2.2 (a) Criteria of purity


1 Describe paper chromatography
2 Interpret simple chromatograms
3 Identify substances and assess their purity from melting point and boiling point information
4 Understand the importance of purity in substances in everyday life, e.g. foodstuffs and drugs


5 Interpret simple chromatograms, including the use of Rf values
6 Outline how chromatography techniques can be applied to colourless substances by exposing chromatograms to substances called locating agents (knowledge of specific locating agents is not required)

2.2 (b) Methods of purification


1 Describe methods of purification by the use of a suitable solvent, filtration, crystallisation, distillation (including use of fractionating column). (Refer to the fractional distillation of crude oil in section 14.2 and products of fermentation in section 14.6.)
2 Suggest suitable purification techniques, given information about the substances involved 


 2.2 (a) Criteria of purity

Before talking about substances, we need to analyze the properties of matter


Pure substances and Mixtures

Substances can be pure or impure.

Pure substances

Pure substances are the ones that have a definite composition, this means that if we take a sample from different zones in the substance, the properties will be the same. 

They can be identified by their intensive properties. Example: Boiling and Melting point, density, etc. 

Elements are pure substances

Elements cannot be broken into simpler substances because they contain only one kind of atoms. Examples: H2, O2, Cl2, Na, Ne, Ar, etc.

These elements can be found in the periodic table.

solids liquids and gases as pure elements

Compounds are pure substances

Compounds that can be broken in simpler substances because they are formed by 2 or more atoms combined together. Examples: water, sulfuric acid, sodium chloride, ethanol, etc

compounds pure substances

Impure Substances

Mixtures are NOT pure substances

When two or more compounds or elements are mixed together, we have a mixture. 

Heterogeneous Mixtures

Their properties very throughout the substance

heterogeneous mixture

Homogeneous Mixtures - Solutions

Their properties are the same in any point of the substance. Homogeneous mixtures are also called solutions. Example: Sea water, brass, bronze, etc. These mixtures can be solid, liquid or gases

homogeneous mixtures


Mixtures need to be separated so we can get pure substances or get rid of the impurities present in the samples. 

Depending on the properties of the substances present, we will utilize different separation methods. 

There are several separation methods. The most important for your IGCSE examination are:



© Analia Sanchez