Why we classify reactions
There are millions of chemical reactions, but most of the ones you meet in high school fall into just five families. Sorting a reaction into its family is useful for two reasons: it helps you predict the products before you ever run the reaction, and it tells you how to balance the equation afterward. The tool above shows balanced equations pulled from a verified dataset and lets you practice naming each type. It is free to embed on your own site or LMS.
Synthesis: A + B → AB
In a synthesis (or combination) reaction, two or more simpler substances join to make a single, more complex product. The formation of water is the classic case: 2 H₂ + O₂ → 2 H₂O. A metal and a nonmetal combining into an ionic compound — the same kind of electron transfer described in types of chemical bonds — is also synthesis, for example sodium and chlorine forming table salt. If you see several reactants collapse into one product, it is synthesis.
Decomposition: AB → A + B
Decomposition is the exact reverse of synthesis: a single compound breaks apart into two or more simpler substances, usually when you add energy as heat or electricity. Heating limestone gives CaCO₃ → CaO + CO₂, and passing current through water splits it back into its elements. One reactant in, two or more products out — that signals decomposition.
Replacement reactions: who swaps with whom
Single replacement follows A + BC → AC + B: a lone element takes the place of another element inside a compound. When zinc drops into hydrochloric acid, Zn + 2 HCl → ZnCl₂ + H₂ — the zinc displaces hydrogen, which bubbles off as a gas. A more reactive metal pushes out a less reactive one.
Double replacement follows AB + CD → AD + CB: two compounds trade partners. Mixing silver nitrate with sodium chloride gives AgNO₃ + NaCl → AgCl + NaNO₃, where insoluble silver chloride drops out as a solid precipitate. Acid–base neutralizations, which form a salt and water, are double replacements too.
Combustion: fuel + O₂ → CO₂ + H₂O
In combustion, a fuel reacts rapidly with oxygen, releasing energy as heat and light. When the fuel is a hydrocarbon, complete combustion always produces carbon dioxide and water: burning methane gives CH₄ + 2 O₂ → CO₂ + 2 H₂O. Because the products are so predictable, combustion is one of the easiest patterns to spot — and to balance, once you know the carbon and hydrogen all end up in CO₂ and H₂O.
Using the pattern to predict products
Once you recognize the family, the pattern hands you the products. Spot a decomposition and you know the single reactant will split apart; spot a double replacement and you can swap the partners to write the products, then check solubility to see what precipitates. The next step is always to make the atoms balance — see balancing chemical equations for that — and if you want to revisit how atoms join in the first place, the molecule builder and Lewis structures show the bonding behind every product.