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Changes Around Us: Physical and Chemical - Combustion

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Combustion is a chemical process in which a substance reacts with oxygen (O2O_2) to give off heat and light. It is a chemical change because new substances are formed.

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A substance that undergoes combustion is said to be combustible, also called a fuel. Examples include wood, coal, and LPGLPG.

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Ignition Temperature is the lowest temperature at which a substance catches fire. A combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.

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Inflammable substances are those which have very low ignition temperatures and can easily catch fire with a flame, such as Petrol, Alcohol, and LPGLPG.

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Types of combustion include: 1. Rapid Combustion (gas burns quickly producing heat and light), 2. Spontaneous Combustion (material bursts into flames without apparent cause), and 3. Explosion (sudden reaction with heat, light, and sound).

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The conditions necessary for combustion are: presence of a combustible substance, presence of air (Oxygen), and attainment of ignition temperature.

📐Formulae

C+O2→CO2+Heat+LightC + O_2 \rightarrow CO_2 + \text{Heat} + \text{Light}

2Mg+O2→2MgO+Heat2Mg + O_2 \rightarrow 2MgO + \text{Heat}

CH4+2O2→CO2+2H2O+EnergyCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}

4Fe+3O2+2nH2O→2Fe2O3⋅nH2O (Rusting process related to oxidation)4Fe + 3O_2 + 2nH_2O \rightarrow 2Fe_2O_3 \cdot nH_2O \text{ (Rusting process related to oxidation)}

💡Examples

Problem 1:

Write the chemical reaction for the burning of Magnesium in the presence of Oxygen and identify the product.

Solution:

2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO

Explanation:

When magnesium (MgMg) burns, it reacts with oxygen (O2O_2) from the air to form Magnesium Oxide (MgOMgO), which appears as a white powdery ash. This is a chemical change because a new substance is formed.

Problem 2:

If a substance has an ignition temperature of 85∘C85^\circ C and its current temperature is 37∘C37^\circ C, calculate the temperature increase required for it to catch fire.

Solution:

85−3748\begin{array}{r} 85 \\ - 37 \\ \hline 48 \end{array}

Explanation:

The substance needs its temperature to be raised by 48∘C48^\circ C to reach its ignition temperature (85∘C−37∘C=48∘C85^\circ C - 37^\circ C = 48^\circ C) before it can undergo combustion.

Problem 3:

Why is the burning of a candle considered a chemical change while the melting of wax is a physical change?

Solution:

Wax (Solid)→HeatWax (Liquid) [Physical Change]\text{Wax (Solid)} \xrightarrow{\text{Heat}} \text{Wax (Liquid)} \text{ [Physical Change]} Wax+O2→CO2+H2O+Heat/Light [Chemical Change]\text{Wax} + O_2 \rightarrow CO_2 + H_2O + \text{Heat/Light} \text{ [Chemical Change]}

Explanation:

Melting is a physical change because only the state changes from solid to liquid. Burning is a chemical change because the wax reacts with O2O_2 to form new products like Carbon Dioxide (CO2CO_2) and water vapor (H2OH_2O).

Combustion Class 7 Notes & Examples | CBSE Science