The entire universe may be considered to be made up of two entities, namely matter and energy. Anything, which occupies space and has mass may be termed as matter. The concept of energy is subtler but its effects can be easily visualized. Light and heat are the two main modes with the help of which the energy of a substance can be changed.
Man endeavours to systematize the knowledge that he gains by studying the various phenomena occurring around him and also from the man-made experiments. Based on these, matter has been classified into various categories.
Based on its physical state, matter has been broadly classified into three categories, namely, the gaseous, liquid and solid states. The classification of matter on the basis of its composition is described below:
Pure Substance and Mixture
Based on its chemical composition, matter may be classified as a pure substance or a mixture. A pure substance is made up of one type of substance whereas a mixture is made up of two or more than two substances.
Element and Compound
A pure substance has been classified into two categories, namely, element and compound. An element contains only one type of substance which cannot be broken further into two or more than two substances of independent existence. A compound contains two or more than two elements which can be broken into simpler substances or their constituents by chemical means or by any other means.
Every pure substance possesses characteristic physical and chemical properties, which identify the substance and help us to distinguish one substance from another. For example, water freezes at 0oC and boils at 100oC at one atmospheric pressure. It can be decomposed into hydrogen and oxygen gases when an electric current is passed through an alkaline solution. This shows that water is a compound and it is made up of two elements - hydrogen and oxygen. Mercury, which is an element, can be characterized from its shiny nature and also the fact that it exists in the liquid form at room temperature.
There are 105 elements discovered so far, out of which 92 are found in nature and the rest have been synthesized in the laboratory. The elements serve as the building blocks of all matter; an enormous variety of compounds as compared to the rest of elements. Some elements like carbon, oxygen, nitrogen and hydrogen form a large number of compounds as compared to the rest of the elements. In fact, carbon forms such a large number of compounds that the study of such compounds constitutes a separate branch of chemistry, known as organic chemistry. It is for this reason that the compounds of carbon have been classified as organic compounds. All other compounds are treated as inorganic compounds.