The differences between chemical and physical reactions are often up for debate among scientists. If you break down scientific reactions into small steps, most reactions will go through both chemical and physical changes before the final result of the reaction. The most definite difference between a chemical and physical reaction, however, is whether the result is reversible.
A chemical reaction is defined as a reaction that results in the creation of a new substance and/or the destruction of existing substances. A chemical reaction occurs at the molecular level of substances. These changes involve either making or breaking the bonds between atoms. A chemical reaction is therefore irreversible.
Examples of Chemical Reactions
There are some changes that can be observed that indicate that a chemical reaction has occurred. These changes are: the temperature rises (exothermic reaction), the temperature decreases (endothermic reaction), a change in colour and a change in odour. Some examples of chemical reactions include iron rusting (an oxide is formed), burning (irreversible change) and milk souring (lactic acid is formed).
A physical reaction is different from a chemical reaction because the substance involved in the reaction has the same chemical composition, but its physical appearance has changed. This change is reversible. A physical reaction is thus defined as a change in the state of matter, i.e., solid, liquid or gas.
Examples of Physical Reactions
Physical reactions are easily observable. Some examples of physical reactions include water freezing, breaking a glass, dissolving sugar in water and folding a piece of paper.