A chemical reaction is usually accompanied by physical effects that are easily observed, such as heat and light emissions, precipitation formation, gas evolution, or color change. The chemical reaction releases heat energy it is called an exothermic reaction. If the chemical reaction absorbs energy it is called an endothermic reaction.
The many different reactions are categorized by chemists into general categories. Chemical reactions are part of the world around us. The boiling egg is a chemical reaction. Once the egg is cooked it cannot be changed back to the way it was.
Chemical equations can be made more concise and useful if we use formula instead of words. Compounds, called reactants, react with each other in a chemical reaction to create various compounds, called products.