Page 30 - ICSE Chemistry 6

Water Oil Salt Sugar

Fig. 2.1: Matter in different forms
The substances may be pure or impure, homogeneous or heterogeneous.

Impure substances contain other substances also in trace amounts.

Pure Substances

Substances which contain par cles of only one kind are called pure substances. Pure substances are of
two types—elements and compounds. Elements are made up of only one kind of atoms. Compounds
are made up of only one kind of molecules. Both elements and compounds have unique proper es like
fi xed mel ng and boiling points, specifi c gravity, etc. They are both homogeneous, i.e., they have uniform
composi on.

The substances that do not have uniform composi on are called heterogeneous substances.

ELEMENTS

Ma er is composed of certain basic substances called
elements. An element is a substance that cannot be broken
into simpler substances by any physical or chemical means.
For example, gold, silver, mercury, sulphur, hydrogen and
oxygen are some elements.

There are 118 known elements today. 92 elements out of
these are naturally occurring and the remaining 26 elements
are made ar fi cially.
Fig. 2.2: A group of atoms making a molecule
An element is composed of minute par cles of its own kind
called atoms. An atom is the smallest unit of an element that shows all the proper es of that element.
Some atoms are capable of independent existence while others are found in combina on with atoms of
the same element or diff erent elements. For example, an atom of sodium can exist freely but an atom
of hydrogen cannot exist independently and hence combines with another atom of hydrogen to form
a molecule. A molecule represents the smallest unit in which a pure substance can be divided without
losing its basic nature or iden ty.

Characteristics of an Element

An element has the following proper es.
• An element cannot be broken into simpler substances by physical or chemical means as it is the
fundamental substance of ma er.
• It is a pure substance as it has atoms of only one kind.

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