Introduction

Why Purification and Separation Matter

There has been a systematic, ongoing development in the techniques chemists use to analyse chemical substances. Chemical substances occurring in nature are almost always found in an impure state, mixed with other materials. Likewise, when a compound is synthesised in the laboratory, it is usually obtained in a crude, impure form. Before the composition and properties of any such substance can be reliably investigated, it must first be brought into a pure form.

The Guiding Principle

Every method of purification and separation covered in this chapter works by exploiting a difference in some physical property between the substances involved — most commonly solubility (in a given solvent), volatility (boiling point), or how strongly a substance is adsorbed onto a surface. None of these methods relies on a chemical reaction; they are purely physical separations.

Techniques Covered in This Chapter

• Filtration and Crystallization — for purifying solids of both soluble and insoluble impurities.
• Distillation (simple, fractional, and under reduced pressure) — for purifying/separating liquids based on differences in boiling point.
• Solvent Extraction — for pulling an organic substance out of an aqueous solution using an immiscible solvent.
• Chromatography (adsorption and partition types) — for separating and purifying the components of a mixture based on differential movement through a stationary phase.