iii.2. Purity: Fixed Structure and types of Pure Substances

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iii.2. Purity: Fixed Structure and types of Pure Substances

Meaning: Pure substances are the substances that are made up of only one kind of particle and have a fixed or constant structure.

Chemical Composition and Transformation: Pure Substances Classified into Elements and Compounds	On the basis of their chemical composition, substances can be classified either as elements or compounds as follows.
Elemental Transformation: Historical and Modern Divisions of Metals, Non-metals and Metalloids:	Historical Evolution of the Term ‘Element’ from Boyle to Lavoisier: Robert Boyle, first to use the term element in 1661, was defined by Antoine Laurent Lavoisier, in the 18th century, as a fundamental form of matter that cannot be further decomposed into simpler substances through chemical reactions. Division: Elements can be normally divided into metals, nonmetals and metalloids.
Properties of Metals: Shining, Conductive, and Resonant Characterstics:	They have a lustre (shine). They have a silvery-grey or golden-yellow colour. They conduct heat and electricity. They are ductile (can be drawn into wires). They are malleable (can be hammered into thin sheets). They are sonorous (make a ringing sound when hit). Examples of metals include gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.
Properties of Non-Metal: Colors, Conductivity, and Malleability	They display a variety of colours. They are poor conductors of heat and electricity. They are not lustrous, sonorous or malleable. Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc

Metalloids: These are elements showing properties between those of metals and nonmetals, examples are boron, silicon, germanium.

Mixtures and Compounds
Mixtures	Compounds
Elements or compounds just mix together to form a mixture and no new compound is formed.	Elements react to form new compounds.
A mixture has a variable composition.	The composition of each new substance is always fixed.
A mixture shows the properties of the constituent substances.	The new substance has totally different properties.
The constituents can be separated fairly easily by physical methods.	The constituents can be separated only by chemical or electrochemical reactions.