Reaction Categories
Middle School
Definition
Basic types of chemical reactions, such as combination (substances join), decomposition (a substance breaks apart), and combustion (a substance burns in oxygen). Classifying reactions helps predict the products that will form.
Worked examples
Combination: \(2\)Mg\( + \)O\(_2 \)→\( 2\)MgO\(\)
Two substances join to form a single product — magnesium and oxygen combine into magnesium oxide.
Decomposition: \(2\)H\(_2\)O\(_2 \)→\( 2\)H\(_2\)O\( + \)O\(_2\)
One compound breaks apart into two or more simpler substances — hydrogen peroxide splits into water and oxygen.
Combustion: \(\)CH\(_4 + 2\)O\(_2 \)→\( \)CO\(_2 + 2\)H\(_2\)O\(\)
A substance burns in oxygen, producing carbon dioxide and water — methane combusts with oxygen.
Common mistakes
- Calling any reaction with oxygen a combustion reaction → Only reactions where a substance burns (rapid oxidation with heat/light) are combustion Rusting is oxidation but not combustion because it happens slowly without flame.
- Thinking decomposition always requires heat → Decomposition can be triggered by heat, light, or electricity Photodecomposition uses light; electrolysis uses electricity to break down compounds.
- Confusing combination with all reactions that have multiple reactants → Combination produces one product; reactions with multiple products are not combination Combination specifically means two or more substances join to form a single product.
Where you'll use it next
You'll apply reaction categories when balancing equations, predicting products in stoichiometry, and analyzing energy changes in thermochemistry and organic chemistry.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026