Crystallisation, Distillation, Chromatography — Revision Notes

⚡ 30-Second Revision

Crystallisation: — Purifies solids. Principle: Solubility difference with temperature. Dissolve hot, cool slow.
Distillation: — Purifies liquids. Principle: Boiling point difference. Vaporize, condense.

- Simple: $\Delta BP > 25^circ\text{C}$. - Fractional: $\Delta BP < 25^circ\text{C}$ (uses fractionating column). - Vacuum: For heat-sensitive compounds (lowers BP). - Steam: For water-immiscible, volatile compounds.

Chromatography: — Separates mixtures. Principle: Differential distribution between stationary and mobile phases.

- TLC: Thin-layer chromatography. $R_f = \frac{\text{Distance by solute}}{\text{Distance by solvent front}}$

2-Minute Revision

Purification methods are crucial for isolating pure substances. Crystallisation is for solids, leveraging their increased solubility in hot solvents and decreased solubility in cold ones. Key steps include dissolving in minimum hot solvent, hot filtration, slow cooling for pure crystal formation, and washing.

Distillation separates liquids based on boiling point differences. Simple distillation is for large boiling point differences ($>25^circ\text{C}$). Fractional distillation uses a fractionating column for better separation of liquids with close boiling points ($<25^circ\text{C}$).

Vacuum distillation is vital for heat-sensitive compounds, lowering their boiling point by reducing pressure. Steam distillation purifies water-immiscible volatile compounds. Chromatography is a versatile technique based on differential interaction with a stationary phase and a mobile phase.

In Thin-Layer Chromatography (TLC), components separate on an adsorbent layer, and their movement is quantified by the $R_f$ value ($R_f = \frac{\text{distance traveled by solute}}{\text{distance traveled by solvent front}}$).

Remember, slow cooling in crystallisation yields purer crystals, and a fractionating column enhances distillation efficiency.

5-Minute Revision

Mastering purification techniques is vital for NEET.

Crystallisation: This method purifies solids. The core principle is that most solids are more soluble in hot solvents than in cold ones. To purify, dissolve the impure solid in the *minimum* amount of a suitable hot solvent.

The solvent should dissolve the desired compound well when hot but poorly when cold, and impurities either very well or not at all. Filter the hot solution to remove insoluble impurities. Then, allow the solution to cool *slowly* and undisturbed.

Slow cooling promotes the formation of large, pure crystals by allowing molecules to arrange themselves perfectly, excluding impurities. Rapid cooling leads to small, impure crystals. Finally, filter the crystals, wash with cold solvent, and dry.

Distillation: This technique separates liquids based on differences in their boiling points.

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Simple Distillation: — Used when the boiling points of liquids differ by more than $25^circ\text{C}$ or to separate a volatile liquid from non-volatile impurities. Example: separating water and salt.
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Fractional Distillation: — Essential for separating liquids with *close* boiling points (difference less than $25^circ\text{C}$). It employs a fractionating column (packed with glass beads or rings) which provides a large surface area for repeated vaporization and condensation cycles, leading to a much better separation. Example: separating ethanol and water.
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Vacuum Distillation: — Used for liquids that decompose at or below their normal boiling points. By reducing the pressure, the boiling point is lowered, allowing distillation at a safer, lower temperature.
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Steam Distillation: — Used for water-immiscible, volatile organic compounds. Steam is passed through the mixture, and the compound co-distills with water at a temperature below the boiling point of either pure component, preventing decomposition.

Chromatography: A highly versatile separation technique based on the differential distribution of components between a stationary phase (fixed material) and a mobile phase (flowing solvent/gas).

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Column Chromatography: — Stationary phase (e.g., silica gel) packed in a column. Mobile phase (eluent) flows through. Components with stronger adsorption to the stationary phase move slower.
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Thin-Layer Chromatography (TLC): — Adsorbent layer on a plate. Sample spotted, solvent rises. Components separate into spots. The **$R_f$ value** (retardation factor) is characteristic for a compound:
$$R_f = \frac{\text{Distance traveled by solute}}{\text{Distance traveled by solvent front}}$$
A higher $R_f$ means greater affinity for the mobile phase.
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Paper Chromatography: — Similar to TLC, but paper acts as the stationary phase (water adsorbed on cellulose).

Remember the key principles and specific applications for each method. Pay attention to apparatus details and the factors affecting separation efficiency.

Prelims Revision Notes

Crystallisation

Principle: — Difference in solubility of solid in a solvent at different temperatures (usually higher solubility in hot solvent, lower in cold).
Purpose: — Purification of solid organic compounds.
Ideal Solvent Characteristics:

* Dissolves desired compound readily when hot, sparingly when cold. * Does not react chemically with the compound. * Dissolves impurities either very well (remain in mother liquor) or not at all (filtered off).

Process Steps: — Dissolution in hot solvent $\rightarrow$ Hot filtration (if insoluble impurities) $\rightarrow$ Slow cooling (for large, pure crystals) $\rightarrow$ Filtration $\rightarrow$ Washing with cold solvent $\rightarrow$ Drying.
Key Point: — Slow cooling is crucial for high purity and good crystal size. Rapid cooling leads to small, impure crystals.

Distillation

Principle: — Difference in boiling points (volatility) of liquid components.
Purpose: — Separation of liquid mixtures or volatile liquids from non-volatile impurities.
Types & Applications:

* Simple Distillation: For liquids with large boiling point difference ($>25^circ\text{C}$) or volatile liquid from non-volatile solid. E.g., water purification, acetone-water separation. * Fractional Distillation: For liquids with close boiling points ($<25^circ\text{C}$).

Uses a fractionating column (provides large surface area for repeated vaporization-condensation cycles). E.g., ethanol-water separation, crude oil refining. * Vacuum Distillation (Reduced Pressure): For liquids that decompose at or below their normal boiling points.

Lowering pressure reduces boiling point. E.g., glycerol purification. * Steam Distillation: For water-immiscible, volatile organic compounds that are steam volatile. Co-distills with water at a temperature below $100^circ\text{C}$.

E.g., purification of essential oils, aniline.

Key Point: — Thermometer bulb should be at the level of the side arm leading to the condenser to measure the temperature of the distilling vapor.

Chromatography

Principle: — Differential distribution (adsorption or partition) of components between a stationary phase (fixed) and a mobile phase (moving).
Purpose: — Separation, purification, and identification of components in complex mixtures.
Types:

* Adsorption Chromatography: Based on differential adsorption on stationary phase. * Column Chromatography: Stationary phase (e.g., silica gel, alumina) packed in a column. Mobile phase (eluent) flows through.

Stronger adsorption = slower movement. * Thin-Layer Chromatography (TLC): Adsorbent layer on a plate. Sample spotted. Solvent rises by capillary action. Components separate into spots. * Partition Chromatography: Based on differential partitioning between two liquid phases.

* Paper Chromatography: Water adsorbed on cellulose paper acts as stationary phase. Solvent as mobile phase.

$R_f$ Value (Retardation Factor): — Characteristic for a compound under specific conditions.

$$R_f = \frac{\text{Distance traveled by solute}}{\text{Distance traveled by solvent front}}$$

$R_f$ is always between 0 and 1. Higher $R_f$ means greater affinity for mobile phase.

Vyyuha Quick Recall

To remember the distillation types and their uses: Simple For Very Strong Differences:

Simple: Significant BP difference
Fractional: Fine (close) BP difference
Vacuum: Volatile, but Very heat-sensitive (decomposes)
Steam: Steam volatile, Separates from water (immiscible)