A chemical change is a process where two substances combine to form one or more new substances. This process is referred to as a chemical reaction, and it is not reversible.
Chemical reactions can either be exothermic or endothermic. Exothermic reactions produce heat, while endothermic reactions require heat for the reaction to occur. When a chemical reaction occurs, the atoms are rearranged, and an energy change also occurs as new products are produced. These new products have different chemical properties from the substances used to cause the reaction.
A change in color, odor or composition can indicate that a chemical change has occurred. For example, paper turns to ash when burned. A change in energy, such as the loss or production of heat or the decomposition of organic matter is a sign of chemical change. When a change is complete, it is irreversible.
Examples of chemical changes include cooking food, burning wood, rusting of iron and rotting of fruits. Another example of a chemical change is the reaction between sodium and water to produce sodium hydroxide and hydrogen.
Chemical changes can be grouped into three main classes: organic, inorganic and biochemical. Neutralization and oxidization are examples of inorganic chemical changes, while condensation and polymerization are examples of organic chemical changes.