4.8. KEYWORDS

The substances which burn in air are called combustibles.

Combustion refers to a chemical process where a substance reacts with oxygen, resulting in the release of heat and light. If a material can burn in air, it is defined as 'combustible'. This means that not all materials can catch fire; only those that can react with oxygen are classified in this way.

Think of burning a piece of wood in a campfire. The wood is clearly a combustible material. However, a stone won't catch fire no matter how hot it gets. Thus, it is not combustible.

Oxygen (in air) is essential for combustion.

For combustion to occur, oxygen is necessary. This means that a fire can only start and continue burning if oxygen is available in the environment. Without oxygen, combustion cannot take place, which is why putting a lid on a burning candle will extinguish the flame.

Imagine trying to light a fire under water—it simply won’t work. Similarly, if you wrap a fire in a blanket to block the air, the fire will go out due to the lack of oxygen.

During the process of combustion, heat and light are given out.

When substances burn, they undergo chemical changes resulting in the release of energy in the form of heat and light. This is why flames give off light and why we feel warmth when we are near a fire.

Think of lighting a birthday candle. Once lit, it not only helps you celebrate but also emits light and warmth to those nearby. This is a direct consequence of combustion.

Ignition temperature is the lowest temperature at which a combustible substance catches fire.

Every combustible material has a specific temperature at which it will ignite, called its ignition temperature. If the material is below this temperature, it will not catch fire, even if it is surrounded by oxygen. For example, wood will only burn when heated sufficiently.

Imagine trying to start a barbecue. You can’t just throw wood on the grill; you need to reach a high enough temperature (adding lighter fluid or paper) to ensure the wood ignites.

Inflammable substances have very low ignition temperature.

Inflammable substances ignite easily because they catch fire at low temperatures. This is important for safety reasons, as these materials pose a significant risk of accidental fires.

Think about how quickly gasoline can ignite compared to a piece of wood. Gasoline is highly flammable (or inflammable), which is why it should be stored safely far from any heat source.

Fire can be controlled by removing one or more requirements essential for producing fire.

Fires can be extinguished by removing one of the three essential components: fuel, oxygen, or heat. This is the principle behind fire-fighting techniques. For example, throwing water on a fire cools the material below its ignition temperature.

If you observe a campfire, you can see how adding logs keeps it burning (fuel). If you throw dirt on it (removing oxygen), the fire will die out.

Water is commonly used to control fires.

Water is effective in putting out fires that involve solid fuels like wood because it cools down the burning material and may also cut off the oxygen supply by creating steam. However, it's not suitable for all types of fires, such as oil fires.

If you’ve ever spilled oil in a kitchen, pouring water on it might actually make it worse because oil floats on water. Instead, using a fire extinguisher that releases carbon dioxide would be safer and more effective.

There are various types of combustions such as rapid combustion, spontaneous combustion, explosion, etc.

Combustion can occur in several ways, such as rapid combustion (like a fireworks explosion), spontaneous combustion (where a material ignites on its own due to heat accumulation), and explosive combustion (where a rapid reaction occurs with a loud noise). Different scenarios and materials lead to different combustion types.

Think of how a firecracker works: it doesn’t gradually burn like a candle; it explodes with a bang due to a rapid and uncontrolled combustion reaction.