What Is a Chemical Reaction?

You encounter chemical reactions all the time. Fire, respiration, and cooking all involve chemical reactions. Yet, do you know what exactly a chemical reaction is? Here's the answer to the question.

Chemical Reaction Definition

Simply put, a chemical reaction is any transformation from one set of chemicals into another set.

If the starting and ending substances are the same, a change may have occurred, but not a chemical reaction. A reaction involves a rearrangement of molecules or ions into a different structure. Contrast this with a physical change, where the appearance is altered, but the molecular structure is unchanged, or a nuclear reaction, in which the composition of the atomic nucleus changes. In a chemical reaction, the atomic nucleus is untouched, but electrons may be transferred or shared to break and form chemical bonds. In both physical changes and chemical changes (reactions), the number of atoms of each element are the same both before and after a process occurs. However, in a physical change, the atoms maintain their same arrangement into molecules and compounds. In a chemical reaction, the atoms form new products, molecules, and compounds.

Signs a Chemical Reaction Has Occurred

Since you can't look at chemicals at a molecular level with the naked eye, it's helpful to know signs that indicate a reaction has occurred. A chemical reaction is often accompanied by a temperature change, bubbles, color change, and/or precipitate formation.

Chemical Reactions and Chemical Equations

The atoms and molecules that interact are called the reactants. The atoms and molecules produced by the reaction are called products. Chemists use a shorthand notation called a chemical equation to indicate the reactants and the products. In this notation, the reactants are listed on the left side, the products are listed on the right side, and the reactants and products are separated by an arrow showing which direction the reaction proceeds. While many chemical equations show reactants forming products, in reality, the chemical reaction often proceeds in the other direction, too. In a chemical reaction and a chemical equation, no new atoms are created or lost (conservation of mass), but chemical bonds may be broken and formed between different atoms.

Chemical equations may be either unbalanced or balanced. An unbalanced chemical equation doesn't account for conservation of mass, but it's often a good starting point because it lists the products and reactants and the direction of the chemical reaction.

As an example, consider rust formation. When rust forms, the metal iron reacts with oxygen in the air to form a new compound, iron oxide (rust). This chemical reaction may be expressed by the following unbalanced chemical equation, which may be written either using words or using the chemical symbols for the elements:

iron plus oxygen yields iron oxide

Fe + O → FeO

A more accurate description of a chemical reaction is given by writing a balanced chemical equation. A balanced chemical equation is written so the number of atoms of each type of element are the same for both the products and reactants. Coefficients in front of chemical species indicate quantities of reactants, while subscripts within a compound indicate the number of atoms of each element. Balanced chemical equations typically list the state of matter of each reactant (s for solid, l for liquid, g for gas). So, the balanced equation for the chemical reaction of rust formation becomes:

2 Fe(s) + O₂(g) → 2 FeO(s)

Examples of Chemical Reactions

There are millions of chemical reactions! Here are some examples:

• Fire (combustion)
• Baking a cake
• Cooking an egg
• Mixing baking soda and vinegar to produce salt and carbon dioxide gas

Chemical reactions may also be categorized according to general types of reactions. There's more than one name for each type of reaction, so that may be confusing, but the form of the equation should be easy to recognize:

• Synthesis reaction or direct combination: A + B → AB
• Analysis reaction or decomposition: AB → A + B
• Single displacement or substitution: A + BC → AC + B
• Metathesis or double displacement: AB + CD → AD + CB

Other types of reactions are redox reactions, acid-base reactions, combustion, isomerization, and hydrolysis. Chemical reactions are everywhere.

Learn More

What Is the Difference Between a Chemical Reaction and a Chemical Equation?
Exothermic and Endothermic Reactions