4.1. What is Combustion?

A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible. It is also called a fuel. The fuel may be solid, liquid or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow.

Combustion is a reaction that typically occurs when a substance combines with oxygen, resulting in the release of energy in the form of heat and light. The substance that burns in this process is referred to as combustible or fuel. Fuels can exist in three different states: solid (like wood), liquid (like petrol), or gas (like methane). During combustion, the energy released can often be seen as a flame or glow. For instance, when you burn wood in a fireplace, the wood reacts with oxygen in the air, releasing heat and creating a visible flame.

Think of combustion as a campfire. When you gather around a fire, the wood (fuel) you burn reacts with the oxygen in the air, producing heat (to stay warm) and light (to see). Without oxygen, the wood would not burn, just like you need air to breathe.

In the reactions mentioned above magnesium and charcoal are combustible substances.

Magnesium and charcoal are two common examples of combustible materials. When magnesium burns, it forms magnesium oxide and releases heat and light, demonstrating vigorous combustion. Charcoal also burns in the presence of air, producing carbon dioxide, heat, and light. The combustion of these materials not only illustrates the process of combustion but also shows the variety of fuels available for use.

Consider how a magnesium flare works. When magnesium is ignited, it burns extremely brightly, which is useful in signaling or illumination. Charcoal, on the other hand, is commonly used in barbecues; when it ignites, it provides a steady heat source for cooking because it burns slowly and steadily.

We find that for combustion, air is necessary.

For combustion to occur, oxygen must be present. In the experiments with a candle and charcoal, it was observed that when air is restricted, the flame either flickers or goes out entirely. This demonstrates that without oxygen, combustion cannot take place. Air contains about 21% oxygen, which is enough for most combustion processes. This principle is critical for understanding fire safety as well, because removing the air supply can extinguish a fire.

Imagine blowing out candles on a birthday cake. When you blow on the candle, you are pushing air away, and the flame goes out. This means that you have removed the necessary oxygen for combustion to occur. It's a simple yet effective way to see the role that air plays in fire.

We find that a combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.

A combustible material has a specific temperature known as its ignition temperature, which is the minimum temperature required for it to ignite and begin burning. Things like kerosene will ignite at a lower temperature compared to wood, which requires a higher temperature to catch fire. Therefore, it is crucial to know the ignition temperatures of various materials to prevent accidental fires, especially in kitchen settings or when handling flammable substances.

Think of trying to start a campfire. You can’t just throw logs in and expect them to ignite right away. You need to kindling (like dry leaves) that ignites at a lower temperature and can transfer that heat to the larger logs, raising their temperature until they reach their ignition point.

Recall the activity of burning of magnesium ribbon performed in Class VII. We learnt that magnesium burns to form magnesium oxide and produces heat and light.

Burning magnesium is a vivid example of combustion in action. When magnesium ribbon is ignited, it burns brightly, producing a significant amount of light and heat while forming magnesium oxide as a byproduct. This reaction not only illustrates the concept of combustion but also shows the transformation of materials during the process, shifting from solid magnesium to gas and solid magnesium oxide. This is a great demonstration of how materials interact during combustion and the energetic nature of such reactions.

Consider magnesium flares used at night for signaling or in photography. When ignited, they provide a brilliant light source due to the combustion of magnesium, demonstrating the practical use of combustion principles in real-world applications.

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