A chemical reaction is a process in which new substances are formed with completely new properties. The substance which reacts in a chemical reaction are called reactant and the new substance produced as a result of chemical reaction are called products.
But how can one identify or recognize that the reaction has been completed. So, below are given some characteristics of a chemical reactions by which we can identify or tell that the reaction has been completed.
Evolution of gas: Some reactions are characterised by the evolution of a gas for ex – When zinc granules are treated with dilute sulphuric acid then bubbles of Hydrogen gas are produced around zinc granules.
We can test this gas by putting a burning candle in front of the place from where the gas is evolving or in front / mouth of the test tube in which reaction is taking place, as hydrogen gas is flammable it will burn with little explosions giving a pop sound
Change in temperature: Some reactions are characterised by change in temperature that is either rise or fall in temperature.
On the basis of change of temperature reactions are of 2 types.
(i) Exothermic Reaction: Those reactions in which heat is evolved or released and there is increase in temperature are called exothermic reactions. For example:
Reaction between quicklime and water is exothermic reaction and in this calcium hydroxide is formed with evolution of heat ex.
(ii) Endothermic Reaction: Those reactions in which heat is absorbed by the reactants to form products are called endothermic reactions. Or we can say decomposition reactions are endothermic reactions as they require heat to decompose or breakdown. For example
Change in colour: In some reaction there is a colour change observed for products that means that reaction has been completed.
If citric acid is added to purple colour potassium permagnate solution, then after mixing its colour disappears.
Formation of Precipitate: Some reactions are identified by the formation of precipitate (a solid insoluble substance) in the reaction mixture. For example