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A compound is a pure substance made up of a chemical combination of two or more elements in a definite proportion. A compound contains two or more kinds of atoms chemically combined together to form new molecules. Water is one such example of a pure substance made up of a combination of hydrogen and oxygen combined in the ratio 2:1 by volume or 1:8 by mass. Hydrogen and oxygen are molecules of elements that exist as gases. But if two atoms of hydrogen are combined with one atom of oxygen under special conditions, then one molecule of water is formed. Therefore water is a compound.





The compound namely, water is formed by the combination of two elements hydrogen and oxygen in the proportion 1:8 by weight.

Compounds are homogeneous materials which contain two or more elements combined in a definite proportion by weight. Examples of compounds are common salt, carbon dioxide, etc.

Some common compounds and their constituent elements



C,H and O


Hydrogen oxide

H and O

Table salt

Sodium chloride

Na and Cl

Quick lime

Calcium Oxide

Ca and O

Slaked lime

Calcium hydroxide

Ca. H and O

Caustic soda

Sodium hydroxide

Na,H and O

Caustic Potash

Potassium hydroxide

K,H and O

Soda ash

Sodium carbonate

Na , C and O

Differences between Elements and Compounds





An element is a simple substance which cannot be further divided into simple ones.

A compound is a chemical combination of two or more elements.


Elements combine to form compounds.

Compounds can be decomposed into elements.


Atoms are fundamental particles of elements.

Molecules are the fundamental particles of compounds.


E.g. Oxygen, Iron, Hydrogen, etc.

E.g. Water, sugar, Hydrogen, Carbon disulphide, etc.


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