The molecules of one reactant are combined with those of another reactant to form a new substance during a chemical reaction. As the chemical bonds are broken, the positions of electrons change, resulting in a product with properties that are different from the properties of the reactants. Various temperatures and catalysts are needed for different chemical reactions to occur.
In a chemical reaction, there is a change of electrons, but there is no change of nuclei. Therefore, the elements present in the product of the reaction are still the same as the elements in its original reactants. There are chemical reactions that take place spontaneously without the input of extra energy, while others need the addition of heat, light or electricity.
A basic example of a chemical reaction is the combustion of a candle, in which oxygen combines with molecules of the wax to melt it into a new product. Similarly, the process of fermentation and the reduction of mineral ores into metals are well-known chemical reactions. A chemical reaction can easily be perceived through the senses because it involves emission of considerable heat, light and gas. In addition, the process forms a precipitate, which features a color that is different from the reactants. Basic chemical reactions involve the combination of harmless elements, while major chemical reactions involve reactants that are often violent and destructive.