COMBUSTION AND FLAME | Class 8 Science Notes

What is Combustion?

Combustion is a chemical process in which a substance reacts rapidly with oxygen to release heat and sometimes light. This process is commonly known as burning. The substance that undergoes combustion is called combustible or fuel. Fuels can be solid, liquid, or gaseous. For example, magnesium burns in air to form magnesium oxide, releasing heat and light, as demonstrated by burning a magnesium ribbon. Similarly, charcoal and coal burn in air producing carbon dioxide, heat, and light. The process of combustion is exothermic, meaning it releases energy. In some cases, light is emitted as a flame or glow during combustion. The chapter introduces the concept that food acts as a fuel for our body, where it reacts with oxygen to release energy in the form of heat, a process studied earlier in Class VII. The section also includes an activity where students test various materials for combustibility, classifying them as combustible or non-combustible based on whether they burn or not. It emphasizes the necessity of air (oxygen) for combustion through experiments with a candle flame under a glass chimney, showing that without air supply, the flame flickers and eventually extinguishes. The section clarifies that the sun's heat and light are not due to combustion but nuclear reactions, a topic for higher classes. It also discusses ignition temperature, the minimum temperature at which a substance catches fire, explaining why a matchstick does not burn by itself at room temperature but ignites when struck due to frictional heat. The history of matchsticks is briefly covered, highlighting the evolution from dangerous white phosphorus to safer red phosphorus in safety matches. The section concludes by explaining that substances with low ignition temperatures, called inflammable substances (e.g., petrol, alcohol, LPG), catch fire easily and require careful handling.

📊 Diagram: Fig. 4.1: Burning of magnesium; We were told that food is a fuel for our body.; (a), (b), (c): Experiment to show that air is essential for burning

🧪 Activity: Activity 4.1: Testing various materials like straw, matchsticks, kerosene oil, paper, iron nails, stone pieces, glass for combustibility under teacher supervision and classifying them as combustible or non-combustible; Activity 4.2: Observing the effect of air supply on a candle flame using a glass chimney and wooden blocks.

🔗 Connection: This section lays the foundation for understanding the conditions necessary for combustion, leading to the study of controlling fire and types of combustion.