In a chemical reaction, the substances that combine are called reactants and the new substances formed are called products. For example, when and (reactants) combine they form (product).
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Most of the reactions, in which the products do not recombine to form reactants, are called irreversible reactions. They are supposed to complete and are represented by putting a single arrow ( between the reactants and products.
On the other hand, reactions in which the products can recombine to form reactants are called reversible reactions. These reactions never go to completion. They are represented by a double arrow ( between reactants and products. These reactions proceed in both ways, i.e., they consist of two reactions; forward and reverse. So, a reversible reaction is one which can be made to proceed in either direction depending upon the conditions.
Let us discuss a reaction between hydrogen and iodine. Because one of the reactants, iodine is purple, while the product hydrogen iodide is colourless, proceedings of the reaction are easily observable. On heating, hydrogen and iodine. vapours in a closed flask, hydrogen iodide is formed. As a result, purple colour of iodine fades as it reacts to form colourless hydrogen iodide, as shown in figure 9.1.