Combustion and Flame: Class 8 Science NCERT Guide

Understanding Combustion: The Science of Fire

Combustion is a chemical reaction where a substance reacts rapidly with an oxidizing agent, usually oxygen from the air, to produce heat and light. We often simply call this process 'burning'. The substance that undergoes combustion is called a combustible substance or a fuel. Examples include wood, paper, kerosene, petrol, and Liquefied Petroleum Gas (LPG). On the other hand, substances that do not burn, like stone, glass, and water, are called non-combustible substances.

For combustion to happen, three essential conditions must be met. Think of it as a 'fire triangle':

1. Fuel: There must be a combustible substance to burn.
2. Oxygen: There must be a supply of oxygen (usually from the air).
3. Heat: The fuel must be heated to its ignition temperature, which is the minimum temperature at which it catches fire. A matchstick won't light up on its own; you need to strike it to generate enough heat from friction to reach its ignition temperature.

Dissecting a Flame: The Three Zones of a Candle

1. Innermost Zone (Dark Zone) — This is the zone right around the wick. It appears black because it contains unburnt wax vapours. There is very little oxygen here, so no combustion takes place. It is the least hot part of the flame.
2. Middle Zone (Luminous Zone) — This is the bright yellow part of the flame, which gives off most of the light. In this zone, there isn't enough oxygen for complete combustion. The wax vapours burn incompletely, producing tiny, solid carbon particles. These particles get extremely hot and glow, creating the yellow light we see. This zone is moderately hot.
3. Outermost Zone (Non-Luminous Zone) — This is the faint, almost invisible blue layer at the very edge of the flame. Here, there is a plentiful supply of oxygen from the surrounding air, allowing for the complete combustion of the fuel. This complete combustion releases a lot of energy, making this the hottest part of the flame. This is why goldsmiths use the outermost zone to melt gold and silver.

Characteristics of a Good Fuel and How to Control Fire

• An ideal fuel should have a high calorific value, meaning it produces a large amount of heat per unit mass (measured in kilojoules per kilogram, kJ/kg).
• A good fuel is affordable, readily available, and easy to store and transport.
• It should burn at a moderate rate and not leave behind harmful residues or polluting gases.
• To control a fire, you must remove at least one of the three elements of the 'fire triangle': fuel, oxygen, or heat.
• Water is an effective fire extinguisher because it cools the combustible material below its ignition temperature. The water vapour also helps cut off the oxygen supply.
• IMPORTANT: Never use water on electrical fires or fires caused by oil/petrol. Water can conduct electricity, and since oil is lighter than water, it will float on top and continue to burn.

Exam Tip: Calorific Value and Fuel Efficiency

In exams, you will often be asked to compare different fuels. Remember that Calorific Value is the key to this. The higher the calorific value, the more 'efficient' the fuel is, as you get more energy from the same amount of fuel. For example, Hydrogen has a very high calorific value (~1,50,000 kJ/kg) while a cow dung cake has a very low one (~6,000-8,000 kJ/kg). LPG has a calorific value of about 55,000 kJ/kg, making it much more efficient than wood (~17,000 kJ/kg). Always mention units (kJ/kg) when discussing calorific value.

Practice Questions with Solutions

• Q: Why does a piece of paper burn easily while a piece of wood requires more effort to light?
  A: Step 1: Recall the concept of ignition temperature. Ignition temperature is the minimum temperature a substance needs to reach to catch fire.
  Step 2: Compare the ignition temperatures of paper and wood. Paper has a very low ignition temperature, so it heats up and catches fire quickly with a small flame like a matchstick.
  Step 3: Wood is denser and has a much higher ignition temperature. It requires a sustained source of heat to reach this temperature before it can start burning.
  Final answer: A piece of paper burns easily because it has a much lower ignition temperature compared to a piece of wood.

• Q: Explain why LPG is considered a better domestic fuel than wood.
  A: Step 1: Compare the fuels based on key characteristics like calorific value, pollution, and residue.
  Step 2: Calorific Value: LPG has a higher calorific value (approx. 55,000 kJ/kg) than wood (approx. 17,000 kJ/kg). This means LPG produces more heat for the same mass.
  Step 3: Pollution and Residue: Burning wood produces a lot of smoke and harmful gases, causing air pollution. It also leaves behind ash as residue. LPG burns cleanly without producing smoke and leaves no residue.
  Final answer: LPG is a better domestic fuel than wood because it has a higher calorific value, burns cleanly without causing pollution, and leaves no residue.

• Q: A goldsmith uses the outermost zone of a flame to melt gold. Why?
  A: Step 1: Identify the different zones of a flame and their properties. A flame has three zones: innermost, middle, and outermost.
  Step 2: Recall the temperature profile of the flame. The outermost zone is where complete combustion occurs due to a plentiful supply of oxygen.
  Step 3: Relate this property to the task. Complete combustion releases the maximum amount of heat, making the outermost zone the hottest part of the flame. Gold has a high melting point, so the hottest part of the flame is needed to melt it efficiently.
  Final answer: The goldsmith uses the outermost zone of the flame because it is the hottest part, which is necessary to achieve the high temperature required to melt gold.

• Q: What are the three essential requirements for combustion? How can a fire be controlled?
  A: Step 1: List the three components of the 'fire triangle'. The three essential requirements for combustion are (i) Fuel (a combustible substance), (ii) Oxygen (usually from the air), and (iii) Heat (to raise the temperature to the ignition temperature).
  Step 2: Explain the principle of fire control. To control a fire, we must remove at least one of these three requirements.
  Step 3: Give examples of controlling a fire. We can cut off the fuel supply. We can cut off the oxygen supply using a fire extinguisher (like CO2) or a blanket. We can remove heat by spraying water, which cools the fuel below its ignition temperature.
  Final answer: The three essentials for combustion are Fuel, Oxygen, and Heat. A fire can be controlled by removing any one of these, for example, by using water to remove heat or a carbon dioxide extinguisher to cut off oxygen.

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