From combination to redox, every type of chemical reaction follows a recognisable pattern. Learn to identify each type, write the correct equation, and answer exam questions confidently.
Chemists classify reactions into types because each type follows predictable rules. Once you recognise the type of reaction, you can predict what products will form — even for substances you have never seen before. This skill is tested directly in exams and forms the foundation for organic chemistry and industrial processes at higher levels.
At Class 6 to O Level, you need to know six main types: combination, decomposition, displacement (single replacement), double displacement, combustion, and redox. Each has a characteristic pattern in its equation.
In a combination reaction (also called a synthesis reaction), two or more substances join together to form one new product. The general pattern is:
A + B → AB
The reactants can be elements, compounds, or a mix of both. Combination reactions often release energy as heat or light.
How to recognise this type: one product is produced from two or more reactants. If you see a single arrow leading to a single substance on the right, it is almost certainly combination.
A decomposition reaction is the reverse of combination — one compound breaks down into two or more simpler substances. Energy is usually needed to drive decomposition, most often in the form of heat (thermal decomposition) or electricity (electrolysis).
AB → A + B
A classic exam clue: if the question says "heated strongly" or "passed electricity through," suspect decomposition.
In a displacement reaction, a more reactive element pushes a less reactive element out of its compound. This follows the reactivity series — an element can only displace another element that sits below it in the series.
A + BC → AC + B
The visual clue in a lab: if you drop a piece of metal into a solution and the solution changes colour — or a new solid appears on the metal's surface — a displacement reaction has occurred.
In a double displacement reaction, two compounds swap their ions to form two new compounds. One of the products is often an insoluble precipitate that falls out of solution as a solid.
AB + CD → AD + CB
Precipitation reactions are used as tests for specific ions in the laboratory. If a white precipitate forms when you add barium chloride to a solution, sulfate ions are present.
Combustion is the rapid reaction of a substance with oxygen, producing heat and light. Most combustion reactions involve fuels (hydrocarbons) burning in oxygen. There are two types: complete and incomplete combustion.
When there is enough oxygen, a hydrocarbon burns completely to produce only carbon dioxide and water.
When oxygen is limited, combustion is incomplete and produces carbon monoxide (CO) and/or carbon (soot) instead of carbon dioxide. Carbon monoxide is colourless, odourless, and highly toxic — this is why poorly ventilated gas heaters are dangerous.
Redox stands for reduction-oxidation. These reactions involve the transfer of electrons between substances. Oxidation and reduction always happen together — you cannot have one without the other.
The memory trick: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons).
Question: Zinc powder is added to copper sulfate solution. A brown solid forms and the blue solution turns colourless. Identify the reaction type and write a word equation.
Reaction type: Displacement (zinc is more reactive than copper and displaces it).
Word equation: Zinc + Copper sulfate → Zinc sulfate + Copper
Observation clue: Brown solid = copper metal depositing; blue colour fading = Cu²⁺ ions being used up.
Question: Write the balanced equation for the complete combustion of ethane (C₂H₆).
Unbalanced: C₂H₆ + O₂ → CO₂ + H₂O
Balance carbon: 2CO₂
Balance hydrogen: 3H₂O
Balance oxygen: 2 + 3/2 = 7/2 — multiply everything by 2 to remove fraction.
Balanced: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Question: Green copper carbonate is heated strongly. Describe what you observe and write a balanced equation.
Observation: The green solid turns black (copper oxide forms). A colourless gas is produced (carbon dioxide) which turns limewater milky.
Equation: CuCO₃ → CuO + CO₂
Question: In the reaction CuO + H₂ → Cu + H₂O, identify what is oxidised and what is reduced.
Copper oxide (CuO): loses oxygen → reduced (copper is the oxidising agent's partner).
Hydrogen (H₂): gains oxygen → oxidised (hydrogen is the reducing agent).
Classifying reactions correctly is the first step to writing accurate equations. Practise with different examples until identifying the type from the equation pattern becomes second nature.