Science · Chemistry foundations
Chemical Reactions and Equations
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Reactants are changed into products.
- A balanced equation has the same number of each atom on both sides.
- Coefficients change particle amounts; subscripts change substance identity.
- Synthesis combines substances.
- Decomposition splits a substance.
- Combustion often uses oxygen and releases energy.
What you’ll be able to do
- Identify reactants and products.
- Use evidence to recognize a reaction.
- Balance simple equations.
- Classify common reaction patterns.
Learn to read reactions, balance simple equations, and recognize common reaction patterns.
Optional review
If you’d like to review the basics, visit Chemical Bonds, Formulas, and Molecules. You can start this lesson without completing those first.
Reactants, products, and evidence
A chemical reaction rearranges atoms. Starting substances are reactants. New substances are products. An arrow means “yields.”
Evidence may include gas formation, a new solid, a lasting color change, or heat or light. A phase change alone is not a chemical reaction because no new substance forms.
Read an equation from left to right. In 2H2 + O2 → 2H2O, hydrogen and oxygen are reactants and water is the product. The plus sign separates substances; it does not mean ordinary arithmetic addition.
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