Compounds

A compound is a pure substance formed from two or more elements chemically combined together in a definite proportion. A compound can only be decomposed by a chemical action into two or more simpler substances. For example, lime is a compound of calcium and oxygen. Its chemical formula is CaO. Similarly, water is a compound of hydrogen and oxygen. The chemical formula of water is $large H_2O$ . Other examples of compounds are ammonia ( $large NH_3$ ), carbon dioxide ( $large CO_2$ ), chalk ( $large CaCO_3$ ), hydrogen chloride ( $HCl$ ) and ammonium chloride ( $large NH_4Cl$ ).

Properties of compounds:

A compound cannot be separated into its constituent elements by mechanical or physical methods because the indivisual property of the element is lost during the formation of a compound. For example, water, iron sulphide, carbon dioxide.

The properties of a compound differ entirely from those of its constituent elements. For instance, water is a compound made up of hydrogen and oxygen. But, the properties of water are different from those of hydrogen and oxygen. Water is a liquid, while hydrogen and oxygen are gases. Similarly, sodium chloride is a compound. It is formed from the elements sodium (solid and highly reactive) and chlorine (gaseous and pungent smelling). Sodium chloride (common salt) can be directly taken with food, but both sodium and chlorine are poisonous.

When a compound is formed, energy is usually released or absorbed (in the form of heat or light) during the chemical reaction. For example water, the constituent elements of water —hydrogen and oxygen —are both gases and do not react with each other unless an electric spark is provided, i.e., energy is supplied. Similarly, the constituents of iron sulphide—iron and sulphur—if kept together, do not react on their own. But, if the mixture is heated, a vigorous reaction occurs and black iron sulphide is formed.

In a compound, the constituent elements are present in a definite proportion by weight. In water, hydrogen and oxygen are present in a fixed ratio of 1: 8 by weight.

A compound has a fixed melting point, a fixed boiling point, etc. For example, ice always melts at 0°C.

A compound is a homogeneous substance, i.e., a compound is such a substance which is the same throughout in its properties and composition. For example, iron sulphide is a compound of iron and sulphur. If we see iron sulphide through a microscope or a magnifying glass, particles of iron or sulphur cannot be separately identified.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Arrange the following substances in the ascending order of the number of constituent elements (S) present in them. A: Sodium Bicarbonate B: Calcium Carbonate C: Water D: Copper
A. D, C, B, A	B. C, B, A, D	C. B, A, D, C	D. A, B, C, D
Right Option : A
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Question : 2

Identify the true statement among the following :
A. Both compounds and mixtures are homogeneous.		B. Both compounds and mixtures are heterogeneous
C. Compounds are homogeneous and mixtures are heterogeneous.		D. Compounds are homogeneous and mixtures can be homogeneous or heterogenous.
Right Option : D
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Question : 3

Which of the following are not the properties of compound ? (1) Substances mix in any ratio (2) Elements combine chemically to form a compounds (3) Their constituents can be separated by simple physical methods. (4) Their constituents cannot be separated by simple physical methods
A. (1) and (2)		B. (2) and (4)
C. (1),(2),(3)		D. (1),(3)
Right Option : D
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