Classification of matter on the basis of chemical composition

Elements compound and mixture of Class 8

Classification of matter on the basis of chemical composition

Pure Substances

  • Substances which are homogenous in nature and contain particles of only one kind are called pure substances.  A pure substance is homogeneous in nature and has definite set of properties. The composition of a pure substances cannot be changed by physical methods.  Pure substances can be further classified as elements and compounds.

ELEMENTS

Pure substances in which molecules are composed of only one kind of atoms are called elements. Example: Silver, Iron, Oxygen, Nitrogen, etc.

Atomicity of elements

The number of atoms present in the molecule of an element is called its atomicity.  Based on the atomicity, the elements are classified into 3 types

  • Monoatomic elements:  The elements each of whose molecules contains only one atom. Example: Cu, Ag and He.
  • Diatomic elements:  The elements each of whose molecules contains two atoms. Example: H2, O2 and N2.
  • Polyatomic elements:  The elements each of whose molecules contain more than two atoms. Example: O3, P4 and S8.

Classification of Elements

It was found that there was a wide variation in the properties of elements. Hence these were further classified into three categories, i.e. metals, non-metals, metalloids based on the properties they exhibit.To score More in your class 8 refer NCERT solutions for class 8 .

 

Classification of matter on the basis of chemical composition

Compounds

Pure substances in which two or more elements combine chemically in a fixed proportion by weight are called compounds.

For example, water (H2O) is considered a pure substance even though it consists of two kinds of atoms, as it has a fixed number of hydrogen and oxygen atoms combined together chemically in a definite proportion by weight.  Hydrogen and oxygen combine in a fixed ratio, 1:8 by weight, to form water.

Mixtures

The systematic study of elements is done by categorising them as metals, non-metals and metalloids.  The different elements combine with each other to form compounds. The formation of compounds from elements results in change in the molecular composition.  Two or more substances either elements or compounds can be mixed together in any proportion. The resultant substances are called mixtures. Since the formation of a mixture involves only physical process, there is no change in the molecular composition.

Compounds

Mixture

Constituent atoms combine chemically.

Constituent substances are mixed physically.

Constituent elements do not retain their properties

The constituents retain their individual properties.

Constituents can be separated by chemical methods.

Constituents can be separated by physical methods.

These are homogenous.

These can be either homogeneous or heterogeneous.

It involves significant energy changes.

No significant change in the energy.


Classification of matter on the basis of chemical composition

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