What this calculator does
Chemical reactions are grouped into a small set of standard patterns based on how many reactants and products are involved and what happens to them: synthesis, decomposition, single replacement, double replacement and combustion. Recognising which pattern a reaction fits is usually the first step to predicting its products or balancing its equation.
This tool works the classification backwards from the usual approach. Instead of memorising five definitions and matching a written equation against each one, answer a few simple questions about the reaction (how many reactants, how many products, whether oxygen is involved) and it identifies the most likely category, along with the general form and a real worked example.
The formula
The classification follows the standard rules taught in introductory chemistry: one reactant splitting into two products is a decomposition; two reactants combining into one product is a synthesis; oxygen reacting with a fuel is combustion; and reactions with two reactants and two products are either a single replacement (one reactant is a free element) or a double replacement (both reactants are compounds that swap partners).
| Term | Meaning |
|---|---|
| Synthesis (combination) | Two or more substances combine to form one more complex product: A + B → AB. |
| Decomposition | A single compound breaks apart into two or more simpler substances: AB → A + B. |
| Single replacement (displacement) | A free element takes the place of an element already in a compound: A + BC → AC + B. |
| Double replacement (metathesis) | Two compounds exchange partners: AB + CD → AD + CB. |
| Combustion | A fuel reacts with oxygen gas, releasing energy and typically producing carbon dioxide and water. |
The inputs explained
| Field | What to enter |
|---|---|
| Number of reactants | How many separate reactant substances are going into the reaction. |
| Number of products | How many separate product substances come out of the reaction. |
| Is one reactant O₂ (oxygen gas) reacting with a fuel? | Answer yes if the reaction is a fuel (such as a hydrocarbon) burning in oxygen gas. |
| Is one reactant a single free element (not a compound)? | For reactions with two reactants and two products, answer yes if one of the reactants is a single free element rather than a compound. |
When to use it
Checking homework or a written equation
Reading off the number of reactants and products from a given equation and answering the oxygen and element questions gives a quick check on which category it falls into.
Predicting products from a pattern
Once a reaction is classified, the general form for that category shows the shape the products should take, which is often the starting point for balancing the full equation.
Studying for an exam
Working through a handful of reactions with this classifier, and comparing the result against the general form and example, reinforces the pattern-recognition skill the categories are testing.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Decomposition vs double replacement: how many reactants decides it
The same two-product setup, with only the number of reactants changed.
| Number of reactants | Most likely reaction type | General form | Worked example |
|---|---|---|---|
| One reactant | Decomposition | AB → A + B | 2H₂O₂ → 2H₂O + O₂ |
| Two reactants | Double replacement (metathesis) | AB + CD → AD + CB | BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl |
Single vs double replacement: what the free element decides
The same reactant and product count, with only whether a free element is involved changed.
| Is a reactant a free element? | Most likely reaction type | General form | Worked example |
|---|---|---|---|
| No | Double replacement (metathesis) | AB + CD → AD + CB | BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl |
| Yes | Single replacement (displacement) | A + BC → AC + B | Zn + 2HCl → ZnCl₂ + H₂ |
Questions
Can a reaction fit more than one category?
Combustion reactions technically also fit the pattern of a synthesis or, less commonly, a decomposition, but they are conventionally classified on their own because the oxygen-plus-fuel pattern and its energy release are the more useful thing to recognise.
What is the difference between single and double replacement?
In a single replacement, one reactant is a free element (not yet bonded to anything) that displaces an element from a compound. In a double replacement, both reactants are already compounds, and they simply swap partners with each other; no free element is involved on either side.
Why does combustion always produce CO₂ and H₂O?
That is specific to the complete combustion of a hydrocarbon fuel in excess oxygen, where every carbon atom ends up bonded to oxygen as CO₂ and every hydrogen atom ends up bonded to oxygen as H₂O. Incomplete combustion, with insufficient oxygen, can produce carbon monoxide or soot instead.
Are these the only types of chemical reactions?
They are the five most commonly taught introductory categories. More advanced chemistry adds further classifications, such as acid-base neutralisation and oxidation-reduction (redox), which can overlap with these five rather than replace them; a double replacement between an acid and a base, for instance, is also a neutralisation reaction.
For the underlying calculation once you know how many moles of each substance are reacting, see the mole calculator. For solution-based reactions specifically, the molarity calculator covers the concentration side.