A compound is a pure substance that is formed by the combination of atoms of two or more elements. The atom in a compound is chemically combined in a fixed and constant proportion.
When compounds are formed, atoms of two or more elements combine chemically, by either transfer or by sharing of electrons. The atoms of the different elements in a compound always combine in a definite proportion. If this proportion is different, the same elements produce entirely a different compound.
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If the atoms of carbon and hydrogen combine in the proportion of 1:4, the resulting compound is methane (CH4). If the proportion is changed to 2:6, ethane (C2H6) is produced, and if the proportion is 6:6, then the product is benzene (C6H6). Similarly, hydrogen and oxygen combine in the proportion 2:1 to yield water. When they combine in the proportion 2:2, hydrogen peroxide is formed. Other examples are potassium nitrate (KNO3) and potassium nitrite (KNO2), carbon dioxide (CO2) and carbon monoxide (CO).
The properties of compounds are absolutely different from the properties of the elements that constitute the compound. For instance, hydrogen is combustible and oxygen is a supporter of combustion. But their product, water, neither burns, nor helps in burning. It actually extinguishes fire. Another familiar example is that of sodium and chlorine. Sodium is a violently reactive metal while chlorine is a highly poisonous gas with a choking and irritating smell. One cannot even think of consuming a piece of sodium or breathing in chlorine as such. But their product, i.e., sodium chloride, is consumed by all of us daily in the form of common salt.
The elements present in a compound can be re-obtained only by chemical processes. Water can be decomposed to hydrogen and oxygen only by electrolysis.
Formation of a compound always involves exchange of energy. For e.g., hydrogen burns in oxygen, to form water, liberating heat. Nitrogen combines with oxygen to form nitric oxide by absorbing heat. A candle burns in air to produce water and carbon dioxide, liberating heat and light. During the process of photosynthesis, carbon dioxide and water combine to form carbohydrate with absorption of light energy.
The combination of two or more elements to form a compound, or of two or more simple compounds to form a complex compound is called Synthesis.
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Examples of Synthesis
The break up of a compound into its elements or into simpler compounds is called Decomposition.
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Examples of Decomposition
