# Types of chemical reactions

Canonical: https://duckyhelper.com/learn/chemistry/types-of-chemical-reactions/
Updated: 2026-10-01

Most reactions in a high school course fit five types. Synthesis: two or more substances combine into one (A + B → AB). Decomposition: one substance breaks apart (AB → A + B). Single replacement: an element swaps into a compound (A + BC → AC + B). Double replacement: two compounds trade partners (AB + CD → AD + CB). Combustion: a fuel reacts with oxygen, usually making carbon dioxide and water.

## Key ideas

**The five reaction types**

| Type | Pattern | Example | How to spot it |
| --- | --- | --- | --- |
| Synthesis | A + B → AB | \(\mathrm{2\,Na + Cl_2 \rightarrow 2\,NaCl}\) | One product |
| Decomposition | AB → A + B | \(\mathrm{2\,H_2O_2 \rightarrow 2\,H_2O + O_2}\) | One reactant |
| Single replacement | A + BC → AC + B | \(\mathrm{Zn + 2\,HCl \rightarrow ZnCl_2 + H_2}\) | An element and a compound on each side |
| Double replacement | AB + CD → AD + CB | \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\) | Two compounds swap partners |
| Combustion | fuel + \(\mathrm{O_2}\) → \(\mathrm{CO_2 + H_2O}\) | \(\mathrm{CH_4 + 2\,O_2 \rightarrow CO_2 + 2\,H_2O}\) | \(\mathrm{O_2}\) reacts with a compound of C and H |

**Will a single replacement happen?** A metal replaces another metal (or hydrogen) only if it is more active, meaning higher on the **activity series**. A shortened version, from most to least active:

$$
\mathrm{Li > K > Ca > Na > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Ag > Au}
$$

**Will a double replacement happen?** Only if it makes something that leaves the solution: a solid (a precipitate), a gas, or water. If every product stays dissolved, there is no reaction.

## Worked examples

**Example 1: predict a single replacement**

Problem: Zinc metal is placed in copper(II) sulfate solution. Predict the products and write the balanced equation.

1. An element (Zn) plus a compound (\(\mathrm{CuSO_4}\)) means single replacement.
2. Check the activity series: Zn is above Cu, so zinc can replace copper.
3. Zinc forms \(\mathrm{Zn^{2+}}\), so it pairs with sulfate as \(\mathrm{ZnSO_4}\). Copper comes out as the metal.

   $$
   \mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}
   $$
4. Count atoms: Zn 1 = 1, Cu 1 = 1, S 1 = 1, O 4 = 4. Already balanced.

Answer: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\), single replacement, coefficients 1, 1, 1, 1

**Example 2: predict a double replacement**

Problem: Solutions of lead(II) nitrate and potassium iodide are mixed. Predict the products.

1. Two ionic compounds means double replacement. The ions are \(\mathrm{Pb^{2+}}\), \(\mathrm{NO_3^-}\), \(\mathrm{K^+}\) and \(\mathrm{I^-}\).
2. Swap partners: \(\mathrm{Pb^{2+}}\) with \(\mathrm{I^-}\) gives \(\mathrm{PbI_2}\). \(\mathrm{K^+}\) with \(\mathrm{NO_3^-}\) gives \(\mathrm{KNO_3}\).
3. \(\mathrm{PbI_2}\) does not dissolve, so it forms a bright yellow solid. That precipitate is why the reaction happens.
4. Balance.

   $$
   \mathrm{Pb(NO_3)_2 + 2\,KI \rightarrow PbI_2 + 2\,KNO_3}
   $$

Answer: \(\mathrm{Pb(NO_3)_2 + 2\,KI \rightarrow PbI_2 + 2\,KNO_3}\), double replacement, coefficients 1, 2, 1, 2

**Example 3: complete a combustion**

Problem: Write and balance the complete combustion of butane, \(\mathrm{C_4H_{10}}\).

1. Complete combustion of a hydrocarbon gives \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\).

   $$
   \mathrm{C_4H_{10} + O_2 \rightarrow CO_2 + H_2O}
   $$
2. Carbon: 4 \(\mathrm{CO_2}\). Hydrogen: 5 \(\mathrm{H_2O}\). Oxygen on the right: \(8 + 5 = 13\), so \(\tfrac{13}{2}\,\mathrm{O_2}\).
3. Double everything to clear the fraction.

   $$
   \mathrm{2\,C_4H_{10} + 13\,O_2 \rightarrow 8\,CO_2 + 10\,H_2O}
   $$

Answer: \(\mathrm{2\,C_4H_{10} + 13\,O_2 \rightarrow 8\,CO_2 + 10\,H_2O}\), coefficients 2, 13, 8, 10

## Common mistakes and how to fix them

- **Calling any reaction with \(\mathrm{O_2}\) a combustion.** \(\mathrm{2\,Mg + O_2 \rightarrow 2\,MgO}\) has one product, so it is synthesis, even though it burns. Combustion in most courses means a fuel with C and H making \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\).
- **Writing products with the wrong formulas.** Fix: build each new compound from the ion charges so the total is zero, then balance with coefficients.
- **Forgetting the diatomic elements.** \(\mathrm{H_2}\), \(\mathrm{N_2}\), \(\mathrm{O_2}\), \(\mathrm{F_2}\), \(\mathrm{Cl_2}\), \(\mathrm{Br_2}\) and \(\mathrm{I_2}\) come in pairs when they are on their own.
- **Predicting a reaction that does not happen.** Fix: check the activity series for single replacement, and look for a solid, gas or water for double replacement.

**Practice: name the type**

1. \(\mathrm{2\,KClO_3 \rightarrow 2\,KCl + 3\,O_2}\)
   A. Synthesis
   B. Decomposition
   C. Single replacement
   D. Double replacement
   E. Combustion

   Answer: Decomposition. One reactant breaks into two products.

2. \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\)
   A. Synthesis
   B. Decomposition
   C. Single replacement
   D. Double replacement
   E. Combustion

   Answer: Synthesis. Two reactants combine into one product.

3. \(\mathrm{Mg + 2\,HCl \rightarrow MgCl_2 + H_2}\)
   A. Synthesis
   B. Decomposition
   C. Single replacement
   D. Double replacement
   E. Combustion

   Answer: Single replacement. The element Mg takes hydrogen's place in the compound, and hydrogen comes out as \(\mathrm{H_2}\). Mg is above H in the activity series.

4. \(\mathrm{BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2\,NaCl}\)
   A. Synthesis
   B. Decomposition
   C. Single replacement
   D. Double replacement
   E. Combustion

   Answer: Double replacement. Two compounds trade partners. Solid \(\mathrm{BaSO_4}\) forms as a precipitate.

5. A strip of copper is placed in zinc sulfate solution. What happens?
   A. Copper replaces zinc
   B. No reaction
   C. Zinc and copper both dissolve
   D. A gas forms

   Answer: No reaction. Copper is below zinc on the activity series, so it cannot replace zinc. Compare Example 1, where the reaction goes the other way.

## Frequently asked questions

### Are there other types of reactions?

Yes. Many courses also teach acid-base (neutralization) reactions, which are a kind of double replacement that makes water, and redox reactions, where electrons move from one substance to another. Synthesis, decomposition, single replacement and combustion are all redox reactions.

### What is incomplete combustion?

When there is not enough oxygen, a fuel burns to carbon monoxide (CO) or soot (C) instead of \(\mathrm{CO_2}\). That is why gas stoves and heaters need air flow and why CO detectors matter: carbon monoxide is poisonous and has no smell.

### How do I know if a product is a solid?

Use a solubility table or rules. For example, nitrates and compounds of group 1 metals almost always dissolve, while most carbonates, phosphates and many sulfides do not. Your teacher may give you a table on tests.

## Sources

- [OpenStax Chemistry 2e, 4.2 Classifying Chemical Reactions](https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions), accessed 2026-10-01

## Related

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- [Acids, bases and pH](https://duckyhelper.com/learn/chemistry/acids-bases-and-ph/)
- [Stoichiometry: how much reacts and how much forms](https://duckyhelper.com/learn/chemistry/stoichiometry/)
- [Chemistry study guides](https://duckyhelper.com/learn/chemistry/)

## Try asking Ducky

- "Look at my list of reactions and tell me which ones I classified wrong."
- "Can you check my predicted products for number 6? I'm not sure about the charges."
- "Quiz me on the activity series: will this metal replace that one?"

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