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.