IGCSE Chemistry 0620 - Separation and Purification

This section focuses on techniques used to separate mixtures and assess the purity of substances using physical properties like melting point and boiling point.

Separation Techniques

Evaporation

Evaporation is used to separate a solid from a liquid when the liquid has a significantly lower boiling point. The mixture is heated, and the solvent evaporates, leaving the solid behind.

• Example: Separating salt (solid) from water (liquid).

Distillation

Distillation is used to separate liquids with different boiling points. The mixture is heated, and the vapor of the more volatile liquid is collected and condensed.

Types of Distillation:

• Simple Distillation: Used for liquids with large boiling point differences.
• Fractional Distillation: Used for liquids with close boiling points. A fractionating column is used to improve separation.

Chromatography

Chromatography is a technique used to separate substances based on their different affinities for a stationary and a mobile phase.

Types of Chromatography:

• Thin Layer Chromatography (TLC): A simple and quick method.
• Column Chromatography: Used for larger quantities of substances.

Sublimation

Sublimation is the process where a solid changes directly into a gas without passing through the liquid phase. This can be used to separate a solid from a mixture if the other components do not sublime.

Example: Separating dry iodine from a mixture.

Assessment of Purity using Melting Point and Boiling Point

Melting Point

The melting point of a pure substance is a sharp and constant temperature. Impurities lower the melting point and broaden the melting range.

Procedure: A small amount of the substance is heated in a capillary tube, and the temperature at which it starts to melt and completely melt is recorded.

Purity Assessment:

• Pure substance: Sharp melting point (e.g., 100°C).
• Impure substance: Broad melting range (e.g., 95-100°C).

Boiling Point

The boiling point of a pure substance is a constant temperature at a given pressure. Impurities raise the boiling point and broaden the boiling range.

Procedure: The substance is heated in a distillation apparatus, and the temperature at which it starts to boil and remains constant is recorded.

Purity Assessment:

• Pure substance: Sharp boiling point (e.g., 100°C at 1 atm).
• Impure substance: Broad boiling range (e.g., 102-104°C at 1 atm).

Technique	Principle	Application	Purity Assessment
Evaporation	Difference in boiling points	Separating solid from liquid	N/A
Distillation	Difference in boiling points	Separating liquids	Sharp boiling point indicates purity
Chromatography	Different affinities for stationary and mobile phases	Separating mixtures	Sharp spot indicates purity
Sublimation	Direct solid to gas transition	Separating sublimable solids	Sharp melting/sublimation point indicates purity
Melting Point	Sharp melting point for pure substances	Assessing purity of solids	Sharp melting point indicates purity
Boiling Point	Constant boiling point for pure substances	Assessing purity of liquids	Sharp boiling point indicates purity

Important Note: When assessing purity using melting or boiling point, it's crucial to use a small amount of the substance and allow it to melt or boil slowly to ensure accurate temperature readings.