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Combination Reaction

A combination reaction is a reaction in which two or more substances combine to form one product.

In one line: Combine means many to one.

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Understand it

This type of reaction is easy to spot because the number of products is one. Many combination reactions release heat or light, so the mixture may feel warm or appear bright. The product is usually a single new substance.

Example

When quicklime reacts with water to form slaked lime, one product is formed.

Rule to remember

General pattern: A + B → AB, often with heat or light released.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1CaO + H2O → Ca(OH)2.
  2. 2Two reactants give one product, so this is a combination reaction.

Method you can reuse

  1. 1Count reactants and products.
  2. 2If several reactants give one product, mark combination.
  3. 3Do not depend only on heat release.

Sketch it in your answer

Write reactants on the left, one product on the right, and mark any heat change above the arrow.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

When quicklime reacts with water and forms one product, which reaction type is shown?

Think first, then show answer
It is a combination reaction because two substances join to form a single product.

Common wrong answer

Saying it is decomposition because heat is involved.

Why it loses marks: Decomposition means one compound breaks into simpler substances.

Don't mix this up

Students think any reaction with heat is combination. Heat release alone does not decide the reaction type.

Exam tip

Check the number of products first. If one product forms from multiple reactants, combination is likely.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask you to identify the reaction type from an equation or from a real-life change that gives one product.
  1. 1

    Enough for 1 mark

    A combination reaction is a reaction in which two or more substances combine to form one product.

  2. 2

    Add for 2 marks

    Example: When quicklime reacts with water to form slaked lime, one product is formed.

  3. 3

    Add for 3 marks

    This type of reaction is easy to spot because the number of products is one.

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