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Combination Reaction

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

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

    Saying it is decomposition because heat is involved. That is wrong because decomposition means one compound breaks into simpler substances.

Definition

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

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.

Memory hook

Combine means many to one.

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

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.

Avoid common mistakes

Common confusion

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

Common wrong answer

Saying it is decomposition because heat is involved. That is wrong because decomposition means one compound breaks into simpler substances.

Exam tip

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

Quick check

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

It is a combination reaction because two substances join to form a single product.

Answer writing and exam use

1-mark answer

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

2-mark answer

A combination reaction is a reaction in which two or more substances combine to form one product. General pattern: A + B → AB, often with heat or light released. When quicklime reacts with water to form slaked lime, one product is formed.

3-mark answer

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. General pattern: A + B → AB, often with heat or light released. CaO + H2O → Ca(OH)2. Two reactants give one product, so this is a combination reaction. Questions may ask you to identify the reaction type from an equation or from a real-life change that gives one product. Saying it is decomposition because heat is involved. That is wrong because decomposition means one compound breaks into simpler substances.
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