Chemistry · Class 11 Science

Ch 3Some Analytical Techniques — Class 11 Chemistry, concept-first.

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.

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Purification of Solids by Filtration and Crystallization

A solid substance may carry two kinds of impurity: those soluble, and those insoluble, in the same solvent as the main compound.

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3.1

Introduction

There has been a systematic, ongoing development in the techniques chemists use to analyse chemical substances.

3.2

Purification of solids

A solid substance may contain two types of impurities: (i) impurities that are soluble in the same solvent as the main substance, and (ii) impurities that are insoluble in that solvent.

3.2.1

Crystallization

When a crude solid consists mainly of one substance along with some soluble impurities, it is purified by crystallization, carried out in four steps.

3.2.2

Fractional crystallization

While ordinary crystallization purifies a single substance of its impurities, fractional crystallization is used to separate two or more soluble substances from one another, provided they have widely…

3.3

Distillation

Distillation is an important method used to separate (i) a volatile liquid from non-volatile impurities dissolved in it, and (ii) two or more liquids that have a sufficient difference in their boiling…

3.3.1

Simple distillation

Simple distillation is used to purify liquids that boil without decomposing at atmospheric pressure. The liquid is first converted into vapour by boiling, and the vapour is then condensed back into li…

3.3.2

Fractional Distillation

If the boiling points of two liquids in a mixture are not appreciably different, simple distillation cannot separate them cleanly.

3.3.3

Distillation under Reduced Pressure

Liquids that either have a very high boiling point, or that decompose when heated to their normal boiling point, are purified by distillation under reduced pressure.

3.4

Solvent Extraction

When an organic substance is dissolved in an aqueous (water-based) solution, it can be pulled out of that solution by shaking the mixture with an organic solvent in which the substance is more soluble…

3.5

Chromatographic techniques

Chromatography is a technique used both to separate the components of a mixture and to purify individual compounds.

3.5.1

Adsorption Chromatography

Adsorption Chromatography works on the principle of Differential Adsorption — different solutes in a mixture are adsorbed onto the (solid) stationary phase to different extents, and this difference is…

3.5.1.1

Column Chromatography

In Column Chromatography, the separation takes place over a column of stationary-phase material — commonly alumina or silica gel.

3.5.1.2

Thin Layer Chromatography

In Thin Layer Chromatography (TLC), a thin layer (about thick) of an adsorbent — silica gel or alumina — is spread over a glass plate, which acts as the stationary phase and is called the TLC plate or…

3.5.2

Partition Chromatography

In Partition Chromatography, both the stationary phase and the mobile phase are liquids, and separation of the mixture's components happens through continuous differential partitioning of each compone…