacket around condenser removes heat.

Step 3: Collect distillate. Why this step? First fractions are ethanol-enriched (~40–60%). Repeat distillation for higher purity.

Result: Distillate is stronger alcohol (brandy ~40–50%). Water remains in flask (discard or re-distill).

Note: Can't reach 100% ethanol by simple distillation due to azeotrope at 95.6% ethanol–water.

3. Chromatography — Separate by Polarity/Affinity

When to use: Complex mixtures (dyes, amino acids, pharmaceuticals) where components differ in polarity, size, or affinity for a stationary phase.

How Chromatography Works

Two phases:

• Stationary phase: immobile material (paper, silica gel, polymer beads)
• Mobile phase: moving solvent (liquid or gas) that carries sample

Mechanism:

1. Spot sample on stationary phase (e.g., dot of ink on paper)
2. Mobile phase flows through stationary phase (e.g., water rises up paper by capillary action)
3. Components partition between phases based on polarity:
   • Polar molecules attracted to polar stationary phase → move slowly
   • Nonpolar molecules stay in mobile phase → move quickly
4. Components separate spatially as they move at different rates

WHY does polarity matter? "Like dissolves like." Polar silica gel attracts polar molecules (H-bonds, dipole interactions). Nonpolar molecules have weak interactions with silica, so they stay dissolved in mobile phase and travel farther.

$0 < R_f < 1$. Low $R_f$: component stuck to stationary phase (polar). High $R_f$: component mobile (nonpolar).

Derivation from first principles: At equilibrium, a component distributes between phases according to partition coefficient $K$: $K = \frac{[\text{component in mobile phase}]}{[\text{component in stationary phase}]}$ High $K$ → prefers mobile phase → travels far → high $R_f$. $R_f \propto \frac{1}{1 + k'}$ where $k' = \frac{K_d V_s}{V_m}$ (capacity factor, $K_d$ is distribution constant, $V_s$ $V_m$ are phase volumes).

Types of Chromatography

• Paper chromatography: Paper is stationary phase (cellulose, polar). Solvent climbs by capillary action. Cheap, quick. Used for inks, dyes, amino acids.
• Thin-layer chromatography (TLC): Silica gel on glass/plastic. Faster, better resolution than paper.
• Column chromatography: Stationary phase packed in vertical column; solvent flows down by gravity. Preparative scale (grams).
• Gas chromatography (GC): Mobile phase = inert gas (He); stationary phase = liquid coating inside capillary. Separates volatile compounds. Used in forensics, drug testing.
• High-performance liquid chromatography (HPLC): High-pressure pump forces mobile phase through fine stationary phase. Separates complex mixtures (pharmaceuticals, proteins).

Step 1: Dot black ink 2 cm from bottom of paper strip. Why this step? Starting point for all components. Must be above final solvent level.

Step 2: Dip paper in solvent (water + alcohol, 1:1) solvent is 1 cm from bottom—below the dot. Why this step? Solvent rises by capillary action, carrying dyes with it. If dot is submerged, dyes dissolve directly into solvent (no separation).

Step 3: Wait 20 minutes. Solvent climbs to 10 cm. Why this step? Different dyes have different polarities:

• Blue dye (polar): sticks to celulose, travels 3 cm → $R_f = 3/10 = 0.30$
• Red dye (medium): travels 6 cm → $R_f = 0.60$
• Yellow dye (nonpolar): travels 9 cm → $R_f = 0.90$

Step 4: Mark positions, measure distances, calculate $R_f$. Why this step? $R_f$ values identify compounds by comparing to reference table.

Result: Black ink separated into three colored bands. Each dye has characteristic $R_f$ in this solvent system.

4. Centrifugation — Separate by Density

When to use: Suspensions of small particles (cells, organelles, precipitates) in liquid where density differences exist but gravity settling is too slow.

How Centrifugation Works

1. Load sample in tubes, place in rotor
2. Spin at high speed (e.g., 5000 RPM = 3000×g)
3. Centrifugal force pushes denser particles outward (to bottom of tube)
4. Less dense liquid (supernatant) stays on top
5. Stop, carefully remove supernatant with pipette; pelet (dense particles) remains at bottom

WHY does density matter? Centrifugal force $F_c = m\omega^2 r$ where $m$ is mass, $\omega$ is angular velocity, $r$ is radius. Denser particles have more mass per volume, so experience greater net force (after subtracting buoyancy).

Derivation: Centrifugal acceleration: $a_c = \omega^2 r$ where $\omega = 2\pi \times \text{RPM}/60$ rad/s. $\text{RCF} = \frac{a_c}{g} = \frac{(2\pi \times \text{RPM}/60)^2 \times r}{g}$ $= \frac{4\pi^2 \times r}{3600g} \times \text{RPM}^2 = \frac{4 \times 9.87 \times r}{3600 \times 9.81} \times \text{RPM}^2$ $= 1.118 \times 10^{-5} \times r\,(\text{cm}) \times \text{RPM}^2$

Sedimentation velocity (Stokes' Law): $v = \frac{2r^2(\rho_p - \rho_f)a_c}{9\eta}$ where $r$ is particle radius, $\rho_p$ is particle density, $\rho_f$ is fluid density, $\eta$ is viscosity. Larger, denser particles sediment faster.

Step 1: Load 5 mL blood in centrifuge tube, spin at 3000 RPM for 10 min (RCF ≈ 1000×g). Why this step? RBCs, being densest, experience highest net force outward. 1000×g is sufficient to pelet RBCs in 10 min (Stokes' Law: $v \propto (\rho_p - \rho_f)$).

Step 2: RBCs form red pelet at bottom. WBCs form thin white layer (buffy coat) above RBCs. Plasma is clear yellow liquid on top. Why this step? Density gradient: RBC (1.1) > WBC (1.07) > plasma (1.03). Centrifugal force stratifies layers by density.

Step 3: Carefully pipette plasma without disturbing pelet. Why this step? Gentle removal preserves separation. Plasma now available for testing (glucose, proteins, antibodies).

Result: Components separated by density. Used in clinical labs worldwide (CBC test).

5. Sublimation — Separate by Phase Transition

When to use: Mixtures where one component sublimes and others don't (or sublime at much higher temperature).

How Sublimation Works

1. Heat mixture gently
2. Sublimable component vaporizes directly from solid
3. Vapor rises, contacts cool surface (cold finger, watch glass with ice)
4. Vapor deposits as solid crystals on cool surface
5. Non-subliming components remain in original container

WHY does sublimation occur? At temperatures below triple point, solid's vapor pressure can exceed atmospheric pressure if intermolecular forces are weak. Molecules escape solid lattice directly to gas phase. For iodine: triple point at 113.7°C, 12 kPa. At room temp (25°C), iodine vapor pressure ≈ 0.3 kPa—enough to sublime noticeably.

Derivation from first principles: At equilibrium, chemical potential of solid = chemical potential of vapor: $\mu_s = \mu_g$ $\mu_s^0 = \mu_g^+ RT\ln P$ Differentiating with respect to $T$ (at constant $P$): $-\frac{S_s}{T} = -\frac{S_g}{T} + R\frac{d\ln P}{dT}$ $\frac{d\ln P}{dT} = \frac{S_g - S_s}{R} = \frac{\Delta H_{\text{sub}}}{RT^2}$ (using $\Delta G = \Delta H - T\Delta S$ and $\Delta G = 0$ at equilibrium). Integrating: $\ln P = -\frac{\Delta H_{\text{sub}}}{RT} + \text{const}$ Higher $\Delta H_{\text{sub}}$ → lower vapor pressure → less sublimation.

Common Subliming Substances

| Substance | Sublimation Temp (1 atm) | Application | |-----------|--------------------------| | Dry ice (CO₂) | -78.5°C | Cooling, fog effects | | Iodine (I₂) | 184°C | Purification, antiseptic | | Camphor | 204°C | Moth repellent | | Naphthalene | 218°C | Mothballs, synthesis |

Step 1: Place mixture in evaporating dish, invert funel above it (stem pluged with cotton), place ice-filled beaker on funel. Why this step? Creates temperature gradient: hot bottom (heat source), cold top (ice). Iodine will sublime and redeposit on cold surface.

Step 2: Heat gently with Bunsen burner to ~190°C. Why this step? Iodine's vapor pressure becomes significant above 184°C. Solid iodine → violet vapor directly. Sand remains solid (melting point 1710°C).

Step 3: Purple iodine vapor rises, contacts cold funel, deposits as shiny gray-black crystals on funel surface. Why this step? Cooling reduces vapor pressure → equilibrium shifts to solid. $\ln P = -\Delta H_{\text{sub}}/RT$ → lower $T$ → much lower $P$ → deposition.

Step 4: Turn off heat, let cool, carefully collect iodine crystals. Why this step? Physical collection. Sand left behind in dish.

Result: Pure iodine crystals. Purity >99% (trace impurities that also sublime may remain—resubline for higher purity).

Summary Table: Choosing the Right Technique

| Property Exploited | Technique | Example | Limitation | |------------------------|---------------|-------------| | Particle size | Filtration | Sand from water | Doesn't remove dissolved substances | | Boiling point | Distillation | Ethanol from water | Ineffective if BP difference <25°C (need fractional) or if compound decomposes | | Polarity/Affinity | Chromatography | Dyes ink | Requires soluble samples; slow for large scale | | Density | Centrifugation | Blood cells from plasma | Requires significant density difference; small-scale | | Sublimation ability | Sublimation | Iodine from sand | Only works if one component sublimes |

Decision tree:

1. Solid in liquid, insoluble? → Filtration
2. Liquids with different BP? → Distillation
3. Complex mixture, small scale? → Chromatography
4. Small particles, density difference? → Centrifugation
5. One component sublimes? → Sublimation

Connections

• States of Matter — phase transitions (solid/liquid/gas) underpin distillation and sublimation
• Solutions and Solubility — "like dissolves like" explains chromatography partitioning
• Intermolecular Forces — weak IMF → easier sublimation; H-bonding → polarity in chromatography
• Vapor Pressure — drives distillation and sublimation
• Colligative Properties — boiling point elevation affects distillation of solutions
• Acids and Bases — ion-exchange chromatography separates by charge
• Thermodynamics — $\Delta H_{\text{sub}}$, $\Delta H_{\text{vap}}$ determine energy requirements
• Stoichiometry — calculate % recovery after separation

Filtration is like using a net. Big LEGOs get caught, small marbles fall through. But if you have LEGO dust mixed with the marbles, the net won't help—particles too small.

Distillation is like boiling water to make steam, then catching the steam in a cold bottle so it turns back to water. If you had salty water, the salt doesn't turn into steam (it's not hot enough to "fly away"), so you get pure water in the bottle and salt left in the pot.

Chromatography is like a race where some runners are wearing Velcro shoes on a Velcro track. The sticky ones (polar molecules) slow down, non-sticky ones (nonpolar) run fast. They finish at different times, so you can tell them apart.

Centrifugation is like spinning a bucket of mudy water really fast. The mud is heavier, so it flies to the bottom. Clear water stays on top. It's artificial super-gravity.

Sublimation is for special substances that go from ice directly to steam without melting. Dry ice does this! It's solid CO₂, and it turns into gas without becoming liquid. If you had dry ice mixed with regular ice, you could separate them by letting the dry ice "fly away" and collecting it on a cold plate.

Alternatively: "Fine Diamond Cuts Can't Scratch" for the order Filtration, Distillation, Chromatography, Centrifugation, Sublimation.

Flashcards

#flashcards/chemistry

What physical property does filtration exploit? :: Particle size. Large particles trapped by porous barrier; small particles and fluids pass through.

What is the distillate in distillation? :: The purified liquid collected after vapor condenses in the receiving flask.

What is the pelet in centrifugation? :: The dense particles that sediment to the bottom of the tube.

Concept Map

Hinglish (regional understanding)